More on Dew Point and Latent Heat

Air parcel dew point temperature (oC)

Approximate amount of water vapor (g) in an air parcel (kg) at saturation (by the way, in text books referred to as the saturation mixing ratio)

Apprpoximate amount of potential heating due to latent heat release (calories) if all the water vapor condenses

0
4
2360
10
8
4720
20
16
9440
30
32
18880
40
64
37760

Latent Heat

A significant source of heating is the latent heat of condensation (discussed in class). For every gram of water vapor that condenses, approximately 590 calories of "heat energy" are given to the immediate surrounding atmosphere.

Note that in the above table, if air at 40C has 100% relative humidity (that is, the parcel's actual mixing ratio equals its value in the table shown above), and if all the water vapor in that air condenses out, 8 times as much heating would occur than if the air had a temperature of 10C and had 100% relative humidity.

This substantiates observations from the middle of thunderstorms and at, say, 18000 feet. Temperatures in the middle of the rising air in cumulonimbi there may be as much as 15-20C (28-38F) warmer than air outside the storm. For Hurricane Rita, observations showed that, at 700 mb (approximately 10000 feet), the interior temperature of Rita was 31C and about 20C warmer than the outside of the storm at the same elevation.

The temperature excess is confusing to conceptualize at first. This is because air of course cools due to expansion as it rises...but this cooling rate is modified by the release of latent heat of condensation.