Slope rain is a form of rain that occurs 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 ascent on the windward side
If air masses hit a mountain range, they are forced to climb. The windward side of elevations is called the windward side. The higher the air rises, the more it cools down – during the dry adiabatic ascent by 1 ° C per 100 m. Dry adiabatic means that the air humidity has not yet reached 100% and accordingly no cloud formation takes place.
The adiabatic principle itself refers to the change of 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 to water as soon as it reaches the dew point. Cloud droplets form, which get bigger and bigger as they collide and merge. If they reach a weight that can no longer be held by air friction and updrafts, rain occurs 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 incline rain or damming rain. The intensity depends on the slope, the height to be overcome and the humidity of the air. Since latent heat is given off when the water vapor condenses, the temperature only drops by 0.4 – 0.7 ° C per 100 m (the average is 0.5 ° C per 100 m).
Since the clouds on the windward side (= windward) rain down before crossing the summit, we speak of the weather side of a mountain range. In this area, temperate rainforests occasionally develop in a climate of temperate latitudes: thanks to the uphill rain, more than 2000 mm of precipitation can occur here per year. They are characterized by a high level of biodiversity and are particularly worth protecting.
Foehn winds on the leeward 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 warms down the slope continuously dry adiabatically – i.e. at 1 ° C per 100 m. A dry, warm downwind occurs on the lee side, which is called the foehn. Factors that favor its development are:
- the low temperature on the ridge of the mountains,
- the slope of the terrain and
- a potential low pressure area on the leeward side as opposed to a high pressure area on the windward side.
As a result of these processes, some characteristic phenomena can occur:
- Foehn wall – a wall of clouds along the ridge where the air flows down, or on the cold front of the low pressure area
- Foehn window – the beautiful 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 leeward side
The best-known example in Germany is the foehn wind in the Alps.
A simple example should explain the cooling of the air masses along the slope.
The air masses have to overcome an obstacle at a height of 2500 m. At the foot 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 incline rain. In the course of the process, the air cools down moist adiabatically by 0.5 ° C per 100 m. After another 500 m the temperature is 2.5 ° C, after 1000 m 0 ° C and at the top of the mountain finally -5 ° C.
If the air masses fall on the leeward side as a foehn wind, they warm up over this distance of 2500 m at 1 ° C per 100 m. This results in a warming of 25 ° C, so that a temperature of 20 ° C is reached at the mutual foot of the slope.