Why air blows

The wind blows because of differences in air pressure. Wind that is caused by a difference in pressure spanning a large area more than about km does not flow directly from the area of high pressure to the depression as in the example of the balloon.

Instead, the wind blows anti-clockwise around the low pressure area in the Northern Hemisphere and clockwise in the Southern Hemisphere. This is the effect of the earth's rotation, which produces a force, called Coriolis, that deflects the wind from its path. The Coriolis force deflects air to the right in the northern hemisphere and to the left in the southern hemisphere.

Around the high and low pressure systems one can clearly see on weather charts, e. If the Earth did not rotate, there would be one convection cell in the northern hemisphere and one in the southern with the rising air at the equator and the sinking air at each pole.

But because the planet does rotate, the situation is more complicated. Air rises at the equator, but as it moves toward the pole at the top of the troposphere, it deflects to the right.

Remember that it just appears to deflect to the right because the ground beneath it moves. This air is cool because it has come from higher latitudes. Both batches of air descend, creating a high pressure zone.

Once on the ground, the air returns to the equator. There are two more convection cells in the Northern Hemisphere. This cell shares its southern, descending side with the Hadley cell to its south. There are three mirror image circulation cells in the Southern Hemisphere. In that hemisphere, the Coriolis Effect makes objects appear to deflect to the left.

Global winds blow in belts encircling the planet. The global wind belts are enormous and the winds are relatively steady Figure below. These winds are the result of air movement at the bottom of the major atmospheric circulation cells, where the air moves horizontally from high to low pressure.

The wind belts are named for the directions from which the winds come. The westerly winds, for example, blow from west to east. These names hold for the winds in the wind belts of the Southern Hemisphere as well.

Besides their effect on the global wind belts, the high and low pressure areas created by the six atmospheric circulation cells determine in a general way the amount of precipitation a region receives. In low pressure regions, where air is rising, rain is common. In high pressure areas, the sinking air causes evaporation and the region is usually dry. More specific climate effects will be described in the chapter about climate.

The polar front is the junction between the Ferrell and Polar cells. At this low pressure zone, relatively warm, moist air of the Ferrell Cell runs into relatively cold, dry air of the Polar cell. The weather where these two meet is extremely variable, typical of much of North America and Europe. The polar jet stream is found high up in the atmosphere where the two cells come together. A jet stream is a fast-flowing river of air at the boundary between the troposphere and the stratosphere.

Jet streams form where there is a large temperature difference between two air masses. A cross section of the atmosphere with major circulation cells and jet streams. The polar jet stream is the site of extremely turbulent weather. Jet streams move seasonally just as the angle of the Sun in the sky moves north and south. The doldrums is an area of calm weather.

The trade winds coming from the south and the north meet near the equator. These converging trade winds produce general upward winds as they are heated, so there are no steady surface winds.

What are the prevailing westerlies? Between thirty and sixty degrees latitude, the winds that move toward the poles appear to curve to the east. Because winds are named from the direction in which they originate, these winds are called prevailing westerlies. Prevailing westerlies in the Northern Hemisphere are responsible for many of the weather movements across the United States and Canada. What are the polar easterlies?

At about sixty degrees latitude in both hemispheres, the prevailing westerlies join with the polar easterlies to reduce upward motion. The polar easterlies form when the atmosphere over the poles cools. This cool air then sinks and spreads over the surface. As the air flows away from the poles, it is turned to the west by the Coriolis effect. Again, because these winds begin in the east, they are called easterlies. What is a sea breeze? On a warm summer day along the coast, this differential heating of land and sea leads to the development of local winds called sea breezes.

As air above the land surface is heated by radiation from the Sun, it expands and begins to rise, being lighter than the surrounding air. To replace the rising air, cooler air is drawn in from above the surface of the sea.

This is the sea breeze, and can offer a pleasant cooling influence on hot summer afternoons. What is a land breeze? A land breeze occurs at night when the land cools faster than the sea. In this case, it is air above the warmer surface water that is heated and rises, pulling in air from the cooler land surface. How is wind helpful to Earth? Wind is the fastest growing source of electricity in the world. It's often one of the least expensive forms of renewable power available.

Some experts say it can sometimes be the cheapest form of any kind of power. Generating power from the wind leaves no dangerous waste products behind. Best of all, its supply is unlimited. How do windmills work? Air pressure depends on the temperature of the air and the density of the air molecules. Atmospheric scientists use math equations to describe how pressure, temperature, density, and volume are related to each other.

They call these equations the Ideal Gas Law. In these equations, temperature is measured in Kelvin. This equation helps us explain how weather works, such as what happens in the atmosphere to create warm and cold fronts and storms, such as thunderstorms.

For example, if air pressure increases, the temperature must increase. If air pressure decreases, the temperature decreases. It also explains why air gets colder at higher altitudes, where pressure is lower. Skip to main content. How Weather Works.