Pressure Systems — Page 41, Lesson 42

I want to walk you through pressure systems, starting with the two main types of depression: frontal and non-frontal. Frontal depressions are also called Polar Front Depressions. They form in temperate latitudes—that's the mid-latitude band—in both the Northern and Southern Hemispheres. The trigger is when warm, moist sub-tropical air masses meet cold polar air masses. That meeting along the polar front is what gives them their name. These depressions move from west to east. In the Northern Hemisphere, they eventually lose their identity—meaning they weaken and dissipate—over the North American or Eurasian land masses. Specifically in the North Atlantic, these depressions originate in the central to western Atlantic, move rapidly eastward, and eventually lose their identity over the steppes of central Asia. As an example, on the analysis chart on the next page, a polar front depression is centred over Greenland and Iceland. Now, non-frontal depressions are different. They are usually formed by surface heating, and when that's the cause, they are known as thermal depressions. They occur over land in summer as a result of strong surface heating. They also occur over the warm sub-tropical oceans, and in that case they are known as tropical cyclones. In winter, they occur over sea areas in cold polar or arctic air masses. The different types of depressions and their formation will be discussed in later chapters. On the analysis chart, thermal depressions—labelled TD—are seen over the Black Sea and the Mediterranean Sea, formed in the cold air coming out of central Asia. Let's move to troughs. A trough is an extension of a low pressure system. On the analysis chart, there are two troughs associated with the polar front depression centred over Greenland and Iceland. One extends north across Greenland and is a non-frontal trough. The other extends southward and is a frontal trough formed along the cold front. Troughs are very often associated with the fronts of polar front depressions. The weather associated with a trough will be similar to that of a depression. Now, depression weather. Let me go through each element. Cloud: extensive, and may extend from low altitude all the way up to the tropopause. Precipitation: may be continuous or intermittent precipitation, or showers, and the intensity can range from light to heavy, dependent on the type of depression. Visibility: poor in precipitation, otherwise good due to ascending air. Temperature: dependent on the type of depression and time of year. For example, a frontal depression coming into Europe from the Atlantic in winter will bring warmer air, but in summer will bring cooler air. Winds: winds are usually strong—the deeper the depression and the closer the isobars, the stronger the wind. Now let's look at anticyclones. An anticyclone, or high, is a region of relatively high pressure shown by more or less circular isobars, similar to a depression but with higher pressure at the centre. Isobars are more widely spaced than with depressions. There are five types of anticyclone: warm, cold, temporary cold, ridges (or wedges), and blocking. Within an anticyclone, at high altitude we have air converging, then descent of air within the anticyclone—that descent is called subsidence—and then divergence at the surface.