Much of what drives the weather is heat that water carries from one place to another. The Sun's warmth turns water into vapor, and when that vapor condenses back into liquid it releases heat, which fuels storms and other weather. Warm air can hold more water vapor than cold air.

The dew point is the temperature at which air is full of water vapor and the vapor starts to turn into drops, and relative humidity compares how much vapor is in the air with the most the air could hold at that temperature. At 100 percent the air is full.

Vapor that condenses is still not rain. Cloud droplets are too small to fall, so to become rain they must grow, either by colliding with other droplets or by collecting more water from the air.

A thunderstorm needs three ingredients: moisture, rising unstable air, and something to give the air a nudge upward, such as hills, mountains, or a front. Most thunderstorms have three stages. In the developing stage, rising warm moist air builds cumulus clouds; in the mature stage there is rain and lightning; in the dissipating stage downdrafts win and the storm fades, and an ordinary one takes about an hour.

As of November 2026, an estimated 16 million thunderstorms happen around the world each year, with about 2,000 going on at any given moment. Storms are sorted by structure into single-cell storms, multi-cell storms, and supercells. Multi-cell storms can line up along a front in a squall line up to 1000 km long. A supercell is a huge thunderstorm with a deep, rotating updraft, and the air rises as fast as 280 km (175 miles) per hour; the storm lasts for hours, and supercells are the least common type.

Lightning comes from the same storms. Inside a thunderstorm cloud, ice crystals and hail crash into each other, and electrons are knocked from some pieces and added to others, so 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. Graupel carries negative charge in the middle of the storm, while ice particles carry positive charge in the upper part. A cloud-to-ground flash begins 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, and this return stroke is the bright flash we see.

Most flashes in thunderstorms stay in the clouds, and lightning can also strike miles from the storm, even where the sky is clear. Lightning does strike the same place more than once, and the Empire State Building is hit about 23 times a year, as of November 2026.

Thunder is the sound of that heat. 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. Thunder rumbles on and on because each part of the long, bending lightning channel makes its own sound, and the sounds reach you at different times.

You can use the delay between flash and thunder to judge distance if you count the seconds between them and then divide by 5 to get miles, although thunder can usually be heard only about 10 miles (16 kilometers) from the flash, and if you can hear it, you are close enough to be struck.

Tornadoes grow out of the same storms. In a supercell, wind shear makes a horizontal spin, and the updraft tilts it upright. Cold downdrafts then help to concentrate the spin at lower levels until a narrow column of fast-spinning air forms, and if it reaches the ground, a tornado is born. The strongest tornadoes come from supercells, thunderstorms with a rotating circulation called a mesocyclone, though most supercells never make a tornado.

The Enhanced Fujita scale rates tornadoes from EF0 to EF5 after the storm, and its wind speeds are estimates worked out from damage to 28 kinds of structures and trees. About 75 percent of the world's tornadoes happen in the United States.

A count of tornadoes is harder than it sounds. Official tornado records in the US only go back to 1950, and spotting and reporting methods have changed a lot since then, so scientists do not know the true yearly average.

Forecasters turn this physics into a prediction by starting with the weather now. Computer models begin with current observations, including weather balloons launched around the globe twice a day, and use physics to calculate how the atmosphere will change. In ensemble forecasting 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, 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.

Words to know

• condenses: turns to liquid
• graupel: icy snow pellet
• supercell: giant rotating storm
• ensemble: group of runs