How does moisture affect evaporation?

Relationship between humidity and temperature

"In summer the cooling effect through evaporation can be quite significant!"
Briemle, p. 97

The air humidity itself indicates the water vapor content of the air. How much water vapor the air can absorb depends to a large extent on its temperature and the air pressure. We therefore speak of relative humidity.
It describes the current water vapor content of the air, as a percentage of the maximum possible intake amount under the existing conditions. The relative humidity does not represent an absolute value of the humidity.
100% relative humidity therefore does not mean that you are under water. It just means that the air can momentarily no longer absorb water vapor and that it is saturated. The absolute humidity is given in grams of water vapor per cubic meter of air:

For example, air with a temperature of
0 ° C absorb 5g water vapor; Air with a temperature of
20 ° C already 17g and at
30 ° C, a water vapor content of 30 g is possible.

Each of these states corresponds to 100% relative humidity.

If the air is not saturated with water vapor, it contains less than 100% humidity. If air with 60% relative humidity is heated from 18 ° C to 25 ° C, it has only 40% relative humidity, although the amount of water in the air remains the same. Conversely, when air is cooled, the so-called dew point is reached at some point. This is the point at which the air reaches 100% humidity and the water it contains no longer remains in vapor form. It condenses in the form of clouds and fog formation. When water condenses, thermal energy is released.

Now, however, the following question arises:

Does the temperature decrease when the humidity rises?

This would mean that house walls that are planted would experience natural cooling by the plants, as plants release water during photosynthesis, which increases the humidity and thus, assuming the thesis just made is correct, lower the temperature would.

On the basis of physical conditions and temperature and humidity measurements, it can be assumed that this thesis is correct, because thermal energy is consumed when water evaporates.

If you pour water on a sunlit terrace in summer, for example, the temperature drops noticeably as the water evaporates.

The evaporation of water given off by plants (and from the open earth beneath the plants) will also have a cooling effect.

For an exact calculation of how much energy (and thus heat) is consumed, however, one would have to collect a large amount of information, e.g. how much moisture does a plant give off during the course of a day, how much water is available to the plant, what is it like in each case Current air humidity (or if it is already at 100%, then no more water could evaporate, ...), how high is the air temperature, ...
These data would have to be collected for each plant and for each point in time.


  • Briemle, Helga: gardens at the house. Bay. Regional association for horticulture and land maintenance. Munich 1995
  • Interview with Prof. Brickl, Berlin. In: OZON, November 2007.