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How is incline rain formed? – Enlightenment for geography

How does gradient rain come about? - Enlightenment; ung für Geography

Slope rain describes a form of rain formation that takes place on geographical obstacles such as mountains. It is therefore also called orographic rain (orographic = relating to the slope and direction of the terrain). The processes of condensation of water droplets with rising air masses (cloud formation) and the growth of the droplets by colliding and merging are essential.

Dry adiabatic rise on the windward side

When air masses meet a mountain range, they are forced to rise. The windward side of elevations is called the windward side. The higher the air rises, the more it cools – by 1 °C per 100 m during the dry adiabatic ascent. Dry adiabatic means that the humidity has not yet reached 100% and accordingly no cloud formation takes place.

The adiabatic principle per se refers to the change in state of a substance without heat exchange with the environment. Due to the falling temperature, the water vapor capacity of the air decreases at the same time. The humidity continues to increase.

Moist adiabatic ascent on the windward side

From a relative humidity of 100%, the water vapor condenses into water as soon as it reaches the dew point. Cloud droplets are formed, which become larger and larger by colliding and merging. If they reach a weight that can no longer be maintained by air friction and updrafts, rain is produced according to the principle of gravity. In the area of ​​the moist adiabatic ascent, cloud formation and precipitation take place along the windward side of an elevation.

It is this precipitation that is referred to as slope rain or congestion rain. The intensity depends on the slope, the height to be overcome and the humidity of the air. Since latent heat is released during the condensation of the water vapor, the temperature only drops by 0.4 – 0.7 °C per 100 m (on average one speaks of 0.5 °C per 100 m).

Since the clouds on the windward side (= windward) rain off before crossing the summit, one speaks of the weather side of a mountain range or mountain. In this area temperate rain forests occasionally develop in the climate of the temperate latitudes: more than 2000 mm of precipitation per year can occur here thanks to the rain on the slopes. They are characterized by a high level of biodiversity and are particularly worthy of protection.

Föhn winds on the lee side

As soon as the air masses have passed the highest point of the obstacle, they drop again. Since no further condensation takes place, the air down the slope continuously warms up in a dry adiabatic manner – i.e. with 1 °C per 100 m. A dry, warm fall wind arises on the leeward side, which is called Föhn. Factors favoring its formation are:

  • low temperature at the crest of the mountain,
  • slope of the terrain and
  • a potential low-pressure area on the leeward side in contrast to a high-pressure area on the windward side.

Some characteristic phenomena can occur as a result of these processes:

  • foehn wall – a cloud wall along the crest where the Air flows down, or on the cold front of the low-pressure area
  • foehn window – the nice weather area between the foehn walls
  • foehn storm – foehn wind with high air speeds
  • Foehn lenses – characteristic clouds due to the vibrations of the air on the lee side

The most well-known example in Germany is the foehn wind of the Alps.

Example

A simple example is the cooling of the air masses along of the slope.
The air masses have to overcome an obstacle of 2500 m height. At the bottom of the slope the temperature is 10 °C. The air cools dry adiabatically by 1 °C per 100 m. This means that after 500 m it only has a temperature of 5 °C. Now the level of condensation has been reached, clouds form and precipitation occurs: the uphill rain. In the course of this, the air cools down by 0.5 °C per 100 m in a moist adiabatic manner. After another 500 m the temperature is 2.5 °C, after 1000 m 0 °C and finally -5 °C at the top of the mountain.

If the air masses on the leeward side fall as a foehn wind, they warm up over this stretch of 2500 m with 1 °C per 100 m. This results in a warming of 25 °C, so that a temperature of 20 °C is reached at the opposite foot of the slope.

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