THE WEATHER-SPHERE: POWERING THE WINDS
By Paul Homewood
Erl Happ has the latest chapter up, this time discussing the role of the upper atmosphere in determining jet stream patterns.
Wind is air in motion flowing from areas of high to low surface pressure. Movement in the air involves the displacement of low density air with high density air. Warm air is less dense. Extreme wind speeds arise when the difference in density is greater and the contrasting air masses closer together. At the surface this manifests as closely packed isobars, lines of equal atmospheric pressure. It follows that wind speed is an indicator of the extent of the difference in density between different parcels of air. In winter cold air from the mesosphere lies on one side of the polar front and ozone rich air is adjacent giving rise to extreme winds, the polar arm of the Jet Stream.
In general, in mid latitudes wind speed increases to about nine kilometres in elevation and reduces thereafter. This is not the situation in the tropics or at the poles. Commercial airliners travel at 10-11 kilometres of elevation to reduce friction. But wind strength and direction in relatively rarefied air is still an important factor determining fuel consumption and journey times.
This fact of increasing wind speed with elevation is peculiar because we think that the air is heated by the surface of the Earth and the greatest differences in air density should manifest close to the surface of the planet. But, in the tropics the air moves sluggishly because the air tends to be uniformly warm. There is a long held notion that heating of the air in tropical latitudes drives the planetary circulation, but as we will see, that notion is incorrect.
The full post can be read here.