Bernard Burton wrote:
The pressure at any level in the atmosphere is governed by the weight of air
above that level. For a given volume, there are more molecules, thus more
mass, when the air is cooler, and less mass when it is warmer. High pressure
then comes about due to a cold anomaly at some level in the atmosphere
above. For the global scale mid-latitude surface high pressure,
tropospheric air descends over these regions, thus warming the troposphere
there, and the cold anomaly must lie above this, and indeed is found in the
lower stratosphere. The temperature regime in the lower stratosphere is
dominated by dynamic effects, with warm anomalies where air is descending,
and cold ones where the reverse is true. Winter continental high and polar
high pressure can be due to radiational cooling of the air near the ground,
in which case the surface high pressure is due to a cold anomaly in the
lower troposphere.
On a global scale, most of the surface pressure pattern can be considered as
an imprint of the pressure pattern at some level in the mid stratosphere,
modulated by the thermal or thickness pattern of the intervening layers.
--
Bernard Burton
Wokingham, Berkshire, UK.

Thanks for that, Bernard. I've often wondered where the
"extra air" that causes high pressure is situated is located and the
answer is in the stratosphere, as it must be. However, I'm still not
sure how that air gets there since the formation of Highs (and Lows)
can be readily correlated with ageostrophic motions in the middle and
upper troposphere. Clearly I'm missing some point or other. Any help?

Tudor Hughes, Warlingham, Surrey.