Pressure Systems — Page 50, Lesson 45

I want to walk you through the general circulation of the atmosphere and how it creates the pressure systems we see on weather charts. We've already talked about the basic idea of pressure systems, but now we need to understand the big picture — the three-cell circulation model. Here's how it works. The circulation of the atmosphere is modified because of the low pressure systems that form in temperate latitudes. This modification gives us three distinct circulation cells. The first is the Hadley cell, which runs between the Equator and the subtropics. The second is the Ferrel cell, which sits between the subtropics and temperate latitudes. The third is the Polar cell, which operates between temperate latitudes and the poles. Now, what does this circulation actually do? At the tropopause — that's the boundary between the troposphere and the stratosphere — we have air flowing outwards from the Equator towards the poles, and also from temperate latitudes towards the Equator. This creates an excess of air at the tropopause in subtropical regions. Because there's too much air piled up there, it is forced to descend. That descending air creates the subtropical high pressure systems, which are permanent features over the subtropical oceans. A classic example is the Azores high in the North Atlantic. Let's move on to a specific pressure system feature called a ridge. A ridge of high pressure is indicated by isobars that extend outwards from an anticyclone. And here's a key point: ridges are always rounded, never V-shaped. If you see a V-shaped pattern in the isobars, that's a trough, not a ridge. You can see this clearly in Figure 4.7. Next, we have temporary cold anticyclones. A temporary cold anticyclone is a ridge of high pressure that is found in the cold air between two frontal depressions. Because the depressions themselves are moving rapidly, the influence of these anticyclones will be experienced for up to a maximum of about 24 hours. So they are short-lived features. Figure 4.8 illustrates this. Now, a very important type is the blocking anticyclone. A blocking anticyclone is one that prevents the usual eastward movement of frontal depressions. Instead of moving east as they normally would, these depressions are forced to take up northerly tracks in the Northern Hemisphere. Blocking anticyclones are usually extensions of the warm subtropical anticyclones. They can persist for weeks. In summer, they give usually warm, dry weather. In winter, they produce gloomy, overcast conditions with a possibility of drizzle. However, over Europe in winter, they may be extensions of the Siberian high, which gives usually cold, clear conditions. Figure 4.9 shows an example of a high extending from the Azores to Scandinavia. Let's talk about anticyclonic weather in general, and I want to break this down by season and type. In summer, and also for cold anticyclones in winter, here's what you get. Cloud: none, except on the edge of the anticyclone. Precipitation: none. Visibility: generally moderate, with haze. Temperature: dependent on the type of anticyclone. Winds: light. In winter, specifically for warm anticyclones, the weather is quite different. Cloud: extensive stratus with a low base and limited vertical extent. Precipitation: possibly drizzle. Visibility: generally moderate to poor, with mist and fog likely. Temperature: relatively warm. Winds: still light. Finally, let's look at cols. A col is a region of almost level pressure between two highs and two lows. It is an area of stagnation, as you can see in Figures 4.10 and 4.11. Col weather is normally settled, but it is dependent on changing pressure. In autumn and winter, cols produce poor visibility and fog. In summer, thunderstorms are common. Figure 4.11 gives an example of a weather forecast for a day when a col influenced the weather over the UK.