Global Climatology — Page 387, Lesson 360

I want to walk you through a new topic: global climatology, focusing on regional wind patterns and wave phenomena that you’ll need to understand as a professional pilot. Let’s start with high-level winds over India. When the southwest monsoon begins, from May to June, there are large changes in the wind pattern at the 200 hPa level — that’s about 12,000 metres or 39,000 feet, a typical cruising altitude for jet aircraft. The axis of the westerly subtropical jet stream moves north of the Himalayas. As a result, the high-level winds across India become easterly. So during the monsoon, instead of the usual westerly jet, you get easterly winds at high altitude over the Indian subcontinent. Now let’s look at the eastern Mediterranean in winter. Here, subtropical jet stream winds are particularly strong at the 200 hPa level. They normally occur in the area around Cyprus and the Egyptian coast. These are westerly winds, and they frequently exceed 100 knots. In fact, 225 knots has been recorded. That’s a very strong jet stream — something you’d need to account for in flight planning, especially for fuel and time calculations on east-west routes across the Mediterranean. Next, polar winds. Near the poles in winter, there are strong westerlies. The reason is that the polar tropopause temperature is lower than the temperature at temperate latitudes. Remember the wind rule for the Northern Hemisphere: low temperature on the left. So with cold air at the pole on your left as you face downwind, you get westerly flow. In summer, the polar tropopause temperature rises and actually exceeds the temperature at temperate latitudes. When that happens, the westerlies reduce in strength and become easterlies. So the polar upper winds reverse direction between winter and summer. Now let’s move to waves — specifically easterly waves and westerly waves. An easterly wave is a wave or trough of low pressure. It originates over West Africa, between latitude 5° North and 20° North, and moves westward towards the Caribbean. Some of these waves continue beyond the Caribbean and into the Pacific. They occur during summer and autumn, and there are usually about 50 of them each year. The weather produced by an easterly wave is similar to that associated with tropical revolving storms — what we call hurricanes or typhoons — but to a much lesser extent in severity. However, easterly waves can develop into tropical revolving storms themselves. So they’re an important precursor to tropical cyclone formation. A westerly wave is very similar to an easterly wave in concept, but it’s simply interconnecting warm front and cold front bands of weather. These bands are associated with a polar frontal depression — a mid-latitude low-pressure system. The westerly wave moves from west to east, creating a pattern that looks very much like a wave. So while easterly waves move from east to west and are tropical, westerly waves move from west to east and are associated with the polar front. Let me also touch on the book’s practice questions that appear at the end of this excerpt. These are the book’s practice questions — let’s try them one at a time.