Meteorology, oriented

Water vapor, the gas behind cloud and rain, ranges from nearly zero over a desert to about 4 percent in the humid tropics. Temperature falls with height through the troposphere up to the tropopause, about 11 to 12 miles high at the equator but under 4 at the poles. Above it, warmer air sits over cooler air and blocks rising motion, which is why a thunderstorm's top flattens into an anvil.

The ground absorbs sunlight and gives the energy back as infrared, much of which water vapor, carbon dioxide and clouds absorb, so an overcast night stays warmer than a clear one. The tropics absorb more than they emit and the poles the reverse, so the surplus is carried poleward. Land heats only its top few inches while water mixes heat through a thick layer, which is why coasts swing less than inland towns.

The trap is assuming temperature always drops as you climb. On a clear, calm night the ground chills the air touching it and forms an inversion, a layer where temperature rises with height. In a valley town at dawn, the stable layer suppresses mixing and pins chimney smoke near the ground until sunshine warms the boundary layer, the lowest kilometer or two of air.

Rule: before predicting what air will do, compare the layer above with the layer below; warmer above means a lid.

Air pressure is the weight of the air overhead, 1013.2 millibars at sea level in the standard atmosphere. Readings are converted to sea level before isobars, lines of equal pressure, are drawn, and crowded isobars mean strong wind.

The pressure gradient force starts air moving toward lower pressure. The Coriolis effect then turns it to the right in the Northern Hemisphere, and above the friction layer the two forces balance, so the wind blows along the isobars. Near the ground, friction slows the wind and weakens that turning, so air cuts across the isobars into a low. Air piling into a low has to rise, bringing cloud and rain, while sinking air around highs brings fair skies.

The trap is giving the Coriolis effect credit for small things. Its turning needs distance and time, so a sea breeze, spanning under about 100 miles and lasting under 12 hours, blows almost straight toward low pressure, and a draining sink is far smaller still. On a sunny coast the land warms faster than the sea, its air rises, and cooler sea air pushes in beneath it, sometimes dropping temperatures 15 to 20 F. After dark a weaker land breeze takes over, and mountain winds run upslope by day and downslope at night.

Rule: find the pressure difference first, then let rotation and friction shape the flow only if the system is large and long-lived enough to feel them.

Rotation breaks the planet's overturning into three cells per hemisphere, with sinking, drying air near 30 degrees latitude, where the main desert belt lies. Jet streams, narrow ribbons of fast wind aloft, run above the sharpest temperature contrasts and are strongest in winter.

Air masses take their traits from where they form: continental means dry, maritime means moist, and polar or tropical gives the temperature. A cold front pushes under warm air like a snowplow and crowds showers and storms into a narrow band. A warm front slopes gently and spreads layered cloud and steady rain well ahead of it. When an upper-level disturbance crosses a stationary front, air spreading apart aloft lifts the air below, and the front kinks into a wave around a deepening low. Later the faster cold front catches the warm front, occluding it and cutting the low off from warm, moist air.

The trap is reading an occlusion as a storm gathering strength; it usually means the low has matured and is starting to weaken. Say you live south of a low's path. Clouds lower and thicken and rain falls on an east wind; the warm front passes and the air turns warm on a gusty south wind; then a line of storms marks the cold front, behind which the wind swings northwest and the temperature drops.

Rule: locate yourself relative to the low's fronts and track, and the sequence of sky, wind and temperature follows.

Air's capacity for water vapor roughly halves with each 10 C of cooling. Relative humidity compares the vapor present with that capacity, so it climbs overnight with no water added. Dew point, the temperature at which the air would saturate if cooled, stays put as the air warms or cools and changes only when moisture is added or removed. Its gap from the temperature shows how close fog or cloud is.

The trap is reading a high humidity number as a lot of moisture. A cool, saturated morning can hold less water than a hot afternoon at 40 percent humidity.

Clouds form when rising air expands and cools to its dew point and vapor condenses on tiny particles of dust, salt or smoke. Fog is cloud at ground level, forming on clear, calm nights over land or when moist air drifts over a colder surface. In warm clouds, bigger droplets fall faster and sweep up smaller ones. In colder clouds, ice crystals grow at the expense of supercooled droplets, still liquid below freezing, in the Bergeron process, and fall as snow or melt into rain.

The brightest rainbow light leaves the drops at about 42 degrees, so the bow appears opposite the sun, with the sun behind you and rain ahead. Ice crystals in thin, high cirrus bend light into a halo about 22 degrees in radius.

Rule: judge moisture by dew point, and judge the next cloud or fog by asking what is cooling the air.

Imagine lifting a bubble of air. Unsaturated, it cools about 5.5 F per 1,000 feet as it expands; once saturated it cools more slowly, because condensation releases heat. If it ends up colder than the air around it, it sinks back and the air is stable; if it stays warmer, it keeps rising. On a sounding, the area where the parcel runs warmer than its surroundings measures its available energy, and a warm layer aloft, a cap, can hold it back until lift breaks through.

A single thunderstorm cell lives about half an hour: its rain drags cool air down beside the updraft, and that downdraft spreads through the cloud and cuts off the warm inflow. Multicell storms last for hours because each cell's outflow joins a gust front that sets off new cells. With vertical wind shear a storm can rotate. A supercell has a deep, persistent rotating updraft, the mesocyclone, and produces nearly all significant US tornadoes, though only about a quarter of supercells make one.

The trap is equating dangerous with severe. In the United States a thunderstorm is severe only with a tornado, wind of at least 58 mph, or hail at least 1 inch across. A slow cluster that dumps rain for hours with frequent lightning, but no hail or strong wind, earns no severe label yet is a real flash-flood and lightning threat.

Rule: ask what lifts the air, whether it is unstable once lifted, and whether shear will organize what grows.

In a storm's middle levels, light ice crystals and heavier soft hail, called graupel, collide. The crystals carry positive charge up and the graupel keeps negative charge low until the air breaks down as lightning. The bright flash you see is the return stroke surging up from the ground. Lightning heats its channel to as much as 50,000 F, and the air's explosive expansion is thunder. Sound takes about 5 seconds to travel a mile, so a 20-second gap puts the stroke about 4 miles away.

The trap is waiting for the rain before taking lightning seriously. Strikes can land more than 10 miles from the rain, about as far as thunder carries, so hearing thunder means you are within reach. A building with plumbing and wiring, or a hard-topped metal vehicle, protects you; in a car it is the metal shell, not the rubber tires. Wait 30 minutes after the last thunder before going back out.

Hail grows as an updraft carries ice through supercooled water, and bigger stones need stronger updrafts: roughly 49 mph for a 1-inch stone and about 103 mph for a 4.5-inch softball, and supercells grow the largest by lifting stones again and again. Downdrafts do damage too: a microburst spreads out over less than 2.5 miles yet can produce violent winds, and a derecho is a fast-moving band of storms whose widespread damaging gusts travel more than about 250 miles.

Rule: judge lightning risk by thunder rather than rain, and judge hail and wind by how strong the storm's updrafts and downdrafts are.

A rotating funnel that has not reached the ground is a funnel cloud, and a whirl in a storm's outflow with no rotation at cloud base, a gustnado, is not a tornado. In supercells, wind that strengthens with height sets air rolling horizontally, and a strong updraft tilts that roll upright into a mesocyclone; how the tornado itself then forms is not fully understood. Landspouts, and most waterspouts, form without a supercell and are usually weaker but can still kill.

Tornadoes are rated on the Enhanced Fujita (EF) scale, from 0 to 5, by comparing their damage with indicators such as homes, mobile homes and trees. The lowest rating means an estimated gust of 65 to 85 mph and the highest one over 200 mph. The trap is treating the rating as a wind measurement. A tornado that crosses open fields and hits nothing cannot be scored, so the National Weather Service lists it as EF-U, strength unknown, however violent it looked.

US tornadoes peak from April through June, and violent ones strike outside Tornado Alley every year. On a highway, an overpass is not shelter: wind can speed up beneath it and debris still reaches you. Flying debris is the main danger, so seek the lowest floor of a sturdy building, an interior room away from windows, and padding such as a mattress or a helmet.

Rule: rotation must reach from cloud to ground, ratings describe damage rather than wind, and shelter is about getting low, inside and covered.

Most winter precipitation starts as snow aloft, and the layers below decide what lands. If the air stays at or below freezing from cloud base to the ground, it stays snow. A warm layer aloft that only partly melts the flakes, above a deep freezing layer, refreezes them into sleet, ice pellets that bounce. A warm layer that melts them completely, above a cold layer too thin to refreeze them, gives freezing rain: supercooled drops that glaze whatever they touch.

The trap is reading the surface thermometer as the answer: two towns both at 30 F can get different precipitation if only one sits under a warm layer aloft. Snow amounts mislead too. In the United States about 13 inches of snow holds one inch of water on average, but the ratio runs from about 2 to 1 in sleet to nearly 50 to 1 in very dry powder, and the familiar fixed rule of thumb is right only about a quarter of the time. Air near freezing, rimed crystals and wind that breaks flakes all push the ratio down.

Lake-effect bands form when cold, dry air crosses a large, warmer lake, and a small wind shift can move a heavy band onto the next town. A blizzard needs sustained winds or frequent gusts of 35 mph or more and visibility often cut to a quarter mile or less for at least three hours, whatever the snowfall.

Rule: read the whole temperature profile for precipitation type, and treat any snow-to-water ratio as a range set by temperature, crystal type and wind.

A tropical cyclone needs ocean water of at least 80 F through roughly the top 150 feet, moist and unstable air, low vertical wind shear and a starting disturbance, and it almost never forms within 5 degrees of the equator. Heat released as vapor condenses warms the column, lowers the surface pressure and pulls in more moist air; over land that supply is cut off and the storm weakens. A system gets a name as a tropical storm at 39 mph of sustained wind and becomes a hurricane at 74 mph.

The trap is reading the category as the danger. The Saffir-Simpson scale rates a hurricane from 1 to 5 on its peak sustained wind alone and says nothing about surge, rain or tornadoes. Storm surge, seawater the storm piles against the coast, is the leading cause of hurricane deaths. It is usually worst just right of the eye, and higher for a larger storm and over a wide, gently sloping shelf. Rain depends on a storm's speed and size, not its wind, so a weak storm that stalls can flood far inland: from 2013 to 2022, inland flooding caused about 57 percent of the 442 US deaths tied directly to tropical systems.

If the forecast cone passes south of you, remember that it maps only the center's likely track, and the center stays inside it only about 60 to 70 percent of the time. Outer-rainband tornadoes and inland flooding can reach far beyond it.

Rule: plan around water and local officials' evacuation orders, never around the category or the edge of the cone.

Rain becomes a flash flood when it falls faster than the ground can soak it up, or fills dry creeks until streams overtop their banks, usually within six hours. Storms that train, passing one after another over the same ground, and cells that back-build on the upwind side pile up rain no single storm could deliver. Steep slopes, rock and clay soils, burn scars and pavement all shed water fast; burn scars can also unleash debris flows, and overloaded storm drains turn underpasses into traps.

The trap is trusting the sky overhead. The storm feeding a flood may sit well upstream, so a creek can rise within minutes under a calm sky. Say you reach a flooded road: more than half of US flood drownings happen in vehicles driven into floodwater, about a foot of moving water can carry most cars away, and the water can hide a washed-out road. Go back the way you came.

Water from the far end of a basin arrives last, so a big river keeps rising after the rain stops, and forecasters route the flood wave from gauge to gauge to predict its crest, sometimes days ahead. Drought creeps in instead of striking: meteorological drought is dry weather, agricultural drought is crops suffering, hydrological drought is a low water supply, and each is judged against what is normal for the region.

Rule: never drive or walk into floodwater, and judge the threat by what is upstream, not by the sky above you.

An airport station reports only the spot where its sensors sit. Weather balloons, mostly launched worldwide at the same two times each day, profile temperature, humidity and pressure aloft. A geostationary satellite hangs 22,236 miles above one spot and can image the same view as often as once a minute, while polar orbiters fly lower for sharper detail but see any one place only a few times a day.

Radar times the echo of a pulse, and its beam climbs with distance until it sits about 12,000 feet up at 100 miles, overshooting low-level weather far away. Doppler velocity shows motion toward and away from the radar; strong inbound wind beside strong outbound wind marks rotation, not the tornado itself, which is why trained spotters' reports still matter.

The trap is treating a watch and a warning as two degrees of the same alert. A watch means conditions are favorable, so plan; a warning means the hazard is happening or imminent, so act. Tornado, severe thunderstorm and flash flood warnings now outline a polygon, often part of a county, rather than whole counties. Under a tornado watch, choose your shelter before any warning comes.

In aviation, marine and fire weather, flight categories rest on ceiling and visibility, a gale warning covers 34 to 47 knots, and red flag criteria for dry, windy fire weather vary by region.

Rule: know what each observation can and cannot see, and match your response to whether a product says possible or happening.

A weather model lays a grid over the globe or a region, starts from an analysis built from observations, and advances the equations of motion seconds to minutes at a time. The analysis is only a best estimate, and because the atmosphere is chaotic its small errors keep growing, even with a perfect model. An ensemble reruns the model from slightly different starting states: tight agreement signals confidence, while a wide spread means several outcomes are still in play.

The trap is misreading a percentage. A 40 percent chance of rain means a 40 percent chance that any given point in the forecast area gets at least 0.01 inch during the period, not 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.

Climate is the same kind of statistics on a longer clock: typical weather measured against 30-year normals, currently 1991 to 2020. El Nino, a weakening of the Pacific trade winds that lets the eastern Pacific warm, shifts weather worldwide and raises wind shear over the Atlantic. A town that sees a 100-year flood two years running has broken no rule: the label means a 1 percent chance in any given year, and the odds do not reset after one occurs.

Rule: read every forecast and climate number as a probability over a stated place and period, never as a schedule.