In a couple of my previous articles we have already looked at the Coriolis force and how it makes low and high pressure systems rotate the way they do (here). But where do low pressures come from in the first place, and how do they develop? Well, just like most things regarding the Earth’s dynamic systems, it is very complicated and still not fully understood by scientists. So, here is just a brief outline.
The pressure in the atmosphere is not uniform all over the planet; it contains semi-permanent areas of higher and lower pressure. These are related to the general large-scale circulation patterns, in turn driven by the uneven heating of the poles and the equator by the Sun. The low pressures that form over the ocean (and produce our surf) are initially derived from these large-scale circulation patterns. But they also need a small, localised perturbation – an initial ‘spark’ – before they can develop into bigger and stronger vortices of circulating air.
In the early twentieth century a group of meteorologists from Bergen in Norway came up with the concept of the polar front – a band on the Earth’s surface where cold air from the poles meets warm air from the equator. It is on the polar front that the initial disturbance takes place that eventually develops into a low pressure system.
The polar front is the boundary between two circulation cells where the surface air is flowing in opposite directions. At the polar front in the northern hemisphere, cold air from the north meets warm air from the south. Because the warm air is less dense it tends to slide over the top of the cold air; so the warm air is constantly being forced upwards.
Now, through a particular combination of circumstances, a perturbation may appear at some point along the front. For example, the north-south air temperature difference may be particularly intense at this point, or there might be some influence from an external factor like the sea surface temperature.
Such a disturbance is known to meteorologists as baroclinic instability, meaning that there is a change in pressure – often closely linked to a change in temperature – over a short distance. As a result, the atmosphere becomes locally unstable.
Around the area of the perturbation, the warm air sliding over the cold air does so with more intensity than along the rest of the polar front. This results in a sharp drop in pressure at the surface over a small area, which leads to surface air being sucked in from outside the area of the perturbation.
And, as I explained in a previous article (here), the air travelling in towards the cell of low pressure is then deflected by the Coriolis force, and begins to circulate around the centre of the disturbance.
Now, for the system to keep growing there has to be something that makes this process persist, some sort of feedback loop that makes the system self-perpetuating. If the initial circulation becomes broad enough and strong enough, it will start drawing in additional cold air from the north (in the northern hemisphere) and warm air from the south.
This extra air being sucked in then increases the temperature contrast in the area of the disturbance, which makes the warm air rise more vigorously, which, in turn, increases the circulation, and so on and so forth. This feedback loop will continue to make the system grow until it becomes a fully-developed low pressure.
The fully-developed system contains a warm front and a cold front, which are remnants of the original section of polar front around the initial disturbance, and which mark the boundary between warm and cold air.
More about this in my book Surf Science: an Introduction to Waves for Surfing.