Having written on controversial, opinion things, I thought I’d take break and write about earth science: the jet stream. For those who are unfamiliar the main jet stream is a high-altitude wind blowing at about 40,000 feet (10 km) altitude at about 50° N latitude. It blows west to east at about 100 km/hr (60 mph), about 12% of the cruising of a typical jet airplane. A simple way to understand the source of the jet stream is to note that the earth spins slower (in mph) at the poles than at lower latitudes, but that the temperature difference between the poles and equator guarantees that air at high altitude is always traveling toward the poles from the lower latitudes.

Consider that the earth is about 40,000 km is circumference and turns once every 24 hours. This suggests a rotation speed of 1667 km/hr at the equator. At any higher latitude the speed is 1667 cos latitude. Thus it’s 1070 km/hr at 50° latitude, 0 km/hr at the north pole; 1667km/hr cos 50°= 1070 km/hr.

Idealized north-south circulation of air around our globe.

It’s generally colder at the poles than it is at lower latitudes — that is nearer the equator (here’s why). This creates a north-south wind where the air becomes more compact as it cools in northern climate (50°latitude  and further north), and this creates a vacuum at high altitudes and a high pressure zone at low altitudes. The result is a high altitude flow of air towards north, and a flow of low altitude air south, a process that is described by the idealized drawing at right.

At low altitudes in Detroit (where I am) we experience winds mostly from the north and from the east. Winds come from the east — or appear to — because of the rotation of the earth. The air that flows down from Canada is moving west to east at a slower speed than Detroit is moving west to east. We experience this as an easterly wind. At higher altitudes, the pattern is reversed. At 9 to 12 km altitudes, an airplane would experience winds mostly from the south-west. Warm air from lower latitudes is moving eastward at 1200 or more km/hr because that’s the speed of the earth. As it moves north, it discovers that the land is moving eastward at a much slower speed, and the result is the jet stream. The maximum speed of the jet stream is about 200 km/hr, the difference in the earth’s east-speed between that at 40°N and at 50°N, while the typical speed is about half of that, 100 km/hr. I’d attribute this slower speed to friction or air mixing.

One significance of the jet stream is that it speeds west-east air-traffic, e.g. flights from Japan to the US or from the US to Europe. Airlines flying west to east try to fly at the latitude and altitude of the jet stream to pick up speed. Planes flying the other way go closer to the pole and/or at different altitudes to avoid having the jet stream slowing them down, or to benefit from other prevailing winds.

I note that Hurricanes are driven by the same forces as the jet stream, just more localized. Tornados are the same, just more localized. A localized flow of this sort can pick stuff up here’s how they pick stuff up. Robert Buxbaum, May 22, 2018