On Fri, 2008-05-23 at 12:21 +1000, Dave Donald wrote: > I'm hoping that someone may be able to explain the relationship between > Temperature inversion, dew point/cloud production and High Pressure (and > it's effect on cloud). High pressure (and its associated stability) is a > cloud killer, but why when all a parcel of air has to do is rise and > condense? Where is the lower layer of an inversion in relation to cloud > production or condensation? Above cloud base? I've seen various diagrams > in relation to temperature inversion and they're pretty vague about where > the inversion is in relation to cloud.
I've written the below assuming we are talking about cumulus clouds. In order to get cloud you have to cool the air until it becomes super saturated with moisture. You can do this either by direct cooling (for example moist air in contact with cold ground makes fog) or cooling by expansion. When a gas expands it becomes colder, so when your thermal takes off it rises and expands due to the lower pressure at altitude and cools, sometimes to the point where it becomes super saturated and a cumulus cloud forms. As you move up through the troposphere (the part of the atmosphere nearest to the ground) the temperature will in general decrease. Sometimes there will be a layer of air where the temperature does not increase, or even where it gets warmer the higher you go, this is an inversion. Inversions are often caused by high pressure systems as they subside (sink) through the troposphere and get warmed up by compression. There are different effects that can contribute to cause a high pressure, search for Hadley/Ferrel/Polar cells on Google for more info on some of them. A thermal is a parcel of air that is rising because it is warmer (and hence less dense) than the surrounding air. As the thermal rises it cools by expansion, but the air around it gets colder as well and so it keeps rising until the air around it is no longer colder. If it reaches an inversion caused by high pressure system subsidence it may no longer be warmer than the surrounding air and so stops rising. If there is no inversion it will keep rising until it hits the tropopause (causing a great thunderstorm), where it stops getting colder and instead starts getting warmer as you pass into the stratosphere. As the thermal rises it may or may not reach a point where it is saturated with moisture (the dew point), if it cools any more it will become super saturated and a cloud forms. If there is an inversion the level where it would become super saturated may be higher than the inversion, causing the thermal to stop rising without forming a cloud (a blue thermal). So in general, the inversion will be above the cloud base, and when the cloud reaches the inversion it will stop growing, sometimes spread out, or you might just see clouds with flat tops as well as flat bottoms. The general exception is in mountainous areas when the inversion is below the tops, in which case you sometimes get thermals coming off the tops of the mountains and forming clouds over just the tops (if there is a cloud on the thermal). I think I've answered most of your questions now. Regards, Mats _______________________________________________ Aus-soaring mailing list [email protected] To check or change subscription details, visit: http://lists.internode.on.net/mailman/listinfo/aus-soaring
