The first public weather forecast was written on 31 July 1861 by Vice-Admiral Robert FitzRoy, who had captained HMS Beagle and had founded Britain's Met Office in 1854. It predicted weather, the short-term condition of the atmosphere, which can change from minute to minute. That differs from climate, the average of the weather over time in a place. Today forecasters collect millions of observations each day from satellites, land stations, ocean sensors, instruments high in the air, radar, and thunderstorm locators. A forecast is still made in three stages: find out what the weather is doing now, feed the data into supercomputer models, and have experts check and refine the result. The models begin with current observations, including weather balloons launched around the globe twice a day, and then use physics to calculate how the atmosphere will change.
Almost all weather happens in the troposphere, the lowest layer of the atmosphere, which holds about 75 to 80 percent of the air's mass and nearly all of its clouds, and at Earth's surface the air above presses down with about 14.7 pounds on every square inch, while higher up there is less air overhead and so the pressure is lower.
Air takes on the temperature and wetness of the place where it forms, and the large bodies of air that result are named for their origins, such as maritime tropical, which is warm and forms over the ocean, or continental polar, which is cold and forms over land far to the north. Cold and warm air masses meet most often in the middle latitudes, such as the United States, where they form fronts and can make big storms. At a cold front, dense cold air pushes under lighter warm air and lifts it, which makes cumulus or cumulonimbus clouds and thunderstorms, and cold fronts move up to twice as fast as warm fronts. A warm front is slower, and high, thin cirrus clouds show up ahead of it before the clouds lower and rain becomes likely. Fronts travel across Earth's surface over several days, often steered by high winds such as the jet streams, and mountains can change a front's path.
The air does not simply drift where it likes, because winds blow toward the center of a low-pressure system, where the air rises, and away from a high-pressure system, where air sinks from above to fill the space. Because Earth spins, the winds around a low swirl counterclockwise north of the equator and clockwise south of it, and a high swirls the opposite way.
The same turning shapes the great steady winds, the trade winds, the westerlies, and the polar easterlies, which blow in the same direction around the planet and bend to the right in the Northern Hemisphere and to the left in the Southern. This is the Coriolis effect, the way Earth's spin turns moving air, and because of it global air circulation splits into three cells north of the equator and three south.
Rain begins with a puzzle, because cloud droplets are too small to fall, and to become rain they must grow, either by colliding with other droplets or by collecting more water from the air. Water vapor also gives back what it took: when it condenses back into liquid it releases heat, which fuels storms and other weather. Air must be lifted for clouds to form, and it is lifted in several ways: sunshine heating the ground, wind pushed up a mountain or slope, air rising at a low-pressure center, and air forced up at weather fronts.
A thunderstorm needs three ingredients, which are moisture, rising unstable air, and something to give the air a nudge upward, such as hills, mountains, or a front. Most thunderstorms pass through three stages: in the developing stage, rising warm moist air builds cumulus clouds, in the mature stage there is rain and lightning, and in the dissipating stage downdrafts win and the storm fades, so that an ordinary one takes about an hour.
Storms are sorted by structure, and the most dramatic type is the supercell, a huge thunderstorm with a deep, rotating updraft, in which the air rises as fast as 280 km (175 miles) per hour and the storm lasts for hours. Supercells are the least common type. A storm is called severe when it can make hail an inch across or bigger, wind gusts over about 58 mph, or a tornado.
Inside a thunderstorm cloud, ice crystals and hail crash into each other, and electrons are knocked from some pieces and added to others, so that the top of the cloud becomes positively charged and the base negatively charged. Scientists think the main charge regions come from graupel, small hail particles, bumping into smaller ice particles, with graupel carrying negative charge in the middle of the storm and the ice particles carrying positive charge in the upper part. That is the accepted outline, but there is a gap in it, because we know what conditions make lightning, and yet scientists still debate exactly how a cloud builds up its electric charge and how lightning begins.
Once the charge is built, a cloud-to-ground flash starts when a path of negative charge called a stepped leader reaches down toward the ground; positive charge flows up to meet it, and a strong current surges back up into the cloud. This return stroke is the bright flash that we see, and most flashes in thunderstorms never reach the ground at all, because they stay in the clouds.
Lightning heats the air in its channel to over 50,000 degrees Fahrenheit in a few millionths of a second, and the air has no time to expand, so its pressure spikes and it blasts outward as a shock wave; within about ten yards the shock wave becomes an ordinary sound wave, which we hear as thunder, and the rumble goes on because each part of the long, bending channel makes its own sound, and those sounds reach you at different times.
The United States gets about 25 million lightning flashes in a year, and lightning does strike the same place more than once, since the Empire State Building is hit about 23 times a year, as of November 2026. Height, pointy shape, and being alone decide where lightning strikes, and metal does not attract it, though metal can carry it. Forecasters can say when lightning is likely to be intense, but it is impossible to forecast individual strikes, because lightning is so widespread, frequent, and random. The safe response is therefore a habit and not a prediction: run to a substantial building or a hard-topped vehicle, do not crouch outdoors, and do not shelter under a tree.
A tornado begins in a supercell, where wind shear makes a horizontal spin, the updraft tilts it upright, and cold downdrafts help to concentrate the spin at lower levels until a narrow column of fast-spinning air forms; if it reaches the ground, a tornado is born. The strongest tornadoes come from supercells with a rotating circulation called a mesocyclone, yet most supercells never make a tornado.
Tornadoes are rated by the Enhanced Fujita scale from EF0 to EF5 after the storm, and the wind speeds are estimates, not measurements, worked out from damage to 28 kinds of structures and trees. Over two-thirds of all tornadoes are weak, rated EF0 or EF1. About 75 percent of the world's tornadoes happen in the United States.
A tropical cyclone is a rotating low-pressure system with organized thunderstorms but no fronts, and it grows over warm ocean water when there are low winds outside the storm and lots of moisture in the air; it weakens when it moves over land or into cooler water. Because of the Coriolis effect, hurricanes spin counterclockwise in the Northern Hemisphere and clockwise in the Southern. The Saffir-Simpson scale rates hurricanes from 1 to 5 using only the top sustained wind speed, so a Category 1 hurricane has winds of 74 to 95 mph, a Category 5 has winds of 157 mph or higher, and Category 3 and up are called major hurricanes.
Research at the National Severe Storms Laboratory led to the national weather radar network, which took forecasters from no warning for threatening weather to about a 15-minute warning lead time. Radar cannot see every tornado, so people still matter, and volunteer storm spotters in the SKYWARN network watch for tornadoes and report to the National Weather Service. Forecasters measure their own uncertainty by ensemble forecasting, in which a supercomputer runs several models at once, or one model many times from slightly different starts. If the runs agree, the forecast is trusted more, and if they differ, forecasters know the weather is hard to predict. Even so, scientists expect a limit: because the atmosphere is chaotic, it will probably always be impossible to predict the weather more than about two weeks ahead, even though forecasts keep getting better. So the forecast stays unfinished. How many days past that wall could better models and more observations ever push it?
Words to know
- troposphere: lowest air layer
- supercell: giant rotating storm
- graupel: icy snow pellet
- ensemble: group of runs
- chaotic: hard to predict
