Pressure Systems — Page 50, Lesson 47

I want to walk you through the movement and terminology of pressure systems. This is a key part of understanding how weather evolves and how we forecast it. Let's start with movement. Frontal depressions — those are the low-pressure systems associated with weather fronts — tend to move rapidly. Non-frontal depressions, on the other hand, depend on their type and location. They may remain relatively static, meaning nearly stationary, or they may move at moderate speeds. Anticyclones, which are high-pressure systems, tend to be slow moving and will persist in more or less the same location for long periods. Cols — the neutral area between two highs and two lows — tend to be static, meaning they hardly move at all. The key point here is that movement of the systems is the key to accurate forecasting. If you know how fast and in what direction each type of system typically moves, you can predict where the weather will be days ahead. Now let's look at the terminology that describes how these systems change. For depressions — low-pressure systems — we have two terms. A depression will fill up or decay as pressure rises. That means if the central pressure of the low increases, the depression weakens and eventually dissipates. Conversely, a depression will deepen as pressure falls. When the central pressure drops, the low becomes more intense. Frontal depressions move rapidly, and their average lifetime is 10 to 14 days. That's a useful planning figure for a forecaster. For anticyclones — high-pressure systems — we have the opposite behaviour. An anticyclone will build up as pressure rises. That means it strengthens when the central pressure increases. An anticyclone will weaken or collapse as pressure falls. When the central pressure drops, the high weakens. Anticyclones are generally slow moving and may persist for long periods, as I mentioned earlier. Finally, cols may last up to a few days before being replaced by other pressure systems. So they are relatively short-lived features compared to anticyclones. Let me summarise the key contrasts. Depressions deepen when pressure falls and fill when pressure rises. Anticyclones build when pressure rises and weaken when pressure falls. Frontal depressions move fast and live about 10 to 14 days. Anticyclones move slowly and can last a long time. Cols are static and last only a few days. The figures in the book show the movement of weather over a period of four successive days, and Figure 4.12 illustrates the maintenance of shape as these systems move. That's the core of what you need to know about pressure system movement and terminology.