We’ve explained the very basics of global weather and why we have cold air moving south and warm air moving north (In the Northern Hemisphere). We call the areas where air of different temperatures collides ‘fronts’. Everything else being equal the warm air rises over the cold air. However what normally happens is somewhere along this front things become unstable. Maybe it’s a small difference in local temperatures meaning the flow of air is slightly different, or simply the chaotic motion of all that air moving when the flow is particularly strong. Either way as the convection cycle runs and warm air rises surface pressure will drop. As surface pressure drops surrounding air will start to rush in, the disturbance will grow. In addition the Coriolis effect we’ve already discussed will mean that this air rushing in won’t do it in a straight line, it’ll start to spin. The faster it goes the faster it grows and pretty quickly we’ve got the swirling low pressure system we’ve seen in satellite photos. This rotating area of high air flow is a storm, that rotating air is the high wind we associate with storms.
With our explanation of atmospheric circulation we explained the division of the atmosphere into ‘cells’ of circulating air. Understanding that storms are generated on the boundaries of these cells by warm air meeting cold air we can start to understand why this sort of storm generally occurs only in particular regions on earth. These ‘mid latitude’ bands (see diagram below) are where most of the storm systems affecting global weather are formed, with the exception of Tropical Storms. It’s these systems that drive the swell we surf.