Pressure and wind

In Step 1.10 you explored why cloud and rain are associated with a depression. But why are they low pressure systems, and how do we get wind? First of all let’s consider atmospheric pressure. This is a measure of how much air is in the column above your head (see Figure 1) and is typically around 1000hPa (or millibars), equivalent to about 10m of water.

Figure 1: Air pressure is a measure of how much air is above your head © Dr Peter Inness

Figure 2: Near the centre of a depression, more air is spreading out at the top than is coming in at the bottom. © Dr Peter Inness

Figure 3: If you have two containers, one with more water in and another with less, and you remove the barrier between them, then the water quickly flows to even out the level.

Exactly the same happens in the atmosphere; if there is more air in one place (high pressure), and less elsewhere (low pressure) then the air moves from high to low. The stronger the pressure gradient the bigger the difference between high and low pressure; or the closer the pressure contours are to each other, the faster the wind. The absolute value of pressure in the centre of the depression is less important than the difference in pressure between the centre and the edge of the system. However, we live on a rotating, spherical planet. So we also need to take the Coriolis force, created by the rotation of the Earth, into account. Let’s take a closer look at this in the next Step.