Wind begins with a difference in pressure. Air has weight, and wind blows from places with high air pressure toward places with low air pressure. Winds blow toward the center of a low-pressure system, and the air rises there; winds blow away from a high-pressure system, and 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.

Some winds blow steadily in the same direction around the planet: the trade winds, the westerlies, and the polar easterlies. Because of Earth's spin they bend to the right in the Northern Hemisphere and to the left in the Southern. This turning is called the Coriolis effect, and because of it global air circulation splits into three cells north of the equator and three south.

Where different kinds of air meet, the weather changes. An air mass takes on the temperature and wetness of the place where it forms, so one may be maritime tropical, warm and over the ocean, or continental polar, cold and over land far 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 moves more slowly, with high, thin cirrus clouds ahead of it and then lower clouds and rain. Fronts travel across Earth's surface over several days, steered by high winds such as the jet streams.

A tropical cyclone is a rotating low-pressure system with organized thunderstorms but no fronts. A hurricane grows over warm ocean water when there are low winds outside the storm and lots of moisture in the air, and it weakens when it moves over land or into cooler water. The Coriolis effect makes hurricanes spin counterclockwise in the Northern Hemisphere and clockwise in the Southern. The eye is a small calm center, and around it is the eyewall, a ring of thick clouds with the strongest winds of the storm.

The Saffir-Simpson scale rates hurricanes from 1 to 5, using only the storm's top sustained wind speed. A Category 1 hurricane has winds of 74 to 95 mph, and a Category 5 has winds of 157 mph or higher. Category 3 and up are called major hurricanes.

Storm surge is a rise in sea level pushed toward the coast by a storm's winds, and it causes much of the flooding along a coast. As a hurricane nears a coast, the sea can rise as much as 20 to 30 feet (6 to 9 meters).

Scientists watch hurricanes with weather satellites and run computer models to predict a storm's likely path. The forecasts let warnings go out to places the storm may hit. A forecast is 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.

Better warnings do not mean a perfect forecast. 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

• Coriolis: spin effect
• eyewall: storm wind ring
• cyclone: rotating storm
• cumulonimbus: storm cloud