Why does the wind blow?

Answer

If air pressure varies between locations, the wind blows.

Example: The air pressure is higher in an inflated balloon than outside it. If a hole is made in the balloon, the air streams out, creating a wind that blows from the greater pressure in the direction of the lower. The wind settles when the pressure is the same inside the balloon as outside.

In the atmosphere the pressure at the earth's surface reflects the weight of air above it, which in turn is determined mostly by its temperature, and as people generally know from everyday life, hot air is lighter than cold. This fits with the fact that depressions (low pressure systems) usually bring warm air.

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 100 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.g. on the TV, there is a system of equilibrium between Coriolis force and the force that pulls air in the direction of lower pressure. Such equilibrium is achieved when wind blows around low pressure systems, and not directly into them.

Translated by Paul Richardson.

Picture: Which way is the wind blowing? Photographer: Paulo Azevedo.