Area Climatology — Page 428, Lesson 393

I want to walk you through the area climatology of a specific region — this is the part of the book that deals with the typical weather patterns you’d encounter as a pilot flying over Europe and the surrounding seas. Let’s start with thermal depressions. A thermal depression, or thermal low, is a low-pressure area caused by strong surface heating. In this region, thermal lows can form in winter to the east of the Alps, over the low-lying Danube area. That area is moist and comparatively warm compared to the surrounding land. The cyclonic circulation — that’s the counterclockwise flow around the low — on the east side of this depression will bring warm air north from the Mediterranean. That warm air forms active warm fronts. These warm fronts can bring extensive low stratus cloud to Germany, and they can bring snowfall as far north as southeast England. Next, we have polar air depressions. These can sometimes affect the extreme northwest sea areas of the region in winter. On the chart, they’re labelled as PL — that stands for polar low — and you can see them in Figure 22.11. Then there’s the Siberian High extension. The Siberian High is a large, cold, high-pressure system that sits over Siberia in winter. When it extends westward, it brings Pc air — that’s polar continental air — which gives cold, dry weather. As that cold air moves over warmer water near the German and Dutch coasts, it can produce steaming fog or low stratus cloud. That happens because the cold air reacts with the warmer water, causing condensation. We also have temporary highs. These are ridges or transient anticyclones — short-lived areas of high pressure — that may exist in the north or northwest of the region, in between travelling polar front lows. Now let’s look at cloud and precipitation. On average, cloud amount exceeds six octas — that means more than six-eighths of the sky is covered. The cloud is frontal, coming from the many polar front depressions, and also from warm fronts moving north from the Mediterranean. However, cloud amounts decrease from west to east. There is much precipitation, and in the east it’s mainly snow. For visibility: radiation fog can form inland when there’s a slack pressure gradient — that means very little wind — and it happens principally in autumn and winter. With a southwest warm, moist wind from the Atlantic, advection fog can form over previously cold, soaked inland areas. Advection fog forms when warm, moist air moves over a colder surface. Smoke haze may reduce visibility to the lee of industrial areas — that means downwind of them. Frontal fog can occur on the warm fronts of deep, active polar front depressions. Finally, winds. Surface winds are generally westerly, although they can be easterly on the north side of depressions. East or northeast winds can occur as an outflow from the Siberian High. Upper winds become increasingly westerly with ascent, due to the increasing westerly thermal component — that’s the wind added by the temperature gradient from equator to pole. Polar front jets are common. They’re located in relation to the moving warm and cold front surface positions, and they’re centred around 300 hPa, which is about 30,000 feet. These jets often exceed 100 knots. The subtropical jet is to the south, near Morocco, and therefore does not affect this region.