Relative humidity and Desert?

Question

there are two derserts , one is polar desert ,and other is the hot desert, both of them are dry and have a very low specific humidity (aboslute humidity ) but the relative humidity in the polar dersert is very high ?> can someoen tell me why >?

Answer 1

The "carrying capacity" of air in regard to water vapour is a total myth that is still taught to unfortunate students even in universities. Air no more "carries" or "holds" water vapour than it does any other gas - nitrogen or oxygen for instance. There is a very good explanation of this on the Bad Meteorology page:
http://www.ems.psu.edu/~fraser/Bad/BadClouds.html

Polar deserts such as Antarctica have very little precipitation but they do have a great deal of ice. The ice sublimates into water vapour raising the relative humidity of the air. A similar thing happens in the oceanic deserts of the horse latitudes. There is very little rain but very high humidity because of evaporation from the ocean.

The major deserts of the world, lie under belts of high pressure around the globe. These are the polar highs and the sub-tropical ridge around 30° to 40° in each hemisphere. In high pressure systems the air descends, it does not rise. Without rising air, you don't get cloud and without cloud you don't get precipitation.

The humid air over Antarctica and in the horse latitudes stays close to the ground. There is no mechanism to transport it higher in the atmosphere to make cloud..

Answer 2

It has to do with the "carrying capacity" of the air in question. The warmer the air is, the more water it can carry as vapor. Deserts are usually defined in terms of amount of precipitation per year. Let's compare two different examples. Say you are in Arizona in the summer and the ambient temperature is 110 F and the dew point is 10 F and the annual precipitation is 5 inches per year. This is a hot desert with a low relative humidity, right? Now compare this to Baffin Island, Canada, where the ambient temperature is 15 F, the dew point is 10 F and the annual precipitation is 5 inches per year. This is a cold desert with a high relative humidity, right? I hope this helps.
P.S. The relative humidity on Mars is usually close to 100% and yet it never rains and almost never snows. this is an extreme example of a cold desert with high relative humidity.

Answer 3

A) less than 10 percent