That's why it always blows from the west

Written by: Jesper Theilgaard, climate communicator and Naturkraft ambassador

Anyone in West Jutland can't avoid noticing that the trees slope towards the east. When the trees grow, they are continuously affected by the constant west wind. Well, there are other wind directions, but wind from the west is definitely the most common.

Geography books tell us that we are in the westerlies, which are by definition the area between 30 and 60 degrees north (or south in the Southern Hemisphere). There is therefore nothing unusual about the prevailing west wind and we are not the only ones to have it.
The background for the westerlies is the general distribution of air pressure – in fact, it is about how high and low the pressures are. It is these pressure differences that cause the air to move, creating pressure differences that make it blow.

Pressure gradient force and coriolis force

It happens because nature always tries to create balance. Differences in pressure are an imbalance and nature will therefore seek to offset this pressure difference. It does this by sending air from high pressure towards low pressure, and the force that makes it happen is called the pressure gradient force. The greater the pressure difference, the greater this force becomes – and the stronger the wind will blow. If it was possible to effectively offset the pressure differences, it would stop the wind from blowing, but it is still blowing, which is why one or more other forces must be involved.

The most important force in this context is the so-called coriolis force, which arises because the earth rotates around its own axis. This rotation means that any movement on the Northern Hemisphere will deflect to the right.

Interaction between the forces

In other words, we now have an interaction between two forces, namely the pressure gradient force and the coriolis force. As mentioned before, nature will try to find a balance, and this means that the two forces will have to cancel each other out. So let's first look at the general pressure redistribution created by the general circulation pattern of the atmosphere. Here it turns out that high pressure is normally around 30 degrees north and a low pressure around 60 degrees north. This means that when the pressure gradient force wants to send air from high pressure towards low pressure, air will be sent from 30 degrees north to 60 degrees north. This movement will then be exposed to the coriolis force, which deflects the north movement to the right, causing the air to move from west to east, thus creating the west wind.

Whenever there is a low-pressure front north of Denmark, the west wind will therefore always be the prevailing wind. We can also formulate a rule of thumb  – if you stand with your back to the wind, there will be an area of low pressure to your left. This rule applies in the Northern Hemisphere, while the reverse is true in the Southern Hemisphere.

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