Geography curriculum

Beneath the oceans the crust is usually only about 5 kilometers thick, while continental crust averages about 30 kilometers and can reach 100 under big ranges. Continental rock is relatively light, so it rides high and resists sinking: where an ocean plate meets a continent, the ocean plate dives. Oceanic rock is also far younger, because it is recycled at trenches.

When continental crust is stretched beyond its limits, tension cracks open at the surface and magma squeezes up through them, sometimes erupting. The rising magma presses on the crust and widens the fractures into a rift zone. East Africa is the live example, where the Indian Ocean may one day flood in; the same process already split Arabia from Africa and opened the Red Sea.

Most mountain building happens where plates converge. When oceanic crust subducts beneath a continent, the overriding plate is lifted, as in the Andes. When two continents collide, neither sinks, so the crust buckles upward, which raised the Himalayas and the Tibetan Plateau. Geologists also use past episodes of crustal upheaval as markers when dividing geologic time into periods.

The rigid lithosphere rides on a slowly flowing layer beneath it, and light continental crust rides high, like an iceberg. Load the surface with thick ice and it sinks, while land just beyond the ice bulges up. North America is still rising where ice-age ice pressed it down, and the old bulge along the East Coast and Great Lakes is still slowly sinking.

The epicenter is only a point on the map, so it says nothing about depth, and depth matters. In 1994 a magnitude 8.3 quake struck 636 kilometers beneath Bolivia; it was felt as far away as Toronto yet caused no major damage. Seismologists locate an epicenter by drawing distance circles around three seismograph stations and finding where they meet.

P waves shake the ground along their direction of travel and arrive first; S waves shake it side to side and arrive later, and the gap between them gives a quake's distance. Rayleigh and Love waves travel near the surface. S waves are stopped entirely by liquid, so the shadow zone they leave on the far side of Earth reveals the liquid outer core.

Silica dissolved in magma links into molecular chains that tangle, so more silica means stiffer magma. As magma rises, dissolved water forms gas bubbles. In runny basalt they float up and escape quietly; in sticky, silica-rich magma they stay trapped under pressure until they rip the magma into ash. Mount St. Helens' dacite held more silica and more water than Kilauea's basalt.

Igneous rock forms when molten rock solidifies: coarse-grained if it cools slowly at depth, fine-grained or glassy if it cools fast at the surface. Weathering breaks any rock into sediment that is buried, compacted and cemented into sedimentary rock. Heat, pressure and hot fluids remake rock as metamorphic rock without melting it; if it melts, it begins again as igneous.

Hardness is resistance to scratching, tested by scratching a mineral against substances of known hardness on the Mohs scale. Mica, at 2.5, yields to a fingernail; calcite yields to a knife; quartz rates 7. Fluorite is often mistaken for quartz, but at hardness 4 a scratch test separates them. Cleavage, luster and streak are the other field clues.

The time scale was first built by relative dating: in undisturbed layers the bottom bed is oldest, and fossils show which layers match. Eras are broad spans defined by the general character of life; periods are shorter spans set partly by major upheavals of the crust. From the mid-1900s, radiometric dating attached reliable numbers to these already-named divisions.

Below the rigid plates, hot, softened mantle flows slowly, kept moving by heat from radioactive decay and heat left from Earth's formation, much like thick soup circulating in a heated pot. That convection was long seen as the main driver. By the mid-1990s most scientists favored forces tied to subduction, chiefly slab pull, the sinking of cold, dense plate, and no proposed mechanism can be tested directly.

The crust is thin, rigid and brittle. Below it the mantle, about 2,900 kilometers thick, is hot, dense, semi-solid rock richer in iron and magnesium. The core is metallic iron-nickel and nearly twice as dense as the mantle; its outer part is liquid and its inner part solid. The spinning liquid outer core creates Earth's magnetic field.

The lithosphere is the crust plus the cool, rigid top of the mantle, averaging at least 80 kilometers thick and thinnest under the oceans. Beneath it the asthenosphere is hot, semi-solid rock that can soften and flow over geologic time. Plates are pieces of the lithosphere, not of the crust alone, so the base of a plate is a change in strength, not a change from crust to mantle.

At divergent boundaries plates pull apart and new crust forms; at convergent boundaries crust is destroyed as one plate dives under another; at transform boundaries plates slide past each other and crust is neither made nor destroyed. A fourth type is the broad boundary zone, such as the Mediterranean-Alpine belt, where microplates make geology and earthquake patterns complicated.

Where an oceanic plate converges with another plate, the denser oceanic slab sinks, carving a trench 8 to 10 kilometers deep. Melting at depth feeds a line of volcanoes: the Andes and Cascades on continents, curved island arcs such as the Marianas at sea. Parts of the sinking plate lock in place for long periods, then slip suddenly, producing large earthquakes.

Continental rock is too light to be dragged down, so when two continents meet head-on the crust crumples and is pushed up or sideways instead. India's collision with Asia about 50 million years ago, followed by slow, continuing convergence, raised the Himalayas and the Tibetan Plateau, mostly within the last 10 million years. The plateau averages about 4,600 meters high.

Along a transform boundary plates grind horizontally past each other. Most transform faults lie on the seafloor, offsetting spreading ridges into zig-zag margins, and are marked by shallow earthquakes; a few run on land. The San Andreas, about 1,300 kilometers long, carries the Pacific Plate northwest past North America at about 5 centimeters a year.

Magma rises through the weak crest of a mid-ocean ridge and erupts as new oceanic crust, which then moves away on both sides. Rock is youngest at the crest and older with distance, and magnetic stripes lie symmetrically on either side. Drill cores collected on a 1968 expedition, once dated, clinched the idea, and the destruction of old crust at trenches explains why Earth does not grow.

Grains of magnetite in cooling lava line up with Earth's magnetic field, and that direction is locked in as the rock solidifies. Normal rocks match today's field; reversed rocks point the other way. Because the field has flipped many times, new crust at a ridge records alternating stripes, a tape recording of reversals that also lets scientists calculate spreading rates.

Alfred Wegener argued in 1912 that South America and Africa fit together and that matching fossils and unusual geologic structures line their facing coasts. He added misplaced climates: coal from tropical plants in Antarctica and glacial deposits in now-arid Africa. Critics rejected the theory because he could not say what force could move whole continents.

Pangaea, Greek for "all lands," was a supercontinent that began to break up about 225 to 200 million years ago. On Alexander Du Toit's reading it first split into Laurasia in the north and Gondwanaland in the south, which then fragmented into today's continents. Spreading over the last 100 to 200 million years grew the Atlantic from a tiny inlet into an ocean.

Hawaii lies more than 3,200 kilometers from the nearest plate boundary. In 1963 J. Tuzo Wilson proposed that a long-lived, hot region beneath the plate feeds a volcano until plate motion carries it away, so the islands age toward the northwest: Kauai's oldest rocks are about 5.5 million years old, the Big Island's under 0.7. Whether hotspots are deep and fixed is now debated.

Shield volcanoes such as Mauna Loa are built of fluid basalt that travels far, leaving broad slopes under about 10 degrees, and they tend to erupt non-explosively. Composite volcanoes such as Mount St. Helens are steep cones of sticky magma that traps gas and erupts violently. A caldera is a collapse rather than a cone: the ground sinks into space emptied by a huge eruption.

The Pacific Plate meets many surrounding plates, and much of its rim is lined with deep trenches where plates subduct, so the Pacific rim is the most seismically and volcanically active zone in the world. Most active volcanoes above sea level sit near such convergent boundaries. Yet about three quarters of all lava erupts unseen at mid-ocean spreading ridges, not around the rim.

A subduction-zone earthquake can let the overriding plate spring seaward, lifting the seafloor and the water above it; submarine landslides and island volcanoes can also set one off. In deep water a tsunami barely raises the surface while racing at over 800 kilometers an hour. It piles up only in shallow water, often after the sea first pulls back for several minutes.

An earthquake has one magnitude, measured from seismic records of its size at the source, though different methods may report slightly different values. Intensity, on the Modified Mercalli scale, rates the shaking and damage people observe at a particular place, so one quake produces many intensities. That makes magnitude the more objective measure and intensity the location-dependent one.

William Smith noticed that certain rock layers in southern England held fossils unlike those in other layers. Using these index fossils as markers, he could recognize a layer wherever it was exposed. Because fossils record the progressive development of life, the method matches rocks of the same age across the world, though it orders events rather than giving their age in years.

Typically around 800 degrees Celsius, a pyroclastic flow is a chaotic, ground-hugging mix of rock fragments, gas and ash that races away from a vent, or from a collapsing lava dome as at Unzen in 1991. Its heat and speed make it lethal: in 1902 one destroyed St. Pierre on Martinique, killing nearly 30,000 people. With mudflows, such flows caused most volcano deaths since 1600.

An explosive eruption can loft sulfur dioxide into the stratosphere, where it combines with water into sulfuric acid droplets that block some sunlight. Pinatubo's 1991 cloud, about 22 million tons of the gas, cooled some regions by up to 0.5 degrees Celsius. Tambora in 1815 lowered global temperatures by as much as 3 degrees, and 1816 became "the year without a summer."

A basin's headwaters lie in the upper basin, low-gradient valleys and floodplains in the middle, and the depositional zone at the lower end. A young channel tends to be straight and fast in a narrow valley carrying boulders and gravel; a mature one meanders slowly across a broad, flat valley and can move only fine sand, silt and clay.

Valleys formed by streams are generally V-shaped, and a young, fast stream sits in a narrow valley. A glacier works differently: it carries rock debris in its base and acts like a giant bulldozer, carving a broad U. So a U-shaped valley in mountains points to past ice, while a V points to running water as the main sculptor.

On a bend the water on the outside moves fastest, eroding a cut bank, while slower water on the inside drops sand as a point bar. The channel shifts sideways this way without changing size, and drifts down-valley because erosion works best on the downstream side. Where two bends meet, the river bypasses the loop, leaving an oxbow lake that sediment slowly fills.

When a stream overflows its banks, water spreads over the flat land beside it and drops sediment, so a floodplain is built by the river itself, sized to its largest floods. Sediment builds natural levees along the channel; when a levee is breached, a coarse, fan-shaped splay spreads across the plain. Point bars, oxbows and terraces are other floodplain features.

The classic triangle, named for the Greek letter, forms where the river's supply dominates, as on the Mississippi. Where waves dominate, longshore currents carry the sediment away and leave only a slight bulge on a straight coast. Where the tidal range is large, ebb and flood currents rework the sediment into ridges and islands fanning out from a funnel-shaped mouth.

Precipitation falling anywhere in a basin, also called a watershed, drains toward the same main channel, so the basin is the basic unit for studying rivers: headwaters above, floodplains in the middle, deposition below. Wetlands inside a basin catch rain and floodwater and release it slowly, lowering flood peaks downstream.

Water landing on one side of a divide flows toward one trunk river and water on the other side toward another, so the divide marks the edge of the area whose runoff, sediment and dissolved material reach a shared outlet. A divide need only be a high point where flow splits; it does not have to be a mountain crest.

Dendritic drainage branches like the limbs of a tree. Trellis drainage looks rectangular and is typical of folded sedimentary rocks, where tributaries cut their channels through the less resistant beds. When streams adjust to geologic structure this way, the pattern on a map hints at the rocks below; Alabama's Cahaba River basin is one such trellis system.

Estuary water is brackish, from 0.5 to 35 parts per thousand salt, and tides push it in and out. Many, such as the Chesapeake Bay, are river valleys drowned by rising seas at the end of the last ice age; others are bar-built, tectonic or fjords. River sediment fills them quickly. A lagoon differs in being little affected by tides.

Waves rarely meet a beach head-on; they arrive at a slight angle and release energy that drives a current running parallel to the shore. That water washes on and off the beach in a sheet, capturing sand each time, so sediment moves along the coast. Steeper wave angles, steeper beaches and higher waves speed it up, and it can badly erode beaches and damage structures.

Waves suspend sediment and currents carry it along the shore, depositing it as a long ridge that extends out from the coast: a spit. A spit can partly block a bay; if it grows all the way across, it cuts the bay off as a lagoon and is then called a bay barrier or bay-mouth bar. Cape Cod ends in such a spit at Provincetown.

On rocky coasts the pounding surf erodes cliffs, and bluffs, cliffs and terraces form as the rock wears back; a wave-cut scarp is the steep bank left by that erosion. A marine terrace is such a surface raised out of the waves' reach, a perched coastline. Old wave-cut terraces now above sea level are one line of evidence for higher seas in past warm periods.

Waves build barrier islands by repeatedly depositing sand parallel to the shore, and they keep reshaping them: the islands erode, grow and can vanish. They tend to form near deltas on flat, tectonically quiet coasts such as the Gulf and East coasts, not the rocky, active West Coast. Their dunes absorb wave energy, which generally means smaller storm surge on the mainland.

Hard corals build reefs after their larvae settle on rock at the edges of islands and continents. A fringing reef, the most common, grows straight out from the shore; a barrier reef runs farther out, behind a lagoon. If a volcanic island ringed by a fringing reef sinks below sea level while the coral keeps growing upward, an atoll remains: a ring of reef around a central lagoon.

Glaciers carve long, narrow, steep-walled valleys; when the ice retreats and seas rise, seawater floods them. A shallow sill near the mouth limits exchange with the open sea, so deep fjord water can become stagnant and low in oxygen. Fjords mark coasts that were once glaciated, from Alaska's Glacier Bay to Norway, Chile and New Zealand.

A valley glacier drags rock debris frozen into its base over the bedrock, grinding the floor and walls into a steep-sided, flat-bottomed U. Streams, by contrast, generally cut V-shaped valleys, so the U shape is a mark of past ice. Small scratches called striations, gouged by rocks in the ice, record the direction the glacier moved.

A cirque is the bowl a glacier scoops out at its head, often later holding a small lake called a tarn. Where two glaciers erode opposite sides of a ridge they leave a saw-toothed arete; a peak carved on three or more sides becomes a horn. A small side glacier cuts less deeply than the main one, so its valley ends high above the main floor as a hanging valley.

Melting ice drops the debris it carried. Where a glacier's end stays in one place, debris piles into a ridge called a moraine; the longer it stays, the bigger the moraine. Terminal and lateral moraines mark a glacier's greatest reach, medial moraines form where two glaciers merge, and a retreat that pauses leaves recessional moraines. Meltwater reworks fine sediment into outwash.

In polar deserts, where temperatures often swing across the freezing point, repeated freezing and thawing sorts the surface into patterned textures as much as 5 meters across. Frozen ground is not permanently stable either: thawing permafrost is one recognized cause of land subsidence, alongside groundwater pumping, underground mining and sinkholes.

Karst develops on soluble rock such as limestone, marble and gypsum, as water entering cracks dissolves them wider. The water may travel far underground before emerging at springs. Sinkholes form when the surface sinks or a cave roof collapses. Karst aquifers supply 40 percent of US drinking groundwater, but water flows through them fast with little filtering, so contaminants spread quickly.

Pumping groundwater from some aquifer systems compacts the subsoil, shrinking the pore spaces that held water and lowering the land, often permanently, along with the aquifer's storage capacity. More than 80 percent of identified US subsidence comes from groundwater use. Draining organic soils, underground mining, sinkholes and thawing permafrost can also lower the ground.

Wind works best where vegetation is sparse and fine sediment is plentiful, though it relies on rivers, glaciers and waves to supply that sediment. It moves sand in short hops called saltation, piles dunes that creep downwind as grains avalanche down the steep slipface, and lays windblown silt down as loess. Blown grains carve rocks into ventifacts and ridges into yardangs.

Wind removes loose, fine particles by deflation, and water helps; what remains is a sheet of closely packed rock fragments, pebbles and gravel. Once in place, that stony layer shields the material beneath it from further deflation, so the pavement limits its own growth. Stony deflation zones make up almost half of Earth's desert surfaces.

Weathering breaks rock down where it sits, through heating and cooling, growing ice or salt crystals, or chemicals such as acid in rain. Erosion is the removal of the products, sand, silt and gravel, by wind or water. They work in sequence, which is why piles of broken rock at the foot of a cliff show weathering that erosion has not yet carried off.

Rock expands when the sun heats it and contracts as it cools, and repeated cycles open cracks until it crumbles. Growing crystals pry rock apart too: ice in winter, and in dry climates salts that water carries to the surface and leaves behind as it evaporates. Collisions between moving pieces break rock as well. None of this changes the minerals, which is what chemical weathering does.

Acid in rain dissolves rocks such as limestone, and oxygen rusts the iron in many rocks. Underground, water moving along fractures decays rock chemically, attacking sharp corners first, so blocks of granite round off into boulders as concentric shells loosen. Through its effect on temperature, climate sets the rate of chemical weathering.

Slope material, whether rock, soil or fill, may fall, topple, slide, spread or flow. A slide moves on a distinct zone of weakness, curved in a rotational slide; debris flows and mudflows move as slurries; creep is imperceptibly slow, betrayed by curved tree trunks and bent fences. Saturation by water, from heavy rain or snowmelt, is a primary cause.

Desertification is the degradation of once-productive land, most often at fragile desert fringes, as livestock compact the soil, grazing and firewood cutting strip plants, and poor farming exposes soil to wind and water. Drought alone is not the cause: well-managed land recovers when rain returns. It has multiple causes and is often reversible, and researchers still disagree about its extent and degree.

The O horizon is decaying plant matter, dark with humus. The A horizon below is mineral soil darkened by organic matter, where most roots work. An E horizon, where present, is bleached as water leaches clay, iron and other material down to the B horizon, the subsoil where it accumulates. The C horizon is partly broken-down parent material, above bedrock.

Rain dissolves minerals such as carbonates and carries them deeper, so leaching depends on how wet a climate is; soils of drier regions can keep calcium carbonate within the profile. Warm, moist climates grow and decompose organic matter quickly, and cold, dry ones do the opposite. Climate is one of five soil-forming factors, with parent material, organisms, landscape position and time.

Shine a flashlight straight at paper and the light makes a bright circle; tilt the paper and the same light spreads into a larger, dimmer ellipse. Sunlight behaves the same way: overhead sun delivers more energy to each square centimeter than low sun. Direct sun also crosses less atmosphere, so less is scattered, which is why the zone under the most direct sun heats most.

Earth's axis is tilted about 23.5 degrees and keeps pointing the same way as the planet orbits, so each hemisphere takes a turn receiving the sun's direct rays, with a higher sun and longer days. Distance cannot be the cause: when the Northern Hemisphere has summer, the Southern Hemisphere has winter, at the same moment and the same distance from the sun.

At the equator daylight stays within a few minutes of 12 hours all year. Moving poleward, summer days lengthen and winter days shorten: in the US mid-latitudes daylight runs from about 15 hours near the June solstice to near nine in December. At the Arctic Circle the sun stays up around the clock near the June solstice, while winter daylight lasts only a little over two hours.

Rising air cools. An unsaturated parcel cools at the dry adiabatic rate, 9.8 degrees Celsius per kilometer; a saturated parcel cools at a moist rate that is not constant but depends on its temperature and pressure, and condensation inside it releases heat. A steep lapse rate in the surrounding air, temperature falling fast with height, signals instability.

In an inversion, temperature increases with height instead of falling. Air rising into that warm layer soon becomes cooler than its surroundings and stops. A warm layer aloft, called a cap, can hold back thunderstorms until it weakens, and the stratosphere, warmed by ozone formation, blocks convection, which is why thunderstorm tops flatten into anvils.

Air that sinks near 30 degrees flows back toward the equator, and Earth's rotation turns it into the northeast trades in the north and the southeast trades in the south; they meet in a belt of calm winds within about 5 degrees of the equator. Air moving poleward from 30 degrees becomes the westerlies. Sailing ships rode the trades between Europe and the Americas, which is how they got their name.

A monsoon is a thermally driven wind that reverses with the seasons because a land mass and its neighboring ocean heat differently. In summer, onshore winds bring heavy rain, as in India. Mountains in the monsoon's path wring out its moisture, and the land beyond, such as the Thar Desert, stays dry.

Sunlight penetrates deep into clear water and wind and weather mix the heat through it, while on land the heat stays in the top few inches of soil. Land therefore heats and cools much faster than water. Places far from the sea feel the full swing: interior mid-latitude deserts have a wide range of annual temperatures, and moist continental climates pair warm summers with severe winters.

When moving air meets hills or mountains, the terrain forces it upward. Rising air cools, and if it cools enough its water vapor condenses into clouds and then rain or snow. That is why higher terrain often gets more precipitation than nearby lowlands, and why summer thunderstorms build over the slopes of the Rocky Mountain West.

Air rising over a range drops much of its moisture on the windward side. Descending the lee side it warms, its clouds dissipate, and precipitation there is reduced. Tall ranges can create whole rain shadow deserts, such as the Turpan Depression behind the Tian Shan in China.

As air descends a mountain's lee slopes it is compressed and warms, arriving warm and dry. In the United States these foehn winds are called chinooks in the Rocky and Sierra mountains; they are common in winter in the mountainous West and can be strong. A bank of cloud along the mountains, the foehn wall or chinook arch, often heralds one.

Albedo is the fraction of incoming radiation a surface reflects. Bright sea ice sends much of the sunlight back, while dark open ocean absorbs it. When warming melts sea ice, more solar energy is absorbed and the water warms, which delays freezing in fall and speeds melting the next spring: a self-reinforcing cycle that helps make the polar regions so sensitive to warming.

Roofs and pavement reflect less sunlight than plants and soil, absorb more heat and release it slowly, so heat islands often grow after sunset. Fewer trees and less water mean less shade and evaporative cooling; tall buildings on narrow streets trap heat and block wind; vehicles and air conditioners add waste heat. US cities run about 1 to 7 degrees Fahrenheit warmer by day than outlying areas.

Each layer is defined by which way temperature changes with height. In the troposphere, home to almost all weather, air cools with height. In the stratosphere it warms, because forming ozone releases heat. The mesosphere cools again and burns up meteors. The thin thermosphere heats strongly as it absorbs solar ultraviolet and X-rays, yet would feel cold. The exosphere fades into space.

Air heated at the equator rises, flows poleward aloft and sinks near 30 degrees north and south, building belts of high pressure; at the surface it returns toward the equator as the trade winds. The sinking air is dry because it already shed its moisture over the tropical rainforests, which is why so many deserts lie near 30 degrees.

In the Ferrel cell, surface air flows poleward and eastward, giving the westerlies between about 35 and 60 degrees; it is driven by friction rather than by heat differences. In the polar cell, air sinks over the poles into polar highs and spreads out as the polar easterlies, the smallest and weakest cell. Low pressure near 50 to 60 degrees brings frequent storms.

Air descending from the Hadley cell builds high-pressure belts near 30 degrees. Sinking air warms, which evaporates clouds, and it already lost its moisture over the tropics, so skies stay clear: deserts of northern Africa, Australia and the American Southwest lie along these latitudes. Most of the world's major deserts sit in areas crossed by the dry trade winds.

The pressure gradient force pushes air from high toward low pressure, and the closer the isobars on a map, the steeper the gradient and the stronger the wind. Earth's rotation turns the flow, and friction with the ground lets air spiral out of highs and into lows. Air converging on a low must rise, cool and form clouds, so lows bring stormy weather and highs fair.

Earth's surface moves east at over 1,000 miles an hour at the equator and not at all at the poles. Air moving poleward keeps its eastward momentum and runs ahead of the slower ground beneath it. Seen from the surface, moving air curves right in the Northern Hemisphere and left in the Southern. The effect is absent at the equator, so gyres and tropical cyclones do not form there.

Jet streams blow west to east around 30,000 feet and follow the boundaries between hot and cold air, shifting north and south as those boundaries move. The bigger the temperature contrast, the stronger the wind, so jets are strongest in winter. Upper-level lows embedded in the jet stream start mid-latitude cyclones, and air masses move with its changing position.

An air mass takes on the temperature and humidity of the surface it sits over, and the longer it stays the more it matches. Continental air is dry and maritime air moist; tropical air is warm, polar colder and arctic coldest. Moving changes it: arctic air crossing a warmer ocean in winter picks up heat and moisture and becomes maritime polar.

A cold front, where colder air replaces warmer, is steep and lifts air abruptly, giving a narrow band of showers and thunderstorms. A warm front slopes gently, giving widespread layered cloud and precipitation ahead of it. A stationary front does not move, and an occluded front forms when a faster cold front overtakes a warm front.

In the Norwegian cyclone model, the cycle starts on a stationary front with warm air to the south and cold to the north. An upper-level low in the jet stream puts a kink in the front, which splits into cold and warm fronts around a deepening low. The faster cold front catches and overtakes the warm front, occlusion cuts off the warm, moist air, and the low dissipates.

A tropical cyclone needs warm ocean water through a layer about 150 feet deep, an atmosphere unstable to moist convection, moist air at mid-levels and weak vertical wind shear. It also needs to be roughly 300 miles or more from the equator, because it relies on Coriolis force to spin; with the rarest exceptions, none forms within 5 degrees of latitude.

Once a tropical cyclone reaches sustained winds of 74 mph, its name depends on the basin: a hurricane in the North Atlantic, Northeast Pacific and South Pacific east of 160 degrees east; a typhoon in the Northwest Pacific west of the dateline; and a severe tropical cyclone in the Southwest Pacific west of 160 east or the Southeast Indian Ocean. Below 39 mph it is a tropical depression.

Luke Howard sorted clouds into hair-like cirrus, heaped cumulus and layered stratus, plus nimbus for rain clouds. Real clouds mix features, so his combinations became the ten basic types, grouped by height: cirrus types are high and made of ice, while altocumulus, altostratus and nimbostratus are mid-level. Despite meaning "high," the prefix alto marks a middle cloud.

Relative humidity compares the moisture in the air with what saturated air at the same temperature would hold, so it changes with temperature even when moisture does not, and by itself it does not show how much water vapor is present. The dew point is the temperature air must cool to for saturation; a higher dew point means more moisture. When the two meet, fog is likely.

A thunderstorm needs moisture, unstable air that keeps rising once nudged, and a source of lift such as a front, a dry line, a sea breeze or a mountain slope. Most tornadoes come from supercells, rotating storms that form where vertical wind shear, a change in wind speed or direction with height, is strong. The central United States gets more tornadoes than anywhere else.

The ocean stores huge amounts of solar heat because light reaches 150 to 200 meters into clear water and turbulence mixes the heat through it, so its temperature changes slowly. Over land the day-night swing is much larger. Along coasts the contrast drives a daytime sea breeze that can cut temperatures by 15 to 20 degrees Fahrenheit, and a weaker land breeze at night.

At the solstices the sun is at its greatest distance from the celestial equator: overhead at the Tropic of Cancer around June 21 and the Tropic of Capricorn around December 21. At the equinoxes it is overhead at the equator, and day and night are nearly equal everywhere. At the Arctic Circle the sun stays up all day near the June solstice, while winter daylight shrinks to about two hours.

There is one connected global ocean, divided into named regions for historical, cultural and scientific reasons. The United States recognizes five: Pacific, Atlantic, Indian, Arctic and Southern. The Southern, the newest, runs from Antarctica's coast to 60 degrees south, but the International Hydrographic Organization has not ratified those boundaries.

Out from land, the continental shelf ends at the continental slope, the break down to the abyssal plain, the relatively flat, sediment-covered floor 3,000 to 6,000 meters deep. The mid-ocean ridge, more than 50,000 kilometers long, rises where plates spread apart, and trenches, the deepest parts of the ocean floor, form where plates subduct.

Seawater averages about 35 grams of salt per liter. Evaporation raises salinity and fresh water lowers it, so surface water is fresher under the rainy tropics near the equator and near the poles where ice melts, and saltiest where evaporation is high and inflow low, as in the Red Sea at around 40 parts per thousand. Forming sea ice leaves salt behind in the water below.

Global winds drag the sea surface along, and the Coriolis effect turns the resulting currents into vast loops: clockwise in the Northern Hemisphere, counterclockwise in the Southern. There are five major gyres, in the North and South Atlantic, the North and South Pacific, and the Indian Ocean. Each has a strong, narrow current on its western side and a weak, broad one on its eastern side.

On the western side of each gyre, along a continent's east coast, runs a strong, narrow current. The Gulf Stream is the classic case: it leaves the Gulf of Mexico through the Strait of Florida, follows the US coast toward Newfoundland and carries warmth toward western Europe. Warm currents like it evaporate more water, which helps make the southeastern US wetter than Southern California.

The current on a gyre's eastern side, along a continent's west coast, is weak and broad, like the Canary Current in the North Atlantic, and the waters along west coasts are cool. Coastal deserts lie on western continental edges near the tropics, where cold currents parallel the shore and upwelling fogs often blanket the land; the Atacama, Earth's driest desert, is one.

Winds drive only the upper 100 meters or so; deeper currents move because of density, set by temperature and salt. Near the poles, forming sea ice leaves its salt behind, so cold, salty water sinks and spreads along the bottom. This global conveyor belt is slow, taking about 1,000 years for a full circuit, and recharges nutrients and carbon dioxide. Added fresh meltwater could slow it.

When wind pushes surface water away from a coast, or apart in the open ocean, deeper water rises to replace it, and that water is colder and rich in nutrients. On the US West Coast, summer winds from the north drive upwelling that chills coastal water and feeds San Francisco's summer fog; in winter the winds reverse and surface water piles against the coast and sinks instead.

Upwelled water is cold and rich in nutrients, so it feeds the algae at the base of the food chain. Good fishing grounds therefore lie where upwelling is common, as off the west coasts of Africa and South America, where it runs year-round. When El Nino weakens upwelling off northwestern South America, fishing turns poor, which is how Peruvian fishermen first noticed the event.

Normally trade winds push warm water west, piling it up near Indonesia and letting cold water upwell off South America. When air pressure at Darwin, Australia, becomes higher than at Tahiti, the trades weaken, warm water piles back east, upwelling falls, and the eastern Pacific warms. Rain shifts with it: heavy rain in normally dry parts of northwestern South America, drought in parts of the western Pacific such as Australia.

La Nina is the cool phase of the same ENSO cycle. Stronger-than-normal trade winds pile warm surface water in the western Pacific, increase upwelling off South America and cool the eastern Pacific, while the prevailing rain pattern shifts farther west than normal. The sea surface changes are small, generally well under 3 degrees Celsius, yet they shift weather worldwide.

The moon's gravity pulls ocean water into a bulge on the side facing it, while inertia raises a second bulge on the far side. Earth turns through both, so most coasts get two highs and two lows every 24 hours and 50 minutes, the lunar day. The sun's tidal force is only about half the moon's, because tide-raising force weakens with the cube of distance.

At new and full moon the sun, moon and Earth line up, the solar tide adds to the lunar tide, and highs are extra high and lows very low: spring tides. About a week later, with sun and moon at right angles, the solar tide partly cancels the lunar one, giving moderate neap tides. Each lunar month brings two of each, and ranges grow again when the moon is closest, at perigee.

In the open ocean the tidal bulges are modest, and mid-ocean islands typically see tides of a meter or less. Where the bulges run onto wide continental margins they grow, and funnel-shaped bays squeeze them further: the Bay of Fundy's tides top 15 meters, the highest in the world. Narrow inlets and shallow water do the opposite, leaving some bays with ocean inlets nearly tideless.

Fetch is the distance wind blows over water in one direction. Big waves need strong wind, long duration and long fetch at once: slow wind makes small waves however long it blows, a brief strong gust builds nothing, and strong wind over a short fetch cannot either. Near shore, friction with the bottom steepens waves until they become unstable and break.

Sea ice already floats in the ocean, so its melting does not raise sea level. Ice sheets and glaciers rest on land, and when they melt, new water flows into the sea. The stakes differ enormously: melting the Antarctic and Greenland ice sheets entirely would raise sea level about 80 meters, while all other glaciers together would add only about half a meter.

Warming raises sea level in two distinct ways: ocean water expands as it warms, and melting land ice such as glaciers and ice sheets adds water. Through the twentieth century sea level rose about 1 to 2 millimeters a year. Ice ages run the process in reverse, storing ocean water on land as ice: sea level stood about 125 meters lower 20,000 years ago, and old wave-cut terraces record higher seas in past warm intervals.

The ocean currently absorbs more carbon dioxide than it releases, so as atmospheric carbon dioxide has climbed, so has the dissolved amount. Reactions in seawater then lower pH, the concentration of carbonate ions and the saturation of calcium carbonate minerals. Fewer carbonate ions make shells and skeletons harder to build for oysters, clams, sea urchins, corals and calcareous plankton.

The sun's energy evaporates water and gravity brings it back down and downhill. Water enters the air by evaporation and evapotranspiration, condenses into clouds, falls as precipitation, then moves as snowmelt, runoff and streamflow or infiltrates to recharge groundwater, which returns to rivers, springs and the sea. Oceans hold 96 percent of Earth's water, and dams, diversions and pumping reshape the cycle.

Below the water table the ground is saturated, and rock there that readily yields water to wells is an aquifer. An unconfined aquifer has no confining layer above it and its water level is the water table; a confined aquifer lies between poorly permeable layers. Recharge rates vary widely, and deep groundwater may take centuries to replace, so heavy pumping can run wells dry.

Groundwater confined between poorly permeable layers such as clay or shale is under pressure, so in a well it rises above the top of the aquifer. If that pressure level stands above the wellhead, the well flows with no pump. Depth alone does not make a well artesian, and recharge must seep down from the surface some distance away and travel sideways into the confined aquifer.

If runoff collects in a lake with no outlet, evaporation is the only way out, and it leaves dissolved salts behind. Over time the lake turns saline, as with the Dead Sea and the Great Salt Lake, which is saltier than the ocean. In deserts such basins hold playas, shallow, short-lived lakes that evaporate quickly and leave some of the flattest land on Earth.

A lake is a low spot where runoff and groundwater arrive faster than outflow, seepage and evaporation can remove them. Glaciers scoured deep basins such as the Great Lakes; a river bend cut off by its meander leaves an oxbow lake; a dam creates a reservoir. Many saline lakes are shrinking remnants: ancient Lake Bonneville was once as big as Lake Michigan.

Wetlands act like sponges, trapping rain, snowmelt and floodwater and releasing it slowly while their plants slow the flow, which lowers flood peaks and erosion. Bottomland wetlands along the Mississippi once stored at least 60 days of floodwater and now hold about 12. Wetlands also improve water quality, store carbon and support more than a third of US threatened and endangered species.

Tropical rainforest stays frost-free all year, with average daily temperatures of about 20 to 25 degrees C and 2,000 to 10,000 mm of rain. Its tall canopy keeps much of the sunlight from reaching the ground, so few plants grow on the forest floor, and fallen leaves rot fast in the warm, wet air. Do not confuse it with temperate rainforest, which grows on cooler coasts farther from the equator.

A biome is a community of plants and animals living together in a certain kind of climate, so biomes are mapped by climate and vegetation across whole regions. An ecosystem is narrower: all the organisms in a particular area plus the air, water and soil they interact with. Ecosystem edges are often blurry, as where a forest meets a meadow or a salt marsh meets a freshwater marsh.

Both are coastal wetlands flooded by the tides, but they split by latitude. Mangroves grow only in tropical and subtropical zones because they cannot survive freezing, while salt marshes are most common at middle to high latitudes. Mangrove prop roots slow tidal water so sediment settles, and marshes buffer waves and trap sediment, so both help hold shorelines against erosion.

Biologists have noted this pattern for more than two centuries, but its cause is still debated. Proposed explanations include faster speciation in the tropics, lineages that arose in the tropics rarely spreading into harsher temperate zones, and the tropics' larger area over time. A study of over 60,000 flowering plant species found tropical lineages did not speciate faster, so treat any single cause as contested.

Air heated at the equator rises and flows poleward aloft, and a belt of high pressure sits near 30 degrees north and south. High pressure brings fair, dry weather, so many deserts line up along that belt. Dry air also lets heat escape fast at night: days average about 38 degrees C, and some deserts drop to about -4 degrees C after dark. Low rainfall, not heat, is what defines a desert.

Coastal deserts lie mostly on the western edges of continents near the Tropics of Cancer and Capricorn, beside cold ocean currents that run along the shore. Upwelling brings cold water to the surface, and winter fogs it produces often blanket these coasts while little rain falls. The Atacama of South America, the driest desert on Earth, can go 5 to 20 years without 1 mm of measurable rain.

The Intertropical Convergence Zone, a band of rising air and showers where the trade winds meet, follows the sun north and south through the year. Near the equator it passes twice, giving two rainy seasons; farther away the wet seasons merge into one wet and one dry season. In the Koppen scheme, savanna climates (Aw and As) stay above 18 degrees C every month but have more than two dry months.

Grasslands get about 500 to 950 mm of rain a year, more than deserts but far less than tropical forests, and they often lie between temperate forests and subtropical deserts. The same biome is the prairie of North America's Great Plains, the pampas of South America, the veldt of South Africa and the steppes of Central Eurasia. Grass height tracks rainfall, and few trees grow except along streams.

Tundra stays below freezing on average for six to 10 months a year, and a layer of permanently frozen ground, permafrost, lies under the surface. It gets only 150 to 250 mm of precipitation, less than many deserts, yet stays wet because cold slows evaporation. Arctic tundra lies at high latitudes; alpine tundra lies above treeline on mountains, where permafrost is patchier.

Cold, strong winds and a short growing season limit tree growth at high elevation, so subalpine forest thins into stunted, twisted krummholz before tundra begins. Treeline height is local: in Glacier National Park it sits near 2,100 m on the west slope but about 1,800 m on the east, while in Rocky Mountain National Park tundra starts around 3,350 to 3,500 m.

The northern boreal forest lies mainly between 50 and 60 degrees north, with tundra to its north and deciduous forest to its south. Winters are long, cold and dry, and summers are short and moderately warm. Evergreen conifers such as spruce, pine and fir dominate because needles help them survive cold, dry conditions. In the Koppen scheme these are the subarctic D climates with cool summers: Dfc, Dfd, Dwc and Dwd.

These mid-latitude forests meet both warm and cold air masses, so they have four distinct seasons, a yearly average near 10 degrees C and about 750 to 1,500 mm of precipitation spread through the year. Broadleaf trees such as oak, maple and beech drop their leaves in fall and go dormant, an adaptation that lets them survive cold winters. Leaf drop, not low rainfall, is the forest's answer to its hard season.

In the Koppen scheme, D climates lie poleward of the C climates, with a warmest month above 10 degrees C and a coldest month below -3 degrees C. Winters bring snowstorms and bitter cold from continental polar or Arctic air masses. Within the group, the humid continental types (Dfa, Dfb, Dwa, Dwb) have hot or warm summers, while the subarctic types have only cool ones.

Koppen codes these climates Cfb and Cfc: no month averages above 22 degrees C, rain falls evenly through the year, and at least one month averages above 10 degrees C. Storms moving around the Earth bring frequent rain to western coasts between 50 and 60 degrees latitude, such as the British Isles. Compare humid subtropical (Cfa), which shares the no-dry-season letter but has hot summers.

Koppen codes it Cfa or Cwa: the warmest months average above 22 degrees C and the coldest stays below 18 degrees C, so it is warm but not tropical. Cfa has rain in every season, while Cwa has a dry winter. In these mild mid-latitude climates, convective thunderstorms dominate summer and mid-latitude cyclones drive winter weather.

Koppen marks the type Cs: the wettest winter month gets at least three times the rain of the driest summer month. It lies on west coasts between about 30 and 40 degrees north and south, in places such as southern California, Chile and the lands around the Mediterranean Sea. Plants there cope with drought and with fires in the dry season, and this climate and vegetation pairing occurs in only five places.

In the MacArthur-Wilson model, larger and less isolated islands hold more species because they have lower extinction and higher immigration. A survey of 23 Boston Harbor islands fit the model, with richness rising with area and falling with isolation, but isolation cut native plants more than exotic ones. Across 5,592 islands worldwide, isolation lowered mammal richness while raising endemism.

Long isolation lets colonizing plants and animals evolve into species and subspecies found nowhere else, as on California's Channel Islands. Of about 1,000 native plant species in Hawaii, nearly all are endemic, and that narrow range makes them more vulnerable to habitat loss and invasive species. Across islands worldwide, more isolated islands hold fewer mammal species but more endemic ones.

Proxy data extend the climate record far beyond instruments. Tree-ring width and density track growing conditions, ice cores hold dust, air bubbles and oxygen isotopes in yearly layers, and lake and ocean sediments preserve pollen and tiny fossils. Each record has limits: trees lay down one ring a year mainly in temperate regions with a distinct growing season.

Wladimir Koppen sorted climates by their temperature profile, which tracks latitude, and later revised the scheme with Rudolf Geiger. The capital letters are A tropical, B dry, C mild mid-latitude, D continental, E polar and H highland. A second lowercase letter adds the precipitation pattern and a third the temperature pattern, so Csa reads mild, dry summer, hot summer. Read the letters in order, not as one label.

Koppen gives mountains their own class, H, because rapid changes in elevation cause rapid changes in climate over short distances, and a mountain can hold any of the other climate types. A single range may therefore contain several biomes along its elevation gradient, and isolated sky island ranges can sit surrounded by a sea of desert.

Climbing a mountain crosses belts of vegetation. In Glacier National Park, prairie and aspen parkland give way to dense spruce-fir forest, then dwarfed subalpine trees, then alpine tundra above treeline, which covers nearly a quarter of Glacier and neighboring Waterton. The top belt echoes latitude: tundra occurs both at high latitudes as Arctic tundra and at high elevations as alpine tundra.

Koppen splits the polar E group in two. EF, ice cap, keeps perennial ice on the ground. ET, tundra, has no true summer, yet its top layer of soil thaws for a short season, so plants can root there. In both, every month averages below 10 degrees C. Polar climates cover the land masses of Greenland and Antarctica and the northern coastal areas of North America, Europe and Asia.

Climate is usually summarized as 30-year averages called climate normals; the current US set covers 1991 to 2020. A single day can come close to its normal on average and still be unusual weather: one Dallas-Fort Worth day peaked near 3 a.m. and fell 31 degrees F by midnight. Large weather swings from one year to the next do not by themselves mean the climate has changed.

In the ice-albedo feedback, melting ice exposes dark ocean that absorbs more sunlight, which warms the surface and melts more ice. In the water vapor feedback, warmer air holds more moisture, and the extra vapor, a greenhouse gas, adds warming; it responds to warming rather than starting it. Feedbacks can compete: from 2000 to 2004, rising Arctic cloudiness appears to have offset the drop in reflected sunlight expected from sea ice loss.

The US government defines an independent state as a people politically organized into a sovereign state with a definite territory, recognized as independent by the United States. Shared identity and statehood need not match: colonial boundaries divided ethnic and linguistic groups, and many states that inherited them lacked geographic, linguistic or ethnic affinity.

The count of countries depends on who recognizes whom. The US State Department lists 197 independent states, including Kosovo and the Holy See, which the United States recognizes although neither is a UN member. Taiwan is listed separately: the United States recognizes the People's Republic as the sole legal government of China, while Taiwan is administered by its own authorities.

Non-sovereign territories differ widely in status. French Guiana, Guadeloupe and Reunion are administrative divisions of France itself, not dependencies. Aruba, Curacao and Sint Maarten are autonomous countries within the Kingdom of the Netherlands, while Bonaire, Saba and Sint Eustatius fall under the direct administration of the Netherlands. Sharing a sovereign does not mean sharing a status.

A landlocked state reaches the sea only across another country's land, so its exports and imports depend on a neighbor's roads, railways and ports. That dependence can be extreme: the Congressional Research Service has described one landlocked country of over 100 million people that relies on a single small neighbor for the transit of about 90 percent of its formal trade, helped by a new rail line between them.

Point Roberts, Washington, sits at the tip of a peninsula below the 49th parallel, so by land it can be reached from the rest of the United States only by traveling through Canada. The Northwest Angle of Minnesota, the northernmost point of the lower 48 states, comes from a 1783 treaty line drawn by mapmakers who did not know the Mississippi rose south of Lake of the Woods.

A very small state keeps full sovereignty but often hands some functions to a bigger neighbor. The State Department describes one tiny European state as linked closely to France through several treaties, and another as forming a customs union with Switzerland, with Swiss authorities applying its import and export rules. Tying customs or trade to a neighbor lets a state too small to do everything alone still run its own affairs.

Rivers make convenient boundaries, but they move. In an 1884 convention, the United States and Mexico agreed that their river boundary follows the deepest channel: if the river shifts gradually, the line moves with it; if it jumps to a new course suddenly, the line stays on the old one. Land stranded by such a shift, the Chamizal tract, took about a century and a 1963 treaty to settle.

A geometric border follows a surveyed line, such as a parallel of latitude, instead of a river or ridge. Applied strictly, it ignores the land: the 49th parallel left Point Roberts, Washington, on a peninsula reachable from the US mainland by land only through Canada. A line drawn from a poor map can also miss its target: a 1783 line meant to run due west to the Mississippi could never reach it.

A buffer state lies between larger rival powers and keeps them from meeting directly. A 1969 US policy statement described Afghanistan as an isolated buffer state between British and Russian power for over a hundred years, a position that changed once British power left the subcontinent. When the rivals around a buffer change, so does its role.

Europe and Asia share one landmass, so the line between them is a convention, not a coastline. NASA describes it as approximate, following a north-south mountain range and a river that runs to an inland sea, and notes disagreement over where it runs in Russia. One common choice puts the Caucasus Mountains in Europe; another puts them in Asia, which changes which peak counts as Europe's highest.

Some capitals are founded by decision rather than grown. The US Residence Act of 1790 placed the permanent seat of government on the Potomac as part of a political bargain, let the president choose the exact site, and made Philadelphia the temporary capital for 10 years. Pierre L'Enfant laid out a grid crossed by wide avenues, yet the city was still sparsely settled when Congress arrived in 1800.

Several states split capital functions among cities. Bolivia uses La Paz as its administrative capital and Sucre for its legislature and judiciary, and Benin names Porto-Novo its capital while keeping the seat of government in Cotonou. When asked for a capital, check which function is meant, because an official capital and the city where the government actually sits can differ.

Federalism divides authority between levels of government. The US Constitution lets states keep the powers needed to regulate daily life, and the Tenth Amendment reserves powers not delegated to the national government to the states or the people. Because states hold their own powers over matters such as education and marriage law, neighboring states often have very different laws, while a county is simply a subdivision of its state.

Primary subdivisions take different names even within one country. Most US states use counties, Louisiana uses parishes, and Alaska uses boroughs plus census areas drawn for statistics. In Maryland, Missouri, Nevada and Virginia some cities belong to no county, and Puerto Rico's municipios serve as county equivalents.

Partisan gerrymandering is drawing legislative district lines to weaken one party's voters and entrench a rival in power. In Rucho v. Common Cause (2019), the Supreme Court ruled 5 to 4 that federal courts cannot hear these claims because no judicially manageable test exists, leaving the issue to state legislatures, Congress and state courts. Racial and one-person, one-vote claims are treated differently.

Partition divides one state into several at once: on the last day of 1992, Czechoslovakia ceased to exist and was succeeded by two new states. Secession is one region leaving a state that carries on: the territory that became Bangladesh was previously the eastern part of Pakistan, which continued as a state without it. Ask whether the original state survives to tell the two apart.

Members of a supranational bloc pool some decisions and keep others national. In the EU, trade and agriculture are run largely through common institutions whose majority-vote decisions bind members, while foreign and security policy mostly needs unanimity, so any one government can block it. Layers also differ in membership: only some EU members use the euro.

Trade blocs deepen in steps. A free trade area drops tariffs among members, who keep their own tariffs on outsiders, so internal customs checks must stop goods entering through the lowest-tariff member. A customs union adds a common external tariff, removing that problem. A common market also frees movement of labor and capital, and an economic union adds coordinated policy and a common currency.

European powers colonized much of Africa and Southeast Asia in the mid to late 1800s, treating the colonies as reservoirs of raw materials, labor and land, though development and settlement there were patchy. The boundaries colonial rule drew often split ethnic and linguistic groups and natural features, laying the foundation for states with little geographic, linguistic or ethnic affinity.

Between 1945 and 1960, about three dozen new states in Asia and Africa gained autonomy or independence, some peacefully and others only after long struggles. The wave reshaped the United Nations, which grew from 35 members in 1946 to 127 by 1970, and the newly independent states became an important factor in changing the balance of power inside it. Many new states also inherited colonial borders.

A frozen conflict pairs halted fighting with an unresolved status. Russia occupies Georgia's Abkhazia and South Ossetia, almost 20 percent of Georgia's territory, and recognizes them as independent, while the United States calls on Russia to reverse that recognition. An unarmed EU mission monitors the 2008 cease-fire, but Russian authorities do not let it operate in either region.

A river that flows from one country into another needs a treaty to divide its water, because the upstream state could otherwise take it first. The 1944 US-Mexico Water Treaty guarantees Mexico 1.5 million acre-feet of Colorado River water a year, and divides the flow of another shared river partly by which country's tributaries supply it. A joint commission with US and Mexican sections applies the treaty and settles disputes.

Two-letter and three-letter codes do not always match a country's English name. GENC, the US government's profile of the ISO 3166 standard and the replacement for FIPS 10-4, codes Germany DE and DEU, Switzerland CH and CHE, and Spain ES and ESP, none of which starts with the English name's first letter. Kosovo carries XK and XKS. Learn a code from the list, not by guessing from the English name.

A coastal state may claim a territorial sea up to 12 nautical miles from its baseline, normally the low-water line, with sovereignty over the water, the airspace above and the seabed below. Foreign ships keep a right of innocent passage through it, but not through internal waters such as harbors and lakes inside the baseline. The United States extended its own claim from 3 to 12 nautical miles in 1988.

Beyond the territorial sea, a coastal state may claim an exclusive economic zone out to 200 nautical miles from its baselines, or to a boundary agreed with a neighbor. There it has sovereign rights to explore and manage living and nonliving resources and to produce energy from water, currents and wind, plus jurisdiction over artificial islands and marine research, rather than the full sovereignty it holds in its territorial sea.

Chokepoints are narrow channels on busy sea routes; some are so narrow that ship size is restricted. By volume of oil carried, the Strait of Hormuz and the Strait of Malacca matter most. Most can be bypassed by longer routes, such as around Africa's Cape of Good Hope, but a blocked chokepoint, even briefly, delays supply and raises shipping costs and world energy prices.

Under the Law of the Sea Convention, only naturally formed land above water at high tide is an island with full maritime zones. A rock that cannot sustain human habitation or economic life gets only a territorial sea and contiguous zone, low-tide elevations and submerged features get no zones of their own, and building on a feature cannot upgrade its status. South China Sea disputes turn on these categories.

Only a state made up wholly of one or more archipelagos may draw archipelagic baselines, which join the outermost points of its outermost islands and drying reefs. The water enclosed must be between one and nine times the land area. Continental states that happen to own archipelagos may not draw such lines, and the enclosed waters carry duties such as respecting traditional fishing rights.

Population density divides a population by land area, leaving out water; the US figure was about 90 people per square mile in 2013. That number assumes people are spread evenly, and they are not: US cities averaged about 1,600 per square mile and unincorporated areas about 35. Density is most meaningful for small areas such as neighborhoods; a national average describes settlement intensity, not how people live.

People cluster rather than spreading evenly across land. In 2020, about 80 percent of the US population lived in urban areas, and everything outside them counts as rural territory. Density also varies widely between cities, from almost 28,000 people per square mile in New York City to about 8,300 in Los Angeles, and within one city high-rise neighborhoods are far denser than blocks of detached houses.

In the demographic transition, death rates fall first and birth rates later, as a stylized five-stage model shows. Because the fall in fertility lags the fall in mortality, population grows rapidly in between. At the end, low fertility can push natural increase below zero. The model is a generalization: much of Africa has moved through it later and more slowly than other regions.

A population pyramid charts the share of each sex in each age group. A wide base that narrows steadily upward signals high fertility and rapid growth; a more rectangular shape shows people spread more evenly across ages. Bulges record events: a baby boom adds a bulge, and a large inflow of labor migrants swells the young-adult bars.

The dependency ratio compares people under 15 and those 65 or older with people aged 15 to 64. Falling fertility first lowers it, which can support faster economic growth, but aging can later push it back up: the US ratio rose from 49.0 in 2010 to 53.7 in 2019 as the 65-and-older group grew. An old-age ratio counts only the older group; Asia's was 13.5 per 100 working-age people in 2020.

The total fertility rate is the number of births a woman would have over her lifetime at current age-specific rates. Replacement level is about two only where mortality is low. It must be higher where fewer girls survive to reproductive age, and it shifts with the sex ratio at birth, typically 103 to 107 boys per 100 girls, so replacement level differs between populations.

A high crude death rate does not mean poor health. Romania's crude death rate, about 15 per 1,000, is high, yet life expectancy at birth there in 2019 was nearly 80 years for women and 73 for men. The death rate is inflated by age structure: decades of low fertility left an unusually large share of people at old ages, where death rates are highest.

Natural increase is births minus deaths; population change also adds net migration, the third component. The two can diverge: Romania's natural increase has been negative for more than 30 years, and emigration has deepened its decline. Early in the demographic transition, when birth rates far exceed death rates, natural increase is at its largest and fertility drives growth.

Migration responds to forces that push people out of one place and pull them toward another, such as an economy's demand for labor or social, political and military upheaval. These flows are harder to project than births or deaths because migration depends less on a population's age structure, and a given flow matters more where the sending or receiving population is small, as on many Pacific islands.

Refugee status has a narrow legal meaning: a person who fled their country of origin because of past persecution or fear of future persecution based on race, religion, nationality, political opinion or membership in a particular social group. Someone who left only to seek a more prosperous life is an economic migrant, not a refugee, and refugees generally must be outside their country of citizenship to qualify.

Rural-urban migration is the move from countryside to city. Researchers describe China's as the largest rural-to-urban migration in history. Migrants there go for better-paid city jobs but often keep their rural household registration, or hukou, so local rules can tie benefits such as health insurance to the village they left.

Remittances are money migrants send home, typically a few hundred dollars per transfer. In 2015 developing countries received about $432 billion this way, nearly three times official development aid, and the flow is steadier than private capital. In nine countries remittances exceeded 20 percent of GDP; in Tajikistan, rebuilding after a 1992 to 1998 conflict, they reached about 42 percent.

The urban share is a snapshot; urbanization is its change over time, and definitions move both. The 1920 US census was the first to count over half the population as urban. In 2020 the urban share slipped from 80.7 to 80.0 percent, yet this was not a move to the countryside: the minimum population for an urban area was raised from 2,500 to 5,000.

A primate city is a country's largest city when it outweighs the rest of the urban system, often because it is the capital and concentrates political power. Such a city is one reason a country's city sizes can stray from the rank-size rule. Some scholars argue primacy is stronger in small countries, but a meta-analysis of rank-size studies found estimates for large and small countries very similar.

The rank-size rule says a city's population multiplied by its rank is roughly constant, so the second city is about half the size of the largest and the third about a third. Zipf made the rule famous in that exact form. Measured slopes vary widely, and much of that comes from method: how cities are delineated and the minimum size counted both change the result. Loosely integrated urban systems tend to stray further from the rule.

Walter Christaller's central place theory says settlements form a hierarchy: larger, higher-order places offer more functions and serve wider areas than smaller, lower-order ones. Each function has a range, the farthest people will usually travel for it, and a threshold, the minimum population needed before it is offered. Its hexagonal pattern assumes uniform land, even population and equal travel costs, so it is an idealized model.

Geographer Jean Gottmann's 1961 study named the string of large metropolitan areas along the US northeastern seaboard Megalopolis, a distinctive region rather than separate cities. The Census Bureau counts about 50 million people, nearly 1 in 6 Americans, in its 103 counties. Railroads, highways and air routes linking the cities fed its growth, and NASA calls such merged regions conurbations.

The term arose in the 1960s for middle- and higher-income households moving into working-class London neighborhoods, bringing investment but displacing earlier residents. US central cities saw it from the mid-1970s, after decades of disinvestment. Not all displacement is gentrification: HUD's look at 1976 housing data found most displacement then came from abandonment and disinvestment, not reinvestment by higher-income newcomers.

Von Thunen's agricultural location theory ties land use to distance from the market town: transport costs decide which crops pay where, producing rings of land use around the town. The rings blur as transport improves. A study of one region of China found that cold-chain storage, better roads and wider markets let cash crops cluster far from the towns they supply.

In the 1960s, new rice and wheat varieties greatly raised world grain production. The key rice line, IR8, was semi-dwarf: older tall varieties fell over, or lodged, when given heavy nitrogen fertilizer, while short, stiff stems stayed upright and turned more fertilizer into grain. Researchers also credit insensitivity to day length for the success of the new rice varieties.

Agglomeration economies are the advantages firms gain from locating near one another, such as specialized labor markets, nearby suppliers and knowledge spillovers. They draw new firms toward dense areas and large cities, but density also means competition: one study of firm entry in France found population density, used as a proxy for agglomeration, discouraged entry by non-creative firms.

The resource curse is the idea that natural resource wealth can bring lower income or more conflict instead of prosperity. The leading explanation is institutional: resource rents feed rent seeking, corruption and violence where institutions are weak, but raise incomes where they are strong. A study of mining districts in Sub-Saharan Africa found stronger institutions increased mining's development benefit and dampened its effect on conflict.

GDP counts production inside a country's borders, while gross national product counts production by the labor and property of its residents wherever it happens. A foreign-owned car plant in the United States adds all its output to US GDP but only the part from US residents' labor and property to GNP. Gross national income is conceptually the same as GNP: GDP plus income received from abroad minus income paid abroad.

Global value chains split production into stages carried out in different countries, so trade in intermediate goods and services now exceeds trade in commodities and finished goods. Each country can focus on tasks where it has a comparative advantage, such as design in one place and assembly in another. The risk is exposure: chains concentrated in one region or reliant on one supplier are vulnerable to disasters and pandemics.

US time zones are set by federal regulation: there are nine standard zones, from Atlantic to Chamorro, and their limits follow state, county and city lines rather than straight meridians. The Atlantic zone, for example, spans 52.5 to 67.5 degrees west longitude but excludes all of Maine. Worldwide, time zone and date boundaries likewise depend on local law.

A flat map needs visual cues to show the shape of terrain. Cartographers have used hachuring, hypsometric (elevation) tints, contour density and pictorial relief. Shaded relief, or hillshading, imitates the sun's light and shadow on hills and canyons; once drafted by pencil, it is now computed from digital elevation models.

A choropleth map shades each area by a value, usually grouped into a few classes because readers can tell only a small number of shades apart. A cartogram instead resizes each area in proportion to a quantity such as population, which fixes the visual weight of large empty areas but distorts shapes or breaks contiguity. Readers often need some explanation before cartograms work well.

Mapping raw counts misleads because areas differ in size and population, so standardize to a rate, ratio or density. Even then, large, sparsely populated areas dominate the eye. Class breaks matter too: the same data can be split many ways, so breaks are easy to manipulate, and too many classes confuse readers while too few hide information.

Latitude measures distance north or south of the equator, up to 90 degrees at the poles, and its parallels stay evenly spaced at about 111 km per degree. Longitude measures east or west of the prime meridian, but meridians converge: a degree spans about 111 km at the equator, 56 km at 60 degrees and zero at the poles. Navigators could find latitude at sea centuries before longitude.

Unlike the equator, the prime meridian is arbitrary. The 1884 International Meridian Conference in Washington chose the line through the telescope at the Royal Observatory in Greenwich, England, because many charts and time zones already used it. Satellite positioning now puts zero longitude about 102 m east of the Greenwich markers, since 19th-century surveyors did not allow for local gravity or Earth's flattened shape.

The date line separates consecutive calendar dates: crossing it westward makes it one day later. It roughly follows 180 degrees longitude, the antimeridian opposite Greenwich, but it has no legal international status, so each country chooses the date it observes. That is why the line zigzags around political borders such as eastern Russia and Alaska's Aleutian Islands.

A topographic map shows the shape of the land with contour lines, each joining points of equal elevation, usually measured from mean sea level. Spacing shows slope: contours crowd together on steep mountainsides and spread apart on flat ground. The same kind of line can also show the depth of the ocean floor.

Remote sensing records energy reflected or emitted from Earth using instruments on satellites or aircraft. Passive instruments detect energy coming from the natural environment, while active instruments send out their own energy and measure the change in the return signal. Synthetic aperture radar, an active method, gives fine-resolution data that can reveal small changes in the surface over time.

The shortest path between two points on a sphere runs along a great circle, any full circumference of the Earth such as the equator or a meridian. On a Mercator map a straight line is a rhumb line of constant direction, which is usually longer than the great-circle route; on a gnomonic map every straight line is a great circle, so navigators have used the two together.

Scale is the ratio of map distance to ground distance, and the larger the second number, the smaller the scale. A 1:24,000 map is large-scale: it covers a small area in enough detail to show footbridges and fence lines. A 1:250,000 map is small-scale, covering a wide area but keeping only major features such as boundaries, parks, roads and streams.

No flat map can match a globe: a projection can keep true directions, true distances, true areas or true shapes in part, but never all four. Equal-area maps distort shapes, while conformal maps keep the shapes of small areas correct but distort sizes, so there is no single best projection, only the one best suited to a purpose.

The Mercator projection, presented in 1569, is conformal and the one map on which any straight line is a rhumb line, a line of constant compass direction, which made it a navigation chart. The cost is size: distances and areas are grossly distorted toward the poles, which cannot be shown at all. Shapes and distances are reasonably true only within about 15 degrees of the equator.

An equal-area projection shows every region at its true relative size, useful wherever a map must show areas in true proportion. The trade-off is shape: shapes are distorted to some degree on every equal-area map. The Albers Equal Area Conic, used for maps of the 48 conterminous states, keeps areas proportional with a maximum scale error of about 1.25 percent.

A compromise projection is neither conformal nor equal-area; it balances errors instead. The Robinson projection, presented in 1963, uses tables of coordinates rather than formulas to make the world look right, showing high-latitude lands closer to true size than Mercator, though Greenland is compressed. Its distortion is lowest along the equator and within 45 degrees of the center and greatest near the poles.

An azimuthal projection maps the globe onto a plane, and directions from its center point are true. The azimuthal equidistant version keeps distances true as well, but only from the center. Azimuthal maps suit regions extending equally in all directions from a center, such as Asia or the Pacific Ocean, and the gnomonic version shows every great circle as a straight line.

Under the Uniform Time Act, US states observing daylight saving time move clocks ahead one hour at 2 a.m. on the second Sunday in March and back at 2 a.m. on the first Sunday in November. A state may exempt itself, and a state in two zones may exempt only part, but federal law overrides any state attempt to use different dates.

A representative fraction such as 1:63,360 means one unit on the map stands for 63,360 of the same unit on the ground, so it works in inches or centimeters alike; at that scale one inch equals one mile. A verbal scale states the relation in words, such as one inch represents 50 feet, which is 1:600. The first number of a representative fraction is always 1.

GPS has three parts: satellites, ground stations that monitor and control them, and receivers. Over 30 satellites orbit about 20,200 km up, and a receiver uses their signals like a tape measure to find its distance from each. With distances to four or more satellites it fixes its position, often within a few yards; processed data from fixed reference stations can reach a few centimeters.

A geographic information system is a computer system that analyzes and displays information tied to particular locations. Its data are organized in thematic layers, such as the eight base layers of The National Map. A finished topographic map is an output of GIS rather than an input, and many GIS programs cannot read the PDF maps.