Upper Winds — Page 186, Lesson 157

I want to walk you through the topic of upper winds, specifically jet streams. As we climb higher into the troposphere, the air density decreases, and the temperature effect starts to overwhelm the surface pressure effect. This means that as altitude increases, winds veer or back until they blow perpendicular to the surface temperature gradient. The practical result is that upper winds will generally be westerly in both hemispheres. The strongest winds are found just below the tropopause. When these winds exceed 60 knots, we call them jet streams. For examination purposes, we assume a jet stream is about 2,000 miles long, 200 miles wide, and 2 miles deep. That gives us a width-to-length ratio of 1:10, a depth-to-width ratio of 1:100, and a depth-to-length ratio of 1:1,000. Speeds in excess of 300 knots have been recorded, though those are rare. Now, what causes jet streams? They are caused by large surface temperature differences — in other words, large thermal components. Where you have a sharp temperature contrast across a region, you get a jet stream. There are several main locations where jet streams form. Let me go through each one. First, the subtropical jet streams. These form in the area of the subtropical anticyclones. They are more or less permanent features, but they move seasonally with the subtropical highs. In the Northern Hemisphere, they occur in the latitude bands 25° to 40° in winter and 40° to 45° in summer. In the Southern Hemisphere, they occur between about 25° and 30°. The jet core is at the 200 hPa level. Second, the polar front jet streams. These are associated with polar front depressions and usually lie parallel to the surface position of the warm and cold fronts. They are found between about 40°N and 65°N in the Northern Hemisphere and around 50°S to 55°S in the Southern Hemisphere. Third, the polar night jets. These occur in higher middle latitudes in mid-winter, near the top of the stratosphere at the 50 hPa level. Their direction is westerly, and speeds average 150 knots. Speeds of 350 knots have been noted. Fourth, the tropical easterly jet, also called the equatorial easterly jet. These are strong easterlies that occur in the Northern Hemisphere's summer between 10° and 20° North. They form where the contrast between the intensely heated central Asian plateau and the sea further south is greatest. This jet runs from the South China Sea westwards across southern India, Ethiopia, and the sub-Sahara. Typical heights are around 100 hPa, which is about 16 kilometres or 50,000 feet. Fifth, the Arctic jet stream. This is found between the boundary of Arctic air and polar air. It typically occurs in winter at around 60° North, but in the USA it is found around 45° to 50° North. The core is at approximately 20,000 feet, which is the 400 hPa level. It is a transient feature found over North America and Northern Eurasia during Arctic air outbreaks.