Another Way of Warming Air

 

1.    Physicists tell us that when water vapor condenses, a certain amount of thermal energy is "given" to the atmosphere.  In short, condensation is a warming process.

 

2.    The amount of heating is proportional to the amount of water that condenses.  At sealevel temperatures and pressures, the warming is about 590 calories for each gram of water that condenses.  The 590 calories is known as "latent heat."

3.     Since air with 100% relative humidity at lower dew point temperatures holds less water vapor than air with 100% relative humidity at higher dew point temperatures, much more heat is liberated in cases for which cloud development is occurring in high dew point environments.

 

For example, air that has 100% relative humidity at 10oC has about 8 g of water vapor per kg of air, while air that has 100% relative humidity at 30 oC has about 32 g/kg.  If the kilogram air parcel initially at 10oC is lifted and cooled so that all of the water vapor condenses out of both, approximately 4720 calories would be liberated.  But if the 30oC were also lifted, 18880 calories would be liberated as it rose. (Illustrated by handout)

 

This tremendous heating due to latent heat release makes the core of hurricanes and monsoonal thermal lows that have access to humid air much warmer, say, at 500 mb level, than can be accounted for by the actual surface temperatures alone.


 

 

Application 1:

 

Air that rises cools at a rade of 5.5F/1000 feet (called the dry adiabatic lapse rate). However, if that air has been cooled to the dewpoint, moisture condenses.  If the air continues to rise, it will continue to expand and cool, but that cooling will be counteracted by the release of latent heat of condensation.  This heat release is not enough to totally counteract the expansional cooling, but it strongly modifies it.  The resulting cooling rate for ascending saturated (air at 100% RH) parcels is about 3.5F/1000 feet (known as the "wet adiabatic rate").

 

Application 2: (See graphical example)

 

Say surface air over the Saharan Desert of Africa has a temperature of 82F and NO water vapor.  The surface temperature may be enough to generate the formation of a thermal (warm core) low there.  However, as the air was lofted from the ground to, say, the 500 mb level, although it would be cooled 99F (cooling at 5.5F/1000 feet) to a temperature of -17F, no condensation would occur.

 

Say the same air moves over the eastern tropical Atlantic and becomes moistened enough so that the dew point temperatures rise to 82F.  A thermal low would form in this case too, and the air would be lofted.  However, when this lofted air reaches the 500 mb level, it will have cooled only 63F because the expansional cooling it experiences would be partially counteracted by the release of latent heat.  It would find itself at a temperature of +19F.

 

In short, for the latter case, the air would be 36F warmer at the 500 mb level than it would have been if moisture had not been condensing.  This 36F warming is due to the release of latent heat.