Pressure Systems — Page 41, Lesson 40

I want to walk you through pressure systems. This is a core part of meteorology for pilots because the patterns of pressure on a weather chart tell us what weather to expect and how the wind will behave. Let's start with the basic building block: isobars. These are lines on a weather map that connect points of equal atmospheric pressure. When you look at a chart, these isobars form recognisable patterns, and we call those patterns pressure distribution systems. The main types we need to know are depressions (also called lows), anticyclones (also called highs), troughs, ridges, and cols. We'll cover each one as we go. Now, there's a very practical rule that links wind direction to pressure patterns, and it's called Buys Ballot's Law. It was developed in the 19th century by a Dutch scientist and meteorologist named Buys Ballot. The law states that if you, as an observer, stand with your back to the wind in the Northern Hemisphere, then the low pressure is on your left. In the Southern Hemisphere, it's the opposite — low pressure is on your right. This is a really useful tool for us, not just in studying wind, but also in altimetry, which is the measurement of altitude using pressure. Next, I want to introduce the term advection. In meteorology, advection simply means the horizontal movement of air. So when we talk about air moving sideways from one place to another, that's advection. Let's focus on depressions. A depression is a region of comparatively low pressure. On a weather chart, it's shown by more or less circular and concentric isobars surrounding a centre, and the pressure is lowest right at that centre. A depression is sometimes called a low or a cyclone. Now, in Europe, the term 'cyclone' is usually reserved for tropical revolving storms, which are intense, small-scale systems. But the term cyclonic circulation is used more broadly to imply any low pressure system. Applying Buys Ballot's Law to a depression tells us something important about the wind direction. In the Northern Hemisphere, the wind will move around a low pressure system in an anti-clockwise direction. So if you picture a low on a chart, the wind is spiralling inwards and turning anti-clockwise around it. There are two types of depression. First, frontal depressions, which are large-scale systems found in our temperate latitudes — these are the ones that bring most of our changing weather. Second, non-frontal depressions, which are small-scale and can occur virtually anywhere. Now, let's look at how the air actually moves inside a depression. At the surface, air is converging, meaning it's flowing inward toward the centre. Because it can't pile up, that air then rises from the surface up to medium and high altitude — that rising motion is called convection. Then, at medium to high altitude, the air diverges, meaning it spreads out and flows away from the centre. So the vertical structure is: convergence at the surface, rising air in the middle, and divergence aloft. This is a key concept for understanding why depressions produce clouds and precipitation — the rising air cools and condenses. Those figures show you exactly what I've described — the anti-clockwise circulation around a Northern Hemisphere depression and the vertical cross-section showing the convergence, rising air, and divergence.