PLEASE ANSWER!!. Why would a descending mass of air tend to become drier???

Question

PLEASE ANSWER!!. Why would a descending mass of air tend to become drier???

Answer 1

Because generally it would warm as it reached the surface of the earth.  Warm air can hold more total moisture, so the relative humidity would decrease.

Answer 2

When air descends, it warms adiabatically because it becomes compressed due to increasing air pressure.   The vapor pressure of water is higher at higher temperatures, so the relative humidity in a given chunk of air decreases when you warm it up (same reason your house feels dry in the wintertime since you have to heat up all that cold air).  The absolute humidity does not change though--that's the total amount of water in the air.  In other words, the water doesn't "go" anywhere, just the relative amount of water that the air can hold changes, and people and weather are senstitive more to relative humidity than absolute humidity.   So the air "feels" drier in a relative sense.  

The real drying effect in weather systems comes when air rises and then falls.  When the air rises, the opposite effect occurs and the relative humidity rises until it hits the dewpoint and clouds form and it starts raining.  When the air falls back down (like coming down the other side of a mountain), it has actually lost water and absolute humidity in the form of rain, and it comes back down drier than it went up.  It also comes back down warmer because of the latent  heat contained in water vapor.  This causes hot and dry deserts on the leeward sides of mountains (death valley) and it causes the deserts north and south of the equator.  

PS: Air doesn't actually "hold" water, rather water comes to equilibrium between vapor and liquid when the partial pressure of vapor equals the vapor pressure.  At higher temperatures, the vapor pressure increases, so more water can evaporate.  But this can happen whether there is air present or not.  When we are surrounded by air, it acts as if the air is "holding water" like a sponge, so we tend to talk about air as if it is a sponge for water, even though this is not completely true.