Global Climatology — Page 387, Lesson 358

I want to walk you through the upper wind patterns that define global climatology at flight levels. These are the jet streams you'll encounter as a professional pilot, and each one has a distinct cause, location, and seasonal behaviour. Let's start with the Subtropical Jets. These blow at the 200 hPa level in each hemisphere. Their latitude changes with the seasons: in winter they sit between 25° and 40° latitude, and in summer they shift poleward to between 40° and 45°. The cause is the upper pressure gradient that forms between the descending warm air and descending cold air on either side of the subtropical high pressure belt. Speeds can be in excess of 100 knots, and near Japan they can reach up to 300 knots. I have figures for you: shows the January pattern, and shows July. Next, the Polar Front Jets. In the Northern Hemisphere, these are transient — they move with the polar front itself, shifting south in winter and north in summer. They are caused by the upper pressure gradient between the Tm — that's Tropical maritime warm air — and Pm — Polar maritime cold air — on either side of the polar front. In the Southern Hemisphere, the polar front jets behave differently: they are more constant and blow around the 50th parallel, but they are less strong than their Northern Hemisphere counterparts. Now the Tropical Easterly Jet, also called the Equatorial Easterly Jet. These are strong easterlies that occur only in the Northern Hemisphere's summer, between 10° and 20° North. The cause is the contrast between the intensely heated central Asian plateaux and the upper air further south — that's where the temperature difference is greatest. The jet runs from the South China Sea westwards, across southern India, Ethiopia, and the sub-Sahara. Its typical height is around 150 hPa, which is about 13 to 14 kilometres, or 45,000 feet. These easterlies can give way to westerlies, especially in January, as the ITCZ — the Intertropical Convergence Zone — moves south. Finally, the Arctic Jet Stream. This is found between the boundary of Arctic air and polar air. It occurs typically in winter at around 60° North, but in the USA it's found further south, around 45° to 50° North. The core of this jet varies between 300 and 400 hPa. It is a transient feature, found over large continents during Arctic air outbreaks. I have figures showing the equatorial upper winds for January and July as well: and the next page's figures. These upper wind patterns are critical for flight planning — they affect fuel consumption, routing, and turbulence encounters at cruise altitudes.