Why do we get zones of very strong winds aloft/jetstreams?
The gradient of temperature from cold/low contour areas to warm/high contour areas tends not to be gentle in mid-latitudes, but often abrupt for a variety of reasons (continental/ocean thermal contrasts; convergence of airstreams etc.), and temperature contrasts are enhanced from time to time – drawn on our maps as fronts. It is important to remember, that although we show fronts as surface features, they extend right through the Troposphere, as a sharp contrast in temperatures in the horizontal, and this gives rise to the strong winds aloft, as I shall try to explain below. The effect of these temperature contrasts is cumulative. Therefore at lower levels, say around 850 hPa, the temperature differences across a frontal surface, which then lead to pressure differences (see opening paragraphs), give winds say around 30 knots. However, by the time we get to 400 or 300 hPa, the warmer column of air will have a greater cumulative expansion that the colder column of air, and the consequent
