Area Climatology — Page 428, Lesson 391

I want to walk you through the area climatology of the North Atlantic and continental Northwest Europe, starting with how the jet streams behave in summer. The polar front jet streams in summer are still found around the 300 hPa level — that's roughly 30,000 feet — and they're positioned in relation to the polar warm and cold fronts just as they are in winter. But there are three key differences: they are less frequent, they are less strong, and they are displaced further north because the polar front itself shifts northward with the summer alignment. Meanwhile, the Subtropical Jet sits at a higher level, around 200 hPa, and it also moves north, settling into a latitude band between 40°N and 45°N. Specifically across the Atlantic in summer, that Subtropical Jet blows from Montreal to Bordeaux, as shown in Figure 22.5. Now let's talk about easterly waves. An easterly wave is similar to a shallow trough — a weak area of low pressure — that extends northward from the equatorial low pressure belt. These waves move slowly from east to west, and they do so under the influence of the anticyclonic wind — that's the clockwise flow around the subtropical high pressure. In the North Atlantic during autumn, West African tornadoes form over Nigeria and drift westwards with these easterly waves. Those tornadoes can become seedlings — the early seeds — of Caribbean hurricanes. That brings us to hurricanes. The term "hurricane" is the name given to tropical revolving storms in the Caribbean and Gulf of Mexico area. Developed hurricanes occur with an average frequency of 3 per year. They happen from August to October, tracking westwards across the Atlantic near a latitude band of 10°N to 15°N, moving at a speed of 10 to 15 knots. Inside the storm, wind speeds can exceed 100 knots. After crossing the Atlantic, they either cross the Bay of Mexico or turn right around the subtropical high pressure, tracking northwest, then north, then northeast up the USA East Coast. Hurricanes are energized by the latent heat of condensation — the heat released when water vapour condenses into cloud droplets — which is why they are more active over the sea. Each season, hurricanes are named alphabetically in order of occurrence, using alternate male and female first names. Next, let's look at tropopause heights. The tropopause is the boundary between the troposphere and the stratosphere. At 70°N latitude, the tropopause sits at about 30,000 feet. At 20°N latitude, it's much higher — around 50,000 feet. Now freezing level height. In January, at 70°N, the freezing level is at the surface — meaning the air at ground level is at or below freezing. At 20°N in January, it's at 12,000 feet. In July, at 70°N, the freezing level rises to 5,000 feet, and at 20°N it goes up to 16,000 feet. Let's move to continental Northwest Europe and its geographical considerations. The mountains of Norway lie to the north, while to the south there are many mountain ranges dominated by the Alps. Between these two regions lies the North European Plain — a flat area with no mountain barrier against the Atlantic winds from the west, nor against the cold winter winds from the east. In winter, polar front depressions move from the Atlantic towards Russia. They travel principally between the mountain barriers to the north and south, although their tracks are variable. Areas to the south of each low will experience frontal weather — the weather associated with the warm and cold fronts. The Alps often block and delay cold fronts, causing frontal and orographical cloud — cloud formed when air is forced to rise over the mountains — to persist on the northern side of the Alps. An active secondary depression — a new low-pressure centre — may develop on such a front, tending to form and intensify there.