How does rain affect the relative humidity?

The humidity and cloud formation

Clouds bring precipitation and are therefore of particular climatic importance. In addition, statements about the state of the atmosphere and further weather developments can be made on the basis of their appearance.

There are basically two basic forms: Cumulus clouds (sharply defined) are created by convection. Stratus clouds (uniform, non-delimited cloud cover) arise through advection.

Convection is the vertical rise of warm air. Advection describes the horizontal sliding of warm air over colder air. To understand cloud formation, one should know the humidity.

humidity

In the atmosphere, water occurs in all its aggregate states: solid (ice, snow), liquid (rain) and gaseous (water vapor). The size of the gaseous water in the air is the humidity. The humidity therefore indicates the water vapor content.

The air humidity is indicated in different ways:

  • maximum humidity (g / m³) ... indicates the maximum amount of water vapor that can fit into one cubic meter of air. It is temperature-dependent, i.e. it is smaller in colder air than in warmer air
  • absolute humidity (g / m³) ... indicates how much water vapor is actually in the air.
  • relative humidity (%) ... is the quotient of absolute and maximum humidity, multiplied by a hundred. LFrel = (LFSection * 100) / LFMax

The diagram opposite is the "dew point curve". The curve itself represents the maximum humidity at different temperatures. It can be seen from it that the maximum humidity is temperature-dependent. The dew point is the temperature to which an air mass has to cool down so that the water it contains condenses.

What happens if the absolute humidity is greater than the maximum, i.e. if the relative humidity exceeds 100%?
If the relative humidity is more than 100%, the air is oversaturated. Then the gaseous water vapor changes to the liquid state. So water vapor condenses to water. Only then can clouds form; Clouds do not consist of water vapor but of liquid water or ice!

Examples for calculating humidity and working with the dew point curve:

Task 1: At an air temperature of 15 C, the absolute humidity is 6.8 g / m³. What is the relative humidity?

given: LFSection = 6.8 g / m³; T = 15 C
tot .: LFrel in %
(The above equation is used for the calculation. We still need the maximum air humidity. It can be taken from the dew point curve. At 15 C it is 12.8 g / m³.)
LFrel = (6.8 g / m³ * 100) / 12.8 g / m³ = 53.125%

The relative humidity in this example is around 53%.

Exercise 2: The following values ​​were measured at a weather station: air temperature = 20 C, absolute humidity = 6.8 g / m³.
How big is the dew point?

(Consideration: dew point = temperature where air condenses; so: How far does the air mass of 20 C given above have to cool down so that the absolute air humidity becomes equal to the maximum and so condensation can start?) Reading the value from the dew point curve

The air must cool down to 5 C for it to condense. The dew point is therefore 5 C.

Exercise 3: At an air temperature of 0 C, the absolute humidity is 17 g / m³. What is the relative humidity?

given: LFSection = 17 g / m³; T = 0 C
tot .: LFrel in %
(For the calculation we use the above equation. We still need the maximum air humidity. It can also be taken from the dew point curve. At 0 C it is 4.8 g / m³.)
LFrel = (17 g / m³ * 100) / 4.8 g / m³ = 354.16%

The relative humidity is around 354%. The air is saturated with it.

The last example shows it clearly: at a certain point in time the air is oversaturated. This happens when the absolute humidity is greater than the maximum, i.e. when the relative humidity takes on values ​​above 100%. This means that there is no more air humidity, but now condensation sets in and liquid water, i.e. clouds, forms.

Cloud formation

With this previous knowledge about the humidity, the cloud formation can now be explained. The formation of clouds by convection, i.e. the formation of cumulus clouds, is explained here. However, advection proceeds according to the same basic principle: air must cool down to the dew point!

As the earth's surface heats up, water evaporates from bodies of water, forests, etc. These water particles rise with the warm air, as the warm air expands considerably and is lighter than the ambient air. When this evaporation flow reaches increasing heights, the water carried in it cools down by 1 C per 100 m. This process is known as dry adiabatic cooling. The air cools down until it does dew point achieved: in one cubic meter (m3) Air contains as much water as possible, the relative humidity is 100%, so the air is saturated. The water in the air package condenses. The height at which this process takes place is called Condensation level designated. When the water particles condense, they change from a gaseous to a liquid state. Then they settle on atmospheric dirt and dust - so-called Condensation nuclei - at. Source / cumulus clouds are formed. These water particles continue to rise, but only cool down by ½ C per 100 m. This is called moist adiabatic cooling designated. If the cloud with its water particles becomes "too heavy", precipitation occurs (rain, snow, hail, sleet).

Matthias Forkel,