Craggenmore,
Consider two columns of air, of equal pressure at Sea Level, and (theoretically) zero pressure at the top where the atmosphere meets space. If they are of the same temperature, they will be of the same height. If different in temperature, the warmer column will 'stretch' upwards, as will the vertical distance between the various intermediate pressure levels. Conversely, the colder column will 'shrink', as will the vertical distance between the various intermediate pressure levels. Thus, at a given geometric elevation, the pressure in the warmer column will be greater than that in the adjacent cooler column. A Pressure gradient therefore exists from the high pressure column towards the low pressure column, and, after coriolis effect, becomes a jetstream.
Some numbers may make this more convincing. At 30,000 feet in ISA, the Temperature is -44.4°C, and the pressure is 300.89 hPa (8.885"Hg). The True Altitude is 30,000 feet.
It is well understood from Altimetry that flying at a given Pressure Height in warmer than ISA temperatures that the True Altitude is higher than indicated. Thus, in a ISA+15°C atmosphere, at 30,000 feet TRUE Altitude, the Pressure Height is 28,364 feet, where the Static Pressure (Ps) will be 323.99 hPa (9.567"Hg), and SAT = -26.2°C.
Now, if the atmospheric temperature is ISA-15°C, at 30,000 feet TRUE Altitude, the Pressure Height is 31,863 feet, where the Static Pressure is 276.24 hPa (8.157"Hg), and SAT = -63.1°C.
Now consider these two adjacent columns of air at a Geometric Altitude of 30,000 feet. The warmer column has a Pressure of 323.99 hPa, and the cooler has a Pressure of 276.24 hPa. That's a significant horizontal pressure gradient.
Now, does anyone have any idea of the mechanics of why the Japanese Jetstream is the "not so divine" wind.
Regards,
Old Smokey