The behaviour of atmospheric pressure systems – highs and lows – is what determines the wind blowing over the ocean, which, in turn, is what generates the waves we ultimately ride. But why do those highs and lows behave the way they do?
To begin, let’s just assume that there are already some areas of the Earth’s surface where the air pressure is higher than average, and other areas where it is lower than average. Never mind how they got there.
Want to check the charts? Go HERE.
Now, in simple terms, high pressure is where the air is pushing down hard onto the surface, and low pressure is where the air is pushing down less hard, or even rising upwards. It is as if there were an excess of air in a high pressure and a deficit of air in a low pressure. As a result, the air flows away from the high, where there is an excess, and towards the low, where there is a deficit. In more technical terms, there is a pressure gradient from the high to the low, and the air flows ‘down the pressure gradient’ to try to equalise it. The greater the pressure gradient, the faster that air will try to flow.
But the air doesn’t go in a straight line from areas of high to low pressure. It turns as it goes, and ends up circulating around those systems. The air circulates around low pressures in a cyclonic direction (hence the word cyclone). Cyclonic means anticlockwise in the northern hemisphere and clockwise in the southern hemisphere. Correspondingly, air circulates around high pressures in an anticyclonic direction (hence the word anticyclone). You guessed it, anticyclonic means clockwise in the northern hemisphere and anticlockwise in the southern hemisphere.
But why does the air circulate that way? Indeed, why does it circulate at all? To explain, we need to bring in the Coriolis force (see previous article HERE). Instead of going in a straight line from the high to the low, the Coriolis force deflects the air to the right in the northern hemisphere and to the left in the southern hemisphere.
Now, think of a cell of low pressure surrounded by high pressure in the northern hemisphere. The air is entering the low from all sides, flowing down the pressure gradient. The air flowing in towards the centre of the low is constantly turning right due to the Coriolis force, resulting in an anticlockwise circulation around the low. Likewise, think of a cell of high pressure surrounded by low pressure. The air flowing out from the centre of the high to the periphery will turn to the right due to the Coriolis force, resulting in a clockwise circulation around the high. In the southern hemisphere, the whole thing is reversed.
If you are confused, think of a low pressure like a roundabout. In Spain, where I surf in the northern winter, people drive on the right-hand side of the road. As a result, cars veer to the right as they enter the roundabout, but they end up circulating around the roundabout in an anticlockwise direction. In South Africa, where I surf in the southern winter, people drive on the left-hand side of the road. Therefore, cars veer to the left as they enter the roundabout, but they end up circulating in a clockwise direction.