Meteorology & Weather curriculum

Leave out water vapor and air is about 78 percent nitrogen, 21 percent oxygen and just under 1 percent argon; with carbon dioxide those make up 99.998 percent of it. Carbon dioxide is only about 0.04 percent, yet it absorbs heat the ground radiates and slows its escape to space.

When dry winds blow over a desert, water vapor makes up nearly none of the air; on hot, humid tropical days it approaches 4 percent by volume. About half of it sits below roughly 6,500 feet. That vapor is the raw material for cloud and rain, and it carries heat that is released wherever it condenses.

Temperature falls with height through the troposphere, rises through the stratosphere, falls again in the mesosphere to as low as -100 C, then climbs in the thin thermosphere. Each layer ends at a “pause.” The thermosphere can reach about 2,000 C yet would feel cold to your skin, because too few molecules are there to carry much heat.

The troposphere is about 11 to 12 miles deep at the equator, about 5.5 miles deep near 50 degrees latitude and under 4 miles deep at the poles, and it is deeper in summer than in winter. Above it, warmer air sits over cooler air and blocks rising motion, which is why a thunderstorm's top flattens into an anvil there.

In the stratosphere, oxygen absorbs ultraviolet radiation, forming ozone and releasing heat, so the temperature climbs from about -56 C at the tropopause to near freezing at the stratopause. That warm-over-cool layering suppresses convection and makes the stratosphere a calm layer, unlike the overturning troposphere below.

Although the atmosphere reaches hundreds of miles up, half of its molecules sit in the lowest 18,000 feet, about where the 500-millibar level is. Density follows pressure, and pressure is greatest at the bottom under the weight of all the air above, so the lowest layers are densest. That level rides higher in warm air than in cold.

Water molecules are too small to bond into droplets on their own, so they condense onto water-attracting particles at least one micrometer in radius, such as sea spray, smoke or wind-blown soil. Every cloud droplet therefore has a speck at its core, roughly a hundredth of the droplet's size, though the droplet is still almost pure water.

The sun's output peaks in visible light, a short wavelength. With the sun overhead, a beam lands on the smallest possible area; as the sun sinks toward the horizon, the same beam spreads over more ground and passes through more atmosphere, where it is scattered and absorbed. That is why a high sun heats the surface so much more.

Of every 100 units of sunlight reaching Earth, about 30 are reflected, mostly by clouds, about 23, rather than the surface, about 7. Fresh snow is an excellent reflector, while dark surfaces such as asphalt absorb more and warm faster. So when snow melts, the darker ground beneath starts soaking up sunlight the snow used to send back to space.

Every object radiates, and the hotter it is, the shorter its peak wavelength. The sun peaks in visible light; Earth, far cooler, radiates in the infrared, which you cannot see directly, though the shimmer over a hot road hints at it. The atmosphere lets much sunlight through but absorbs most of the infrared the surface emits, and that difference shapes the energy budget.

Water vapor, carbon dioxide, methane and ozone absorb infrared radiated by the ground, so more energy stays near the surface than would with no atmosphere at all. Clouds do the same on a small scale: an overcast night stays warmer than a clear one because less of the ground's heat escapes to space.

Sunlight strikes low latitudes more directly, so the tropics absorb more than they emit while high latitudes emit more than they absorb; the two balance near 35 degrees latitude. If nothing moved the heat, the tropics would keep warming and the poles keep cooling. Winds, storms and ocean currents carry the surplus poleward instead.

Earth's axis is tilted about 23.5 degrees and points the same way all year, so each hemisphere takes turns facing the sun more directly, with longer days and a higher sun. The solstice brings the most sunlight, yet the warmest days come later, because heating the surface takes time, so warmth keeps building after the sun's peak.

Air is a poor conductor, so conduction warms only a thin layer touching the ground; rising thermals then carry that heat upward by convection, and wind moves it sideways, which is called advection. Evaporation is the big hidden carrier, moving about 24 units of energy from the surface for every 7 carried by rising warm air.

Temperature rises for as long as incoming sunlight exceeds the infrared the ground gives off. Sunlight peaks around noon, but that surplus continues into the afternoon, and the air near the ground needs time to warm. At night the ground keeps losing heat until shortly after sunrise. The swing is largest over land under clear, dry, calm skies.

Water needs about five times as much heat as quartz sand to warm by the same amount, sunlight penetrates it deeply, and currents mix the heat through a thick layer, while land heats only its top few inches. That is why a beach is hotter than the sea on a sunny afternoon, and why San Francisco's temperatures vary far less than St. Louis's.

On average the troposphere cools about 6.5 C per kilometer, about 3.6 F per 1,000 feet, and that figure defines the standard atmosphere. Real air seldom matches it: some layers hold steady or even warm with height. Treat the standard rate as an average, not a rule, especially when you use it to estimate where the freezing level is.

On clear nights with light wind the ground cools fastest and chills the air touching it, forming a surface inversion. Inversions also form aloft where sinking air warms by compression, or where warm air rides over cold air at a front. The layer is very stable and suppresses mixing, so smoke and haze stay trapped below it.

Pavement and roofs reflect less sunlight than vegetation and store heat, dense buildings trap it and block cooling breezes, and vehicles and air conditioners add waste heat. In the United States this makes cities roughly 1 to 7 F warmer by day and 2 to 5 F at night, as stored heat leaks out slowly. Calm, clear weather strengthens the effect.

Wind chill reflects how fast wind strips heat from exposed skin, so it cannot cool a car radiator or a water pipe below the actual air temperature, only bring it there faster. The heat index combines temperature and humidity, because humid air slows the evaporation of sweat. Its values assume shade; full sun can add up to 15 F.

Frost forms like dew, but on surfaces that radiate their way below 32 F, so it can coat grass while a thermometer a few feet up reads in the mid-30s. Cold, dense air also drains downhill at night and pools in valleys. The growing season runs from the last killing frost of spring to the first one of autumn.

Countless molecules striking a surface add up to pressure, and it pushes in every direction, not only down. At sea level the standard value is 1013.2 millibars, the same as 1013.2 hectopascals, or 29.92 inches of mercury. The two metric units are identical; inches of mercury survive mainly in US aviation and broadcasts.

A barometer in a mountain city reads lower than one on the coast just because it sits higher, with less air overhead. To find real highs and lows, each reading is converted to the value it would have at sea level, correcting for elevation and the air column's temperature. Near the ground, pressure rises roughly 1 inch of mercury per 1,000 feet of descent.

Near the surface the air is dense, so a short climb leaves a lot of air below you; higher up, thinner air means you must rise much farther to lose the same pressure. That is why the pressure lines on an upper-air diagram spread apart toward the top. Temperature matters too: pressure falls more slowly with height in a warm column than a cold one.

On a surface map, each isobar joins places with the same sea-level pressure. The change in pressure over a distance is the pressure gradient, and wind speed rises with it, so you can read wind strength from the spacing alone: crowded lines mean a steep gradient and strong wind, while widely spaced lines mean light wind.

Near the ground, friction lets air spiral into a low, and since air cannot pile up, it rises, cools and forms cloud and rain. Around a high, air spirals outward and air from above sinks to replace it, warming by compression so clouds evaporate. The weather follows the vertical motion, not the pressure number itself.

Weather reports carry a three-hourly pressure tendency, and a remark flags pressure falling rapidly at 0.06 inch or more per hour. Pressure also dips and rises twice a day on its own, lowest around 4 a.m. and 4 p.m., so one reading means little. In the tropics, outside tropical cyclones, day-to-day changes are too small for barometer rules to work.

The distance between the 1000-millibar and 500-millibar surfaces grows in warm air and shrinks in cold, dense air, so thickness works as an average temperature for the layer, and packed thickness lines mark fronts. A thickness of 5,400 meters, the 540 line, roughly follows the surface freezing point, but season and terrain create exceptions.

Wherever pressure differs across an area, a force pushes air straight across the isobars toward lower pressure, and it is stronger where the isobars are closer. It is the force that actually drives the wind. Coriolis deflection and friction both grow with wind speed, so they only shape air that the pressure difference has already set moving.

Earth turns beneath anything in motion, so a path that is straight seen from space looks curved from the ground. The apparent deflection is zero at the equator and greatest at the poles, and at a given latitude it doubles when wind speed doubles. It acts at right angles to the wind, so it changes the wind's direction, not its speed.

The turning builds up only as air travels far and for a long time. Sea and valley breezes span under about 100 miles and last under 12 hours, too small and brief for much deflection, so they blow almost straight toward low pressure, and a sink is far smaller still. Hurricanes depend on it: they rarely form within 5 degrees of the equator, where Coriolis is too weak to make winds circle a low.

Air starting toward low pressure is turned by Coriolis until, after a full 90 degrees, the two forces cancel. The result, the geostrophic wind, flows along the height contours rather than across them, with higher pressure on the right as you look downwind in the Northern Hemisphere, and its speed is set by how tightly the contours are packed.

Within the lowest few thousand feet, friction slows the wind, which weakens the Coriolis deflection while the pressure force stays the same. The wind therefore cuts across the isobars toward lower pressure, by about 10 degrees over water and as much as 45 degrees over rugged terrain. That inflow is what makes air converge and rise in a low.

The pattern comes from the pressure force, Coriolis and friction acting together. South of the equator the deflection reverses, so the spirals turn the other way: clockwise into lows and counterclockwise out of highs. Most tornadoes share the cyclonic sense of their hemisphere, though some spin the opposite way.

A west wind blows from the west. A vane shows direction and an anemometer measures speed; the sustained wind is a two-minute average, while a gust is a rapid swing of at least 10 knots between peaks and lulls. In aviation, direction is given in degrees, so a north wind is 360 and an east wind is 90.

By day the land warms the air above it, which rises, and cooler, denser sea air pushes in beneath it. Its leading edge, the sea-breeze front, acts like a small cold front: temperatures can drop 15 to 20 F as it passes, and storms often form along it. At night the pattern reverses, but the land breeze is weaker and shallower.

Sunlit slopes warm the air touching them faster than air at the same height over the valley, so a valley breeze climbs the slopes by day, sometimes building clouds above them. After dark the slopes cool first, and the denser air slides downhill as a mountain breeze, collecting on the valley floor, where it can produce fog by morning.

Air climbing the windward slope cools and drops moisture as cloud and precipitation. Going down the far side it warms by compression at the faster dry rate, so it arrives warmer and much drier than it started; in one worked example a parcel starts at 15 C and ends at 23 C. Chinooks are common in winter in the West and can be strong.

A range lying across the prevailing wind lifts moist air up its windward slope, where it cools to saturation and builds cloud and rain. Descending the lee slope, the air warms and dries, so clouds dissipate. The Cascades show it plainly: western slopes are cloudy and wet while areas to the east are semiarid, and the dry zone can reach hundreds of miles.

Land and ocean heat differently through the year, so the wind between them flips direction with the seasons; rain is a result, not the definition. In West Africa, southwesterly winds carry the wet season far inland once the tropical rain belt moves north. Away from the equator, climates become monsoonal, with one wet season and one dry season.

When wind is forced through a narrow opening between high ground, it accelerates, a process called funneling. Along the Pacific coastal ranges and the Inside Passage, strong gap winds pour through low passes and river valleys out onto the water when pressure inland is much higher than pressure over the ocean.

Polar air flowing over open, ice-free water picks up heat and moisture from below and becomes unstable, building bands of showers over and downwind of the lake, mostly in autumn and winter. The convection strengthens as the water-air temperature difference grows, as wind speed increases, and as the incoming air gets drier. The Great Lakes are the best-known US case.

On a planet with no rotation, one loop would lift air at the equator and sink it at the poles, but rotation breaks it into three: the Hadley cell in the tropics, the Ferrel cell in midlatitudes and the small, weak polar cell. Between them lie surface belts of low pressure near the equator and 50 to 60 degrees, and high pressure near 30 degrees and the poles.

Northeast and southeast trade winds converge and force air upward, building a ragged belt of showers and storms around the globe; about 40 percent of tropical rainfall rates exceed an inch an hour. The band moves north in the Northern Hemisphere summer and south in winter, so places near the equator get two wet seasons and two dry ones.

Air sinking in the subtropical highs warms by compression, which dries it and dissolves clouds, and it often forms an inversion aloft that caps convection. Look along 30 degrees north and south and you find the American Southwest, northern Africa and Australia's deserts, while the stormier band from 50 to 60 degrees is far wetter.

In the tropics the trade winds blow mainly from the east and curve toward the equator. Between roughly 35 and 60 degrees the westerlies prevail, and the troughs and ridges riding in them generally travel west to east. Closer to the poles, air spreading out from the polar highs forms the polar easterlies.

The polar jet lies near 50 to 60 degrees latitude and the subtropical jet near 30 degrees, typically 4 to 8 miles up, with cores that can top 275 mph. Both are strongest in winter, when the contrast between warm and cold air is greatest, and both follow the sun, with the polar jet shifting north to near the US-Canada border by summer.

Usually four or five of these waves, each thousands of miles long, circle the hemisphere and drift slowly east, sometimes stalling. Unsettled weather tends to fall between a trough and the ridge downwind of it, and fair weather between a ridge and the next trough. Because the waves move slowly, a wet spell in one place often comes with a dry spell elsewhere.

Winds blowing steadily around semi-permanent pressure centers push on the sea surface and drive its currents. Along the west coasts of continents they carry cool water toward the equator, as the California Current does off the US West Coast; along east coasts, warm tropical water flows poleward. Those currents strongly shape the long-term climate of the coasts beside them.

Each gram of water that evaporates takes up about 2,500 joules without warming, and gives the same amount back when it condenses into cloud, wherever that happens. This is how much of the surface's surplus heat reaches the atmosphere. The scale is enormous: an average hurricane releases about 52 million trillion joules a day this way.

In one worked example, air at sea level holds up to about 27 grams of vapor at 30 C, but only 15 grams at 20 C and 8 grams at 10 C, so each 10-degree drop roughly halves the capacity. Cooling moist air even a little brings it near saturation, and that sets up dew, fog and cloud.

It is a ratio, not an amount. Air holding 8 grams of vapor sits at 30 percent humidity at 30 C, 53 percent at 20 C and 100 percent at 10 C, with not a drop added. That is why humidity climbs overnight as the air cools, and why, at the same moisture content, warm air shows a lower humidity than cold air.

Dew point is the temperature air must cool to before it saturates. It holds steady as the day warms or cools, changing when moisture is added or removed. With a dew point of 45 F, humidity is 100 percent at 45 F but 34 percent at 75 F. The spread between temperature and dew point shows how close the air is to fog or cloud.

On a clear night, grass and car roofs radiate heat away and chill the air in contact with them; once that air reaches its dew point, water condenses as dew. When the surface is below freezing, thin ice crystals form instead as frost. Dew that forms first and freezes later is called frozen dew, not frost.

Mixing ratio is the mass of water vapor per kilogram of dry air; specific humidity is the vapor's share of the whole moist sample's mass. Unlike relative humidity, neither depends on temperature. A parcel that rises or sinks without saturating keeps all its vapor, so these values hold steady until condensation removes water.

Evaporating water into air cools it until the air saturates, and the wet-bulb temperature is the lowest reading reached that way. The wet bulb globe temperature adds wind and sunshine to gauge heat stress on people. Forecasters also watch the height where the wet-bulb temperature drops below 0 C, which helps judge the risk of large hail.

The ocean supplies most of the vapor, about 47 inches of evaporation a year, and plants pass about 99 percent of the water they draw up into the air. Winds carry the vapor over land, where it falls and runs off or soaks in. A lake with no outlet loses water only by evaporation, so its salts build up, as in the Great Salt Lake.

These bands carry most of the water vapor moving outside the tropics; an average one transports about as much water as flows from the mouth of the Mississippi, and strong ones up to 15 times that. Most are weak and refill water supplies, but the strongest can stall over a watershed and flood it, as the Pineapple Express can on the US West Coast.

Most often, air rises and expands, cooling about 5.5 F per 1,000 feet until it saturates. Four common lifts do it: air forced over mountains, air converging into low pressure, air pushed up at fronts, and buoyant air warmed by the ground. Droplets are always forming and evaporating, so a cloud grows only while condensation wins.

Air lifted without saturating cools about 3 C per 1,000 feet while its dew point drops only about 0.5 C, so the gap closes at a predictable height. Air with similar moisture saturates at the same height, giving flat, level bases. The moister the air, the lower the base; in very dry air, parcels may rise two miles before clouds form.

Cirrus, wispy and made of ice crystals, is often the first cloud to appear ahead of a low-pressure system. Cirrostratus spreads as a thin veil and nearly always makes a halo around the sun or moon, which tells it apart from thin stratus or fog. In midlatitudes these clouds sit above roughly 16,500 feet.

Altostratus is a gray sheet through which the sun looks as if seen through ground glass, with no halo. As it thickens and its base lowers, it can become nimbostratus and begin steady rain or snow. Altocumulus, the most common middle cloud, forms patches of rounded masses and makes a colored corona when the moon shines through its edge.

Stratus is a flat gray layer whose base may sit only a few hundred feet up; if thick enough it yields drizzle, and when it lowers onto hills it becomes fog. Stratocumulus forms lumpy, patchy layers. Nimbostratus brings steady rain or snow, and its base sags as precipitation continues, which is why it is often wrongly classed as low.

Cumulus grows from rising thermals, with flat bases and cauliflower tops, often appearing in the morning and fading by evening. With enough instability it keeps climbing into cumulonimbus, whose top flattens into an anvil at the tropopause. A thunderstorm is always a cumulonimbus, and it is the cloud that brings hail and tornadoes.

Lenticular clouds hover near or downwind of ridges in mountain waves. Mammatus pouches hang under anvils but bring no severe weather themselves. A shelf cloud rides a storm's gust front, while a wall cloud lowers under the updraft and can precede a tornado. A contrail forms when hot, humid jet exhaust meets cold air aloft; in dry air it vanishes within minutes, in humid air it lingers and can spread into cirrus.

Radiation fog forms over land on clear, calm nights; advection fog when moist air moves over a colder surface; upslope fog when stable air climbs terrain. Steam fog forms when very cold air crosses warmer water, and frontal fog when rain falling into cold air saturates it. Below freezing, liquid droplets can glaze whatever they touch as freezing fog.

Cloud droplets are so small that one falling from a base 3,300 feet up would need about 48 hours to land and would evaporate long before. Droplets of different sizes fall at different speeds, so bigger ones collide with and absorb smaller ones until they are heavy enough to fall. This warm-rain process dominates in tropical air, where the freezing level is high.

When ice crystals and liquid droplets share a cloud below freezing, vapor deposits onto the ice more readily than it condenses onto the droplets, so the crystals grow while the droplets shrink. The crystals become heavy enough to fall, reaching the ground as snow if the air stays cold or melting into rain if not. Called the Bergeron process, it dominates at middle and high latitudes.

Small, pure droplets resist freezing and can stay liquid down to about -40 C. Between 0 and -10 C, clouds are mostly supercooled droplets, which freeze when they strike an aircraft. Freezing rain works the same way: rain from a warm layer aloft falls through a shallow subfreezing layer and freezes as glaze on contact with the ground.

Drops smaller than 0.5 millimeter that fall close together are drizzle; larger ones are rain. Drizzle forms in stable air, falls from stratiform cloud such as stratus and often comes with fog. Because its tiny drops are so numerous, drizzle usually cuts visibility more than light rain does.

Snow crystals grow as vapor deposits directly onto ice, and temperature, with humidity, picks the basic form: long needles near 23 F, flat plates near 5 F. The air's temperature profile also sets how much water a snowfall holds. The familiar 10-to-1 snow-to-water ratio is right only about a quarter of the time; fresh snow has measured anywhere from 3-to-1 to 100-to-1.

Rainfall can vary enormously over short distances; in one storm, totals across a Texas county ran from under 5 inches to nearly 37. Many automated home gauges under-report over months, sometimes by 25 percent or more, and most cannot collect snow. Radar estimates fill the gaps between gauges but run high where hail is mistaken for heavy rain.

Streaks of rain hanging below a cloud base show that the air beneath is dry. Evaporation removes heat, so the air around the falling precipitation cools, grows denser and sinks. In dry environments this can produce a dry microburst, with damaging wind but no rain at the ground, and shafts of virga may come before one.

A parcel is a bubble of air assumed to keep its moisture and gain no outside heat as it moves. Lift it and compare: if it ends up colder and denser than the air around it, it sinks back and the air is stable; if it stays warmer, it keeps rising and the air is unstable. Real air mixes, so the parcel is a simplification, not a literal object.

A rising parcel expands into lower pressure, and the work of expanding takes energy from it, so it cools; a sinking parcel is compressed and warms. Because air conducts heat so poorly, almost no heat crosses the parcel's boundary, which is what adiabatic means. As long as the parcel stays unsaturated, the process is fully reversible.

The dry rate, 9.8 C per kilometer or 5.5 F per 1,000 feet, holds until the air saturates. After that, condensation releases heat that offsets part of the cooling, so the moist rate is lower, about 4 C per kilometer near the surface. Cold air holds little vapor, so high in the troposphere the moist rate approaches the dry one.

If the environment cools more slowly than the moist rate, any lifted parcel ends up colder, and the air is absolutely stable; inversions are the extreme case. If it cools faster than the dry rate, the air is absolutely unstable. In between lies conditional instability: a parcel must be lifted past its level of free convection before it rises on its own.

On a Skew-T diagram the red temperature line and blue dew-point line run up from the surface; the closer they are, the higher the humidity, and where they touch the air is saturated. A layer where temperature climbs with height is an inversion, and a large one marks the tropopause. A wind staff along the side shows speed and direction at each level.

CAPE is the area on a sounding where a lifted parcel is warmer than its surroundings; values over 1,000 joules per kilogram are common near thunderstorms and can exceed 5,000. Lower down, a warm layer aloft called a cap can leave the parcel colder than the air around it. That negative area, convective inhibition, can delay storms until lift breaks it.

Unstable air still needs a push. Winds converging into a low force air upward; fronts and drylines wedge denser air beneath lighter air; mountains force air up their slopes; and uneven heating makes some air buoyant, as along a sea breeze. Rain-cooled outflow from earlier storms acts like a small cold front and can lift air into new storms.

Moisture, usually from warm oceans, feeds the cloud; instability lets a nudged parcel keep rising; lift supplies the nudge. Many storms get only those three and stay short-lived. Forecasters looking for tornadic storms add vertical wind shear as a fourth ingredient, and even a very favorable setup does not make every storm severe.

In the towering cumulus stage, updrafts dominate. The mature stage begins when precipitation reaches the ground and drags air down beside the updraft, and hazards peak late in this stage. Then the rain-cooled downdraft spreads through the cloud and cuts off the warm, moist inflow, and the storm dissipates, often leaving only an anvil.

Each cell lives about the usual half hour, but its cold outflow merges with its neighbors' into one gust front, and convergence along it sets off fresh cells every 5 to 15 minutes. When new cells keep forming upwind and passing over the same ground, called training, rain piles up; when they form so fast the cluster seems to stall or creep backward, called back-building, flash floods follow.

Wind that strengthens and veers with height sets the whole storm rotating, and its updraft can last for hours and exceed 100 mph. Supercells produce nearly all significant US tornadoes and most hail larger than golf balls, and nearly all of them bring severe weather of some kind, yet only about a quarter produce a tornado.

New cells keep forming at the leading edge, with rain and hail behind, so a line can last for hours across hundreds of miles, often fronted by a shelf cloud. Most of its damage comes from straight-line wind, though about a quarter of US tornadoes form in squall lines. Where outflow surges ahead, the line bows outward on radar.

A mesoscale convective system can span a whole state and last more than 12 hours; squall lines count, as do large, round complexes that peak overnight, when the main threat shifts to heavy rain. Some leave a mesoscale convective vortex, a low only 30 to 60 miles wide that can outlive its parent storms by up to 12 hours and seed the next round.

The thresholds are a tornado, convective wind of at least 50 knots, which is 58 mph, or hail at least 1 inch across, about quarter size. Heavy rain and lightning are not on the list, so a storm can flood streets and strike repeatedly without being classed severe. Wind damage to buildings can count as evidence that the wind threshold was met.

Falling rain drags air down, and evaporation chills it, so it grows denser and sinks faster. At the surface the cold air spreads out like a small cold front, bringing a sharp temperature drop and a jump in wind, often well ahead of the storm. That gust front can lift warm air into new cells or cut older ones off from their supply.

In the middle of the cloud, light ice crystals and heavier soft hail, or graupel, collide. The crystals pick up positive charge and are carried up; the graupel gains negative charge and stays lower. Air insulates until the charge difference grows so large that it breaks down as lightning. Storms that make little ice usually make little lightning.

Because the electric field inside the storm is far stronger than the one between cloud and ground, roughly 75 to 80 percent of lightning occurs within the cloud. Most cloud-to-ground flashes carry negative charge. Under 5 percent are positive, coming from the anvil or upper storm, and they carry more current and can strike more than 25 miles from the rain.

The leader is a channel of negative charge that moves down in steps tens of meters long, invisible to the eye. As it nears, positive charge reaches up from trees, poles or the ground to meet it. Once they connect, the return stroke surges from the ground up into the cloud and makes more than 99 percent of the light you see.

Lightning heats its channel to about 50,000 F in a fraction of a second, and the air expands so fast that it makes a shock wave. Light arrives at once, but sound takes about 5 seconds per mile, so count the seconds and divide by 5. A sharp crack means the channel passed close by; a long rumble means it was several miles off.

Lightning can strike more than 10 miles from the rain, and thunder is audible to about that range, so hearing it means you are within reach. A building with plumbing and wiring, or a hard-topped metal vehicle, offers protection; picnic shelters, dugouts, tents and convertibles do not. Many people are struck before or after a storm's peak.

Tall, pointed, isolated objects get hit again and again; the Empire State Building averages about 23 strikes a year. What protects people inside a hard-topped car is its metal shell, not its tires. Metal neither draws nor deflects a strike, since height, shape and isolation decide where lightning lands. Crouching or lying flat outdoors is not safe either.

A small ice nucleus collides with supercooled drops and grows, sometimes cycling in and out of the updraft several times. Where the air is just below freezing, the water spreads and freezes slowly, letting bubbles escape and leaving clear ice; in much colder air it freezes at once, trapping bubbles as cloudy ice. The layers record that trip.

Roughly, a 1-inch stone needs an updraft near 49 mph, a 2.75-inch baseball about 81 mph, and a 4.5-inch softball around 103 mph. Multicell storms produce plenty of hail but rarely the largest, because each cell's mature stage is short. Supercells keep lifting stones back into the cold upper cloud, giving them time to grow.

A pea is about a quarter inch, a penny or dime three quarters of an inch, a quarter 1 inch, a hen's egg 2 inches, a baseball 2.75 and a softball 4.5. These comparisons let spotters report size quickly and consistently. Hail at least 1 inch across makes a thunderstorm severe, though smaller stones can still damage crops.

Florida has the most thunderstorms, yet New Mexico, Colorado and Wyoming usually have the most hailstorms, because Florida's high freezing level lets stones melt on the way down. Hail falls in swaths up to about 10 miles wide and 100 long. Stones three quarters of an inch and larger can badly damage aircraft, and giant hail can total cars and kill.

A microburst covers an area under 2.5 miles across with peak winds lasting a few minutes, yet can reach 168 mph; a macroburst is wider and lasts 5 to 20 minutes, and intense ones can do tornado-like damage. Wet bursts come with heavy rain. In dry air the rain evaporates first, leaving a dry microburst marked only by virga or a ring of blowing dust.

A bow echo, or a series of them, qualifies when it travels more than about 250 miles with widespread gusts of 58 mph or more. Within the damage path, smaller pockets of extreme wind can resemble a tornado track, so one house may be wrecked beside lightly damaged neighbors. About 70 percent of derechos occur from May through August.

As rain-cooled air pools beneath a storm complex, it draws air into the trailing side of the system. That inflow tilts the updraft rearward, which spreads rain over a wider area, chills more air and strengthens the gust front so it bows out. The cycle continues as long as new cells keep forming along the advancing edge.

Both contacts are required. A rotating funnel that has not reached the ground is a funnel cloud, and a whirl at ground level with no rotation at cloud base, such as a gustnado, is not a tornado either. The visible funnel need not touch down; a debris whirl beneath it means the circulation already has.

Wind that speeds up with height sets the air rolling horizontally; a strong updraft tilts that roll upright, and veering winds favor its counterclockwise half, forming a mesocyclone. How a tornado then forms is not fully understood. One leading idea involves temperature contrasts in downdraft air wrapping around the mesocyclone, but some violent tornadoes formed without them.

Shear is the change in wind speed or direction with height, and winds that veer, turning clockwise, while strengthening best support supercells. Helicity folds that shear and turning into one number, usually for the lowest 3 kilometers relative to storm motion; values near 150 square meters per second squared or more favor mesocyclones.

A landspout forms without organized storm-scale rotation, often under a towering cumulus, and is the land version of a waterspout; both are usually smaller and weaker than supercell tornadoes but can still kill. A gustnado is an eddy in a storm's outflow with no link to rotation at cloud base, though it can still break windows and limbs.

Surveyors compare damage with 28 indicators, such as homes, mobile homes and trees, each with eight degrees of damage, then assign an estimated three-second gust: EF0 is 65 to 85 mph and EF5 is over 200 mph. Because the rating comes only from damage, a tornado that crosses open fields cannot be scored; the Weather Service lists it as EF-U, strength unknown.

About 1,200 tornadoes hit the United States in a typical year, and the central part of the country gets more than anywhere else in the world, with the peak from April through June. The season shifts north over the year, from spring on the Gulf Coast to June or July in the northern Plains. Violent tornadoes occur outside Tornado Alley every year.

Flying debris causes most of the harm, which is why thick padding such as a mattress, or a helmet, helps even in an interior room. Mobile homes, sheds and vehicles give little protection. Under a bridge, wind can speed up and debris still reaches you, and people have died there.

Violent tornadoes have crossed the Mississippi and almost every major river east of the Rockies, and have climbed mountains and crossed canyons. Downtowns are rarely hit only because they cover so little area; St. Louis has been struck at least four times. Opening windows gains nothing: you lose time and risk flying glass.

The small letter gives moisture: "c" for continental air that formed over land and is dry, "m" for maritime air that formed over water and is moist. The capital gives temperature: A for arctic, P for polar, T for tropical. So cP is cold and dry and mT is warm and moist. The United States makes a poor source region, because passing storms rarely let air sit still long enough to take on the ground's traits.

Air takes on the traits of whatever lies under it. Arctic air crossing a warmer winter ocean picks up heat and moisture and becomes maritime polar, while the same air moving south over dry land warms but stays dry. Warm, moist air drifting over a cold surface turns stable instead, which is why it brings fog, drizzle and flat layered cloud.

Dense cold air pushes under the warm air like a snowplow and forces it upward abruptly, so showers and storms crowd into a narrow band along or just ahead of the boundary. Whether that band holds thunderstorms depends on the warm air: unstable air builds storms, while stable air gives overcast and rain. Cold fronts typically move 25 to 30 mph, faster than warm fronts, and skies often clear quickly behind them.

Because the boundary slopes gently, warm air climbs slowly over a broad area, so layered cloud and light to moderate precipitation spread well ahead of the surface front. Warm fronts move slowly, typically 10 to 25 mph, which is why the gray, damp spell can last hours. Clouds lower and thicken as the front nears; once it passes, the rain usually ends and temperatures rise.

A stationary front forms when neither air mass can push the other, so its clouds and rain can sit over the same area for days. An occluded front forms because cold fronts usually move faster than warm fronts and catch up to them, lifting the warm air off the ground. An occlusion is usually a sign that the parent storm has matured and is beginning to weaken.

A dryline divides moist Gulf air from dry desert air, usually across the southern and central High Plains in spring and early summer. Dry air is denser, so it pushes under the moist side and lifts it, and severe storms often fire along it or just to its east. An outflow boundary is the leading edge of rain-cooled air spreading from earlier storms, and it can lift air to start new ones.

A cold front is drawn as a blue line with triangles and a warm front as a red line with half circles, each pointing the way the front is moving; a stationary front alternates the two, and an occluded front is purple. Analysts draw isobars, lines of equal pressure, from the station plots. Cold fronts nearly always trail south and west of a low's center and never extend from a high.

Ahead of a cold front the dew point is high and the pressure falls. As the front passes, pressure bottoms out, winds turn gusty and variable, and temperature and dew point drop fast. Afterward pressure keeps rising and the wind settles from the west or northwest. A warm front's passage is gentler: the temperature rises steadily while the pressure levels off.

Worked out by Norwegian meteorologists in the 1910s and 1920s, the model starts with a stationary front. An upper-level disturbance passing over it kinks the front into a wave, splitting it into a cold front and a warm front around a deepening low. The faster cold front then catches the warm front and occludes, cutting the low off from warm, moist air so that it slowly dies.

Cyclogenesis is the forming or strengthening of a low. Winter storms often spin up in southeastern Colorado in the lee of the Rockies, then head east or northeast, drawing cold air south from Canada and moisture north from the Gulf. Others strengthen in the lee of the Canadian Rockies and move southeast, and nor'easters develop near the Carolina coast, fed by warm, moist Atlantic air as they move north.

When winds high in the atmosphere spread apart, air rises from below to replace it, and that rising motion feeds clouds and precipitation. This happens downstream of an upper-level disturbance, or ahead of a trough, which is why a surface wave tends to form when such a disturbance passes over a front. It also explains why most foul weather lies between a trough and the ridge to its east.

A jet streak is a local wind-speed maximum that travels along the jet stream. Air speeds up as it enters the streak and slows as it leaves, and that sets up divergence aloft in the right half of the entrance region and the left half of the exit region, viewed looking downwind. Those two quadrants, the right rear and left front, are where rising air and storm potential tend to increase.

Ahead of the warm front, clouds lower and thicken and steady rain falls on an east wind. Once the warm front passes you are in the warm sector, the warm air between the two fronts: the rain ends, skies turn partly cloudy, temperatures climb and winds turn gusty from the south. A line of storms often marks the cold front, and behind it the wind swings northwest, temperatures drop and skies clear.

The required pressure fall depends on latitude: 24 millibars in 24 hours at 60 degrees, but only about 17.8 millibars at the latitude of New York City. Bombs can form where a cold air mass collides with warm air, such as air over warm ocean water, and nor'easters fed by warm Atlantic air can grow this explosively. The label measures how fast the pressure drops, not how much snow or wind a storm brings.

Surface lows tend to follow the upper-level wave pattern, so the longwaves mark out their tracks. A Colorado low, for example, forms in winter in southeastern Colorado or northeastern New Mexico and moves northeast across the central Plains. Your side of the track sets your weather: Panhandle lows usually drop heavy snow north of their surface track, while thunderstorms may form on the warm side to the south.

Most winter precipitation starts as snow high in the cloud, and what reaches you depends on the layers it falls through. It stays snow only if the air is at or below freezing from cloud base to the ground. A warm layer aloft that partly melts it gives sleet, one that melts it completely over a shallow freezing layer gives freezing rain, so a surface reading alone cannot tell you the type.

Snow melts completely in a warm layer aloft, then falls through a thin freezing layer near the ground without time to refreeze. The supercooled drops freeze on contact with anything at or below freezing, coating roads, trees and power lines in glaze. An ice storm leaves at least a quarter inch of ice, and that weight snaps branches and lines, so power can be out for days.

Both start as snow that meets warm air aloft. Sleet forms when the flakes only partly melt and then refreeze in a deep freezing layer, reaching the ground as ice pellets that bounce. Freezing rain forms when they melt completely and the cold layer near the ground is too thin to refreeze them, so they glaze on contact. Sleet piles up like gritty snow; freezing rain coats every surface.

In the United States about 13 inches of snow equals one inch of water on average, but the ratio runs from about 2 to 1 for sleet to nearly 50 to 1 for very dry powder. Lacy dendrite and plate crystals pile up fluffiest. Warm air near freezing, crystals coated with frozen cloud droplets, and wind that breaks flakes apart all push the ratio down, toward 10 to 1 or lower.

Cold, dry air crossing a large, warmer lake picks up heat and moisture and becomes unstable, building bands of snow showers downwind. A longer fetch, the distance the air travels over water, adds more warmth and moisture and usually means stronger bands. Wind direction decides where the snow lands, so a small shift in the wind can move a heavy band from one town to the next.

A blizzard needs sustained wind, or frequent gusts, reaching 35 mph or higher, together with snow in the air that often cuts visibility to a quarter mile or less, for at least three hours. Heavy snowfall and severe cold often come with it but are not required. Strong wind lifting snow that has already fallen can produce a ground blizzard under clear skies.

Most winter storm deaths are not caused directly by the storm, and about 70 percent of ice and snow injuries happen in vehicles. Black ice, a clear glaze on the road that is hard to see, is most common in early morning when meltwater from the day refreezes overnight. Wind chill measures how fast exposed skin loses heat; frostbite still requires the actual air temperature to be below freezing.

A tropical cyclone needs ocean water of at least 80 degrees F through the top 150 feet or so, moist air in the middle troposphere, air unstable enough for deep convection, and low vertical wind shear. It also needs a starting disturbance, usually a tropical wave. Storms almost never form within 5 degrees of the equator, because the Coriolis force there is too weak to start the spin.

A tropical disturbance is an organized cluster of storms, often 100 to 300 miles across, that holds together for at least a day. Once its storms organize around a defined circulation it is a tropical depression, with peak one-minute sustained winds of 38 mph or less. At 39 mph it becomes a tropical storm and gets a name, and at 74 mph it becomes a hurricane in the Atlantic and eastern Pacific.

Air spirals inward, counterclockwise in the Northern Hemisphere, and rises in the eyewall, a ring of tall thunderstorms with the heaviest rain and usually the strongest winds. Some air sinks in the center, forming a calm, often clear eye typically 20 to 40 miles wide. Spiral rainbands trail outward with gaps between them. The storm is warm-core: warmer at its center than around its edges.

Warm, moist air rises, cools and condenses, and the condensation releases heat that warms the column and makes it rise faster. That lowers the surface pressure and pulls in more moist air, a chain reaction that can build to hurricane force. Over land the storm loses the ocean's moisture and heat, so the thunderstorms near its center fade and it weakens quickly.

Forecasters count it as an increase of at least 30 knots, about 35 mph, in peak sustained wind over 24 hours. It draws on the ingredients that let any storm strengthen: a deep layer of warm water, since a storm feeds on the whole warm layer above the cooler water below rather than the surface alone, and low vertical wind shear, which keeps its core intact.

In intense storms some outer rainbands can organize into a second ring of thunderstorms. As this outer eyewall slowly contracts inward, it starves the inner eyewall of moisture and momentum, and the storm weakens for a while. Once the outer ring fully takes over, the hurricane can return to its earlier strength or even surpass it, so a temporary drop in wind is no sign the threat has passed.

Vertical wind shear is a change in wind speed or direction with height. When it is strong, the storm's core tilts with the shear, which disrupts the flow of heat and moisture that a tropical cyclone runs on. Too much shear can make a young disturbance fall apart. El Nino tends to raise shear over the Atlantic, which is one reason it cuts the number of Atlantic hurricanes.

A tropical cyclone is a warm-core low with no fronts, powered by heat released as vapor condenses. An extratropical cyclone draws its energy from the temperature contrast between warm and cold air masses and usually trails fronts. As a tropical storm moves poleward it can transition into one, and it may still carry hurricane-force winds and heavy rain afterward, so the label is no all-clear.

The scale rates a hurricane from 1 to 5 using only its peak one-minute sustained wind, and it estimates property damage from that wind. It says nothing about storm surge, rainfall flooding or tornadoes, which can be deadly in any category. A larger storm pushes a higher surge and a slower one drops more rain, so a weaker, slow-moving storm can flood more than a stronger, faster one.

In deep water the wind-driven circulation leaves no visible surge, but in shallow coastal water the sea floor blocks it, so water is pushed up and inland. Stronger, larger and onshore-moving storms raise more water, and the worst surge is usually just right of the eye. Storm surge is the leading cause of hurricane deaths: two feet can lift a car and ten can flood a home's first floor.

Storm surge is the rise caused by the storm itself; storm tide is the actual water level, surge plus the normal astronomical tide. A surge that arrives at high tide can produce storm tides of 20 feet or more. The sea floor matters too: a wide, gently sloping shelf, like that off Louisiana and Mississippi, allows a higher surge than a steep shelf such as Florida's southeast coast.

Rainfall depends on a storm's speed, size and the terrain beneath it, not on its wind category, so a weak tropical storm that stalls can flood more than a fast major hurricane. Of 442 U.S. deaths tied directly to tropical systems from 2013 to 2022, about 57 percent came from inland flooding, mostly people who walked or drove into the water. The danger can last days after the wind fades.

Tornadoes form most often in the right-front quadrant relative to the storm's motion, frequently in rainbands far from the center. They are usually weaker and shorter-lived than Great Plains tornadoes, and they seldom come with hail or much lightning, which removes familiar warning signs. They can keep forming for days after landfall while the remnant circulation survives.

Each circle along the forecast track is sized so that two-thirds of the official track errors from the past five years fall inside it, so the storm's center stays within the cone only about 60 to 70 percent of the time. The cone says nothing about storm size. Impacts can span hundreds of miles, and places well outside it often get tornadoes or inland flooding.

A hurricane watch goes out about 48 hours before tropical-storm-force winds are expected, and a warning about 36 hours before, because preparing becomes hard once those winds arrive. That is also why evacuations are planned to finish before tropical-storm-force winds, not hurricane-force winds. Every category can bring deadly surge, so leaving a surge-prone area follows local officials' orders rather than the storm's category.

A flash flood starts when rain falls faster than the ground can soak it up, or fills dry creeks until streams overtop their banks, usually within six hours of the rain. A failed dam, levee or ice jam can cause one with no rain at all. Water can rise within minutes, and the storm that feeds it may sit well upstream, so the sky overhead can look harmless.

Training means storm after storm passing over the same ground, like railroad cars on a track, piling up rain no single storm would deliver. Back-building is a related setup in which new cells keep forming on the upwind side, often the west or southwest, so the storm complex seems to stand still or even move backward. Most flash floods come from slow-moving or repeating storms.

Steep slopes, rock and shallow clay soils shed water fast, so a creek six inches deep can become a ten-foot torrent in under an hour. Ground burned by wildfire absorbs little, so rain runs off almost at once and can trigger debris flows. Pavement and roofs do the same in cities, where overloaded storm drains turn underpasses and basements into traps.

Rain does not reach a river all at once; water from the far end of a basin arrives last, so a river keeps rising after the rain stops. Forecasters model that lag, how the flood wave spreads as it moves downstream, and how much rain the soil absorbs, then route the water gauge to gauge. The crest is the highest stage as the wave passes, and big rivers can be forecast days ahead.

More than half of U.S. flood drownings happen when a vehicle is driven into floodwater, and walking into it is next. Six inches of moving water can knock an adult down, about a foot can carry most cars away, and two feet can carry off SUVs and trucks. The water can hide a washed-out road, and it is never safe to drive or walk through it.

Drought is a moisture shortage over a season or longer, and it creeps in rather than striking. Meteorological drought means dry weather patterns dominate; agricultural drought means crops are being hurt; hydrological drought means the water supply itself is running low. What counts as drought depends on a region's normal climate, so the same rainfall can be drought in one place and normal in another.

ASOS is the nation's main automated surface network, run jointly by the NWS, FAA and Defense Department, with about 950 stations, mostly at airports. AWOS is similar but usually reports fewer elements. Each station measures only its own spot, and its sky sensor sees cloud only up to 12,000 feet, so a valley station can report a ceiling high enough for visual flight while hikers on the ridge sit in fog.

Temperature sits upper left and dew point lower left, both in degrees F. Pressure, upper right, drops its leading 9 or 10 and its decimal, so 998 means 999.8 mb. The center circle shows cloud cover in eighths, and the wind barb points to where the wind blows from, each long barb worth 10 knots and each short one 5. A tendency figure shows the three-hour pressure change.

A balloon lifts a small instrument package that radios back pressure, temperature and humidity, while GPS tracking of its drift gives the wind. Most launches worldwide happen at 00Z and 12Z, so stations sample the whole atmosphere at the same moments. A flight can last over two hours and climb above 115,000 feet, and the data are plotted on a Skew-T chart.

Each chart shows the height where the pressure equals one value, so lines on it are height contours, low in cold air and high in warm air. The 850 mb chart, near 5,000 feet, tracks low-level moisture; 700 mb, near 10,000 feet, shows humidity and rising air for precipitation; 500 mb shows troughs, ridges and spin; 300 and 200 mb locate the jet stream.

A geostationary satellite orbits 22,236 miles up over the equator once every 24 hours, so it seems to hang over one spot and can image the same view as often as once a minute. That steady view lets forecasters track cloud motion and watch storms develop. The tradeoff is height: its images are less detailed, and the Earth's curve limits its view of the polar regions.

A polar orbiter passes over both poles about 14 times a day at roughly 520 miles up, much lower than a geostationary satellite, so its images are far more detailed. The cost is coverage: it never sees the whole Earth at once, no two passes view exactly the same place, and any one spot gets only a few looks a day.

Visible imagery shows reflected sunlight, so it goes black at night. Infrared measures the temperature of cloud tops and the ground, so it works around the clock; cold high clouds look white, but low clouds can match the ground and vanish. Water vapor channels also read temperature, at wavelengths water vapor absorbs, so they map mid- and upper-level moisture and reveal jet streams and troughs where no cloud shows.

The radar sends a brief pulse, listens for the echo, and times the round trip to find the target's distance. Reflectivity, in dBZ, measures how much power comes back, and it grows with drop size so steeply that one large drop can echo as strongly as hundreds of drops a third its width. The beam also rises with distance: 100 miles out, the lowest scan sits about 12,000 feet up.

A Doppler radar reads only the part of the motion toward or away from it, so the measured speed is usually less than the true wind and drops to zero where the wind crosses the beam. By convention green means toward the radar and red means away. Strong inbound winds right next to strong outbound winds form a couplet, the signature of intense rotation, though the radar sees the rotation, not the tornado itself.

Dual-polarization radar sends pulses in both horizontal and vertical planes. Differential reflectivity compares the two: flattened large raindrops read above zero, while tumbling hail looks round and reads near zero. Correlation coefficient measures how alike the targets are; tornado debris is so jumbled that it drops sharply, but it counts as a debris signature only when it sits on a strong velocity couplet.

A model lays a grid over the globe or a region and starts from an initial state built from balloon, satellite, surface, ship and aircraft observations. Equations of motion then step the atmosphere forward a few seconds to minutes at a time, each step's result feeding the next. Finer grids and shorter steps add accuracy but multiply the computing; doubling grid points in each direction quadruples the work.

Every run needs a starting picture of the atmosphere, and data assimilation builds it by folding in observations from radiosondes, satellites, surface stations over land and sea, and commercial aircraft. The result, the analysis, is only the best estimate of the true state, and its small errors grow during the forecast. Rapid-update models assimilate fresh radar data as often as every 15 minutes.

Global models such as the GFS cover the whole Earth, while regional models such as the NAM and RAP cover part of it with finer detail. The HRRR goes further: a 3-kilometer, hourly-updated, convection-allowing model built for the next several hours to two days. More grid points buy detail but cost computing, so each type fits a different job.

The atmosphere is chaotic, so any small error in the starting analysis grows during a forecast until all predictive information is lost, even with a perfect model. How fast that happens depends on the season, the region and the weather pattern. In practice, models are generally reliable for about four days, after which run-to-run differences grow.

An ensemble reruns the model many times from slightly different starting states, each within the uncertainty of the analysis. Every run is deterministic, but together they map the range of outcomes. Averaging the members gives a more reliable forecast than any single run, and the spread shows how far to trust it: tight agreement signals confidence, while a wide spread means several outcomes are still in play.

A 40 percent chance means a 40 percent chance that any given point in the forecast area gets at least 0.01 inch during the period. It does not mean rain over 40 percent of the area or for 40 percent of the time. Forecasters reach it by multiplying their confidence that rain will form by the share of the area expected to get it, so 80 percent sure of 50 percent coverage gives 40 percent.

A watch means conditions are favorable and the hazard is possible: plan and be ready to act. A warning means it is happening or imminent: act now. A tornado watch usually covers many counties or whole states, while a warning covers an area about the size of a city or small county. Winter storm watches typically go out 12 to 48 hours ahead and warnings 12 to 24 hours ahead.

An advisory covers weather that is less severe than warning level but can still cause real trouble, such as a flood that is a nuisance or snow and ice that make travel hazardous. Outlooks look further ahead: winter storm outlooks come 3 to 5 days early, the Storm Prediction Center issues severe outlooks for days 4 through 8, and the Weather Prediction Center forecasts the chance of rain heavy enough to flood.

The five risk levels run marginal, slight, enhanced, moderate and high, set by how widespread and intense organized severe storms are expected to be. Behind them are probabilities of severe weather within 25 miles of any point. A 15 percent tornado chance sounds low, but if the usual daily odds are near 1 percent, it means about 15 times the normal risk.

Since October 2007, NWS warnings for tornadoes, severe thunderstorms, flash floods and marine hazards have outlined the actual threat area as a polygon, often just part of a county, instead of warning whole counties. A polygon can shrink during the warning but never grow; if a storm heads outside it, a new warning is issued for the newly threatened area.

Warnings carry tags beyond the base level. A considerable tag marks an unusually dangerous hazard, and a catastrophic tag adds the words flash flood or tornado emergency for a hazard capable of catastrophic damage. Source tags matter too: radar indicated means radar suggests the hazard but nobody has confirmed it, while observed means radar or reliable reports confirm it.

NOAA Weather Radio broadcasts warnings around the clock from more than 1,000 transmitters, but it needs a special receiver, and a transmitter can be degraded or knocked off the air. Wireless Emergency Alerts reach WEA-capable phones automatically with no sign-up, sent by authorized agencies through mobile carriers. Through the Emergency Alert System, Weather Radio also carries non-weather emergencies such as AMBER alerts.

Radar can detect strong rotation but not the tornado itself, because its beam rises above the ground with distance. Trained SKYWARN spotters, a volunteer corps of 350,000 to 400,000, report what is actually happening, and warnings can be based on a spotter or police report as well as radar. A report can upgrade a warning from radar indicated to observed.

The ceiling is the height of the lowest cloud layer reported as broken or overcast, or the vertical visibility when the sky is totally obscured. Prevailing visibility is the greatest distance visible through at least half the horizon. Together they set the flight categories: VFR needs a ceiling above 3,000 feet and visibility above 5 miles, while LIFR means a ceiling below 500 feet or visibility under 1 mile.

Convective turbulence comes from rising and sinking air on warm afternoons and is strongest beneath or inside cumulus clouds. Mechanical turbulence forms as wind tumbles over buildings, trees and mountains, growing with wind speed and rough terrain. Clear-air turbulence strikes in cloudless air, usually above 15,000 feet near the jet stream, where sharp wind shear between the jet core and nearby air gives no visual warning.

Structural ice forms when supercooled droplets, still liquid below freezing, strike an airframe and freeze. Small drops freeze instantly into rough, milky rime, the type pilots report most often. Larger drops in warmer cloud freeze only partly at first and smear back over the surface before hardening into clear ice, which is denser, harder to see and disrupts airflow more. Mixed ice layers the two.

A microburst is a small, intense downdraft that hits the ground and spreads out in all directions, typically less than 2.5 miles across and lasting 5 to 15 minutes. An aircraft flying through first gains headwind, then loses it and meets a sinking column of air, losing 30 to 90 knots of headwind. The only clue may be a rain shaft, virga or a ring of blowing dust.

Wind waves grow with three things: wind speed, how long it blows, and fetch, the open distance it blows over without changing direction. That is why storms of equal size raise bigger waves in the open Pacific. Waves that outrun their storm become swell, smoother and longer-period, able to travel thousands of miles, so a storm far offshore can bring dangerous surf to a sunny beach.

Marine wind products step up by strength: small craft advisory thresholds vary by region, a gale warning covers 34 to 47 knots, a storm warning 48 knots or more, and a hurricane force wind warning 64 knots or more, the last three for winds not tied to a tropical cyclone. Fog at sea is usually advection fog, moist air cooled from below as it drifts over colder water.

A red flag warning signals weather that can drive extreme fire behavior. Criteria vary by region, but they typically follow a dry spell when fire danger is already high, with sustained winds of 15 mph or more and humidity at or below 25 percent expected. A large fire can make its own weather: its heat can build pyrocumulus or thunderstorm clouds whose lightning and stronger winds spread it further.

Gas molecules are far smaller than light's wavelengths, so they scatter short wavelengths more than long ones, and blue scatters about four times more strongly than red. Violet scatters even more but mostly high up, so blue dominates what you see. At sunrise and sunset light crosses far more air, losing its blue, then yellow and orange, until mostly red is left.

Sunlight bends entering a raindrop, reflects once off the back, and bends again on the way out, and because each color bends a different amount the light fans into a spectrum. The most concentrated rays emerge at about 42 degrees, so the bow always appears opposite the sun and you need the sun behind you and rain ahead. If the sun is higher than 42 degrees, no bow appears.

Some light reflects twice inside each drop before leaving. Those rays exit at about 50 degrees instead of 42, so the second bow sits outside and above the first, and the extra reflection reverses its colors, putting red on the inside. Light can reflect more than twice inside a drop, but those higher-order bows are typically never seen in normal conditions.

Thin, high cirrus or cirrostratus clouds are made of ice crystals that act like prisms, bending sunlight or moonlight into a ring usually about 22 degrees in radius. Sun dogs are colored spots about 22 degrees to either side of the sun at the same height, red nearest the sun. Halo clouds often form ahead of rain or snow, but a halo is no reliable forecast.

A corona is a small ring of light hugging the sun or moon, blue inside and red outside, produced by diffraction of light around tiny droplets in thin cloud or mist. A halo is much larger and comes from refraction in ice crystals, with its colors in the opposite order. A glory is a set of colored rings around the shadow of your head when it falls on cloud below you.

Light bends as it passes through air of changing density. Over a sun-heated road or desert, air near the ground can be hotter and less dense than the air above, producing an inferior mirage, the false "puddles of water" seen on superheated highways. In a temperature inversion, with cold air under warm, light passing at low angles bends so that looming lifts objects from beyond the horizon into view.

Crepuscular rays are alternating bands of light and shadow that appear when the sun shines through gaps in clouds. Near sunrise and sunset, sunlight takes a long, low path through the densest air, so blue is scattered out first, then yellow, then orange, leaving reds and oranges on the clouds. Civil twilight is the stretch between sunset and civil dusk, and between civil dawn and sunrise.

The planetary boundary layer is the lowest one or two kilometers of air, where friction with the ground slows the wind and temperatures react most to daytime sun and nighttime cooling. By day, surface heating drives convection that mixes the layer, rising higher as heating increases. At night, radiational cooling often builds an inversion in the lowest few thousand feet, and an inversion traps pollutants beneath it.

The mixing height is how high a rising parcel or smoke column can climb before it stops spreading; below it, pollutants are trapped. Multiply the mixing depth by the wind speed that carries air away and you get the ventilation index, a measure of how well the air can disperse pollution from a source. A shallow layer with light winds scores low, so pollution builds.

In an inversion, temperature rises with height, so warm air sits like a lid over colder air below and pollutants cannot mix upward. Inversions form most often near the ground on clear nights through radiational cooling, along fronts, and where cold air settles into valleys. They can trap smog and haze that lingers until the inversion breaks.

PM10 covers inhalable particles 10 micrometers or smaller, and PM2.5 the fine fraction 2.5 micrometers or smaller, about 30 times thinner than a hair. Some come straight from roads, fields, smokestacks and fires, but most form in the air from gases like sulfur dioxide and nitrogen oxides. Fine particles reach deep into the lungs and even the bloodstream, so they pose the greatest health risk.

Ground-level ozone is not emitted directly. It forms when nitrogen oxides and volatile organic compounds from cars, power plants and industry react in sunlight, which is why it peaks on hot, sunny days in cities. Wind can carry it far, so rural areas can see high levels too. High overhead, stratospheric ozone is helpful, shielding the surface from ultraviolet light.

The AQI runs in six color-coded categories from good, 50 or below, to hazardous, above 300. A value of 100 roughly matches the national short-term health standard for that pollutant, so readings above it are unhealthy, first for sensitive groups and then for everyone. An index is computed for each major pollutant, and the highest one becomes the reported AQI.

Wildfire smoke can spread thousands of miles from the blaze; West Coast smoke has crossed more than 3,000 miles to New York and out over the Atlantic. Its fine particles can penetrate deep into the lungs, and very large fires can loft smoke high in towering fire clouds. Forecasters use satellite fire detection with models such as HRRR-Smoke to predict where smoke will go up to 36 hours ahead.

The difference is time. Weather is the state of the atmosphere at a given moment, while climate is the average or typical weather over years or longer, usually measured against 30-year normals. A single day can look normal on average yet swing wildly in weather, and a big swing in one day, month or year does not by itself mean the climate has shifted.

A climate normal is the plain average of a value, such as a day's high temperature or rainfall, over three consecutive decades. The current set covers 1991 to 2020, and a new set based on 2001 to 2030 is due in 2031. So "above normal" compares today with that 30-year baseline, which itself shifts each decade as the window moves.

Wladimir Koppen grouped the world's climates by temperature patterns tied to latitude, and with Rudolf Geiger refined them into the Koppen-Geiger system. Its main groups are tropical, dry, moist subtropical mid-latitude, moist continental mid-latitude, polar, and highland, the last set by elevation. Thresholds define each group; in moist tropical climates every month averages above 64 degrees F.

Normally, trade winds push warm water west across the Pacific, letting cold water upwell off South America. When air pressure at Darwin, Australia, rises above that at Tahiti, the trades weaken, upwelling slows and the eastern Pacific warms: El Nino. Stronger trades cool it instead: La Nina. Sea temperatures change only a few degrees, yet weather shifts worldwide, and El Nino raises Atlantic wind shear.

The North Atlantic Oscillation is a seesaw in pressure between the Azores high and the Icelandic low that steers winter storms across the Atlantic. The Arctic Oscillation's negative phase lets cold air plunge into the Midwestern United States and western Europe. The Madden-Julian Oscillation is a tropical rainfall pattern that cycles every 30 to 60 days, far faster than El Nino.

A seasonal outlook gives the odds that conditions averaged over the period will be below normal, near normal or above normal. Leaning toward above normal shifts the odds; it does not promise a warm day on any particular date. Where forecasters see no signal, the map shows equal chances, meaning each of the three outcomes is as likely as it is in the long-term record.

A 100-year flood is a climate average: it has a 1 percent chance of happening in any given year. It is not scheduled once a century, so two can strike in back-to-back years, and the odds do not reset after one occurs. The same idea rates heat: one country's October 2021 average temperature, almost 7 degrees F above its 1991 to 2020 average, corresponded to a one-in-6,250-year event.

Event attribution asks whether, and by how much, climate change altered the odds or severity of a particular extreme, using both historical observations and model simulations. Results come as changed likelihoods: NOAA research found the 2020 to 2021 California and Nevada heat and drought was made six times more likely. Studies cover only the events chosen for analysis, so a set of them is not a full census of extremes.