Re: Why does warm air hold more water than cold air?

In a technical sense, it is not true that warmer air "holds" more water 
vapor than cold air.  Actually, it is the temperature of the water vapor 
itself that governs the amount of water vapor that may be held in the 
atmosphere. The warmer the water vapor, the greater its maximum vapor 
pressure.  Vapor pressure is the portion of atmospheric air pressure 
attributable to water vapor.  The greater the maximum (saturation) vapor 
pressure is the greater the capacity of the mixture of air and vapor to 
hold water vapor.  

Since the amount of water vapor in the air is quite small compared to the 
rest of the gases in the atmosphere, the temperature of the water vapor is 
governed by the temperature of the rest of the air in which it resides.  
This leads to the somewhat inaccurate but very convenient notion that 
warmer air holds more water vapor.

In order to explain this to 4th graders, we won't differentiate 
between the notion of vapor pressure versus "air capacity."  It is probably 
sufficient to say that the air is like a sponge.  When air temperature 
increases, that sponge grows a little and the air can hold more water 
vapor.  When air temperature decreases, the sponge shrinks and the air can 
hold less vapor.  

Keep in mind though the reality of the situation:  If air temperature 
increases, water vapor temperatures does too.  This results in a higher 
saturation (or maximum) vapor pressure.  If there isn't enough vapor in the 
air to meet the maximum, evaporation occurs as the atmosphere strives to 
reach balance.  If air temperature decreases, the saturation vapor pressure 
decreases as well.  If there is more vapor present than this maximum value 
can support, the condensation occurs as the atmosphere strives to reach 
balance.