Why Does the Winds Blow?

To understand what makes the wind blow, we first need to understand what atmospheric pressure is. Pressure at the earth's surface is a measure of the 'weight' of air pressing down on it. The greater the mass of air above us, the higher the pressure we feel, and vice-versa. The importance of this is that air at the surface will want to move from high to low pressure to equalise the difference, which is what we know as wind.

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.

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.

So wind is caused by differences in atmospheric pressure - but why do we get these differences? It's down to the rising and sinking of air in the atmosphere. Where air is rising we see lower pressure at the earth's surface, and where it's sinking we see higher pressure. In fact if it weren't for this rising and sinking motion in the atmosphere then not only would we have no wind, but we'd also have no weather.

This rising and sinking of air in the atmosphere takes place both on a global scale and a local scale. One of the simplest examples of a local wind is the sea breeze. On sunny days during the summer the sun's rays heat the ground up quickly. By contrast, the sea surface has a greater capacity to absorb the sun's rays and is more difficult to warm up - this leads to a temperature contrast between the warm land and the cooler sea.

As the land heats up, it warms the air above it. The warmer air becomes less dense than surrounding cooler air and begins to rise, like bubbles in a pan of boiling water. The rising air leads to lower pressure over the land. The air over the sea remains cooler and denser, so pressure is higher than inland. So we now have a pressure difference set up, and air moves inland from the sea to try and equalise this difference - this is our sea breeze. It explains why beaches are often much cooler than inland areas on a hot, sunny day.

A similar process takes place on a global scale. The sun's rays reach the earth's surface in polar regions at a much more slanted angle than at equatorial regions. This sets up a temperature difference between the hot equator and cold poles. So the heated air rises at the equator (leading to low pressure) whilst the cold air sinks above the poles (leading to high pressure). This pressure difference sets up a global wind circulation as the cold polar air tries to move southwards to replace the rising tropical air. However, this is complicated by the earth's rotation (known as the coriolis effect).

This is primarily because the sun heats the earth's surface unevenly. As  heat is transferred to the air, we get regions of warm and cool air  which can turn into regions of low and high pressure. This difference in  pressure makes a force that causes the wind to blow.

Posted by: Lusubilo A. Mwaijengo

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