Other Depressions — Page 351, Lesson 320

I want to walk you through a few more types of depressions that don't fit the classic polar-front model. We'll start with what happens when a cold front meets a mountain range. Sometimes a cold front will approach a mountain range, and much of the cold air is initially held back by the range itself. When that unstable air finally breaks over the mountains, lifting occurs — and you get additional lifting from what's called the orographic low. The result can be heavy banks of Cumulonimbus clouds, with line squalls, very heavy showers, thunderstorms, hail, and poor visibility. A good example of this happens over the Alps in northern Italy in winter, where the cold front is part of a polar front depression. Now let's move to thermal depressions. The basic theory is straightforward. As the air at the surface is heated, it expands. That expansion causes the pressure surface to be lifted — so at altitude, you get higher pressure. That higher pressure at height results in an outflow of air. In turn, that outflow causes a fall in surface pressure, and the air then moves cyclonically — that is, it rotates in the same sense as a low-pressure system in your hemisphere. A key point: the thermal depression often weakens with height because pressure tends to be higher aloft. This can cause upper winds to reverse direction. However, if the thermal depression develops in unstable air, it can be active all the way up to tropopause heights. The weather associated with thermal depressions includes Cumulus and Cumulonimbus clouds — possibly with hail and thunderstorms — heavy showers, good visibility except inside the showers themselves, and moderate or severe turbulence. Next, the monsoon low. Over large continents in summer, a large thermal low develops that controls the circulation of air. The weather pattern is variable — it's affected by topography, for example the Himalayas, and by the air masses that get drawn into the circulation. Then we have polar air depressions. These are thermal lows that form when Arctic Maritime air is subject to lifting on a large scale. This usually happens when the mAc air — that's maritime Arctic air — moves south over a warmer sea in winter. It gives Cumulus, Cumulonimbus, heavy showers, and sometimes secondary cold fronts develop. Don't confuse these with polar front depressions. Polar front depressions are at the joining point of the Tropical Maritime and Polar air masses; polar air depressions are a different beast. We also see thermal lows develop over inland waters. In winter, thermal lows develop over the Caspian Sea, the Black Sea, and the Mediterranean Sea. A cold outflow of cPc air — continental Polar cold air — from the Siberian high flows over those warmer seas. Convection and the development of depressions result. Similar lows develop over the Great Lakes of North America. Finally, thermal lows over land in summer. During summer, shallow lows will appear over land due to surface heating. If the air is already unstable, or if there are old frontal zones in the area, thunderstorms, widespread rain, or squalls may result. These lows occur regularly in summer over the American mid-west, giving heavy thunderstorms.