Pressure Systems — Page 41, Lesson 44

Let’s start with the two main types of anticyclone, because the book introduces them by contrast. First, cold anticyclones. These occur as permanent features at the poles and as seasonal features over continental land masses in winter. In simple terms, the air at the surface is cooled, which increases its density. That denser air draws more air down from above, and that process increases the surface pressure. So a cold anticyclone is essentially a high-pressure system built by surface cooling and the resulting subsidence of air from aloft. Now, warm anticyclones — to understand their formation, we need to look at the global circulation of air. In the 19th century, a British scientist named George Hadley proposed a global circulation model. He suggested that hot air rises at the Equator, then flows toward the poles at the tropopause, descends at the poles, and flows back to the Equator at the surface. That’s the Hadley cell in its original form. But this model wasn't quite correct, because in our temperate latitudes, pressure is predominantly low due to large-scale frontal depressions. So an American scientist, William Ferrel, proposed a modification to Hadley’s model, introducing what we now call the Ferrel cell — a mid-latitude circulation that helps explain the warm anticyclones we find in subtropical regions. The book shows this in Figure 4.6, which illustrates the Hadley cell, the polar front, and the associated wind flows. So the key takeaway: cold anticyclones are thermally driven by surface cooling; warm anticyclones are dynamically driven by the global circulation, specifically the descending branch of the Hadley cell in the subtropics.