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How a rain shadow is formed
Krishna: What causes a rain shadow? High mountain ranges!
A rain shadow is a patch of land that has been forced to become a desert because mountain ranges blocked all plant-growing, rainy weather. On one side of the mountain, wet weather systems drop rain and snow. On the other side of the mountain—the rain shadow side—all that precipitation is blocked.
The Tibetan Plateau is the best example of a rain shadow. Rainfalls from the southern South Asian monsoon do not make it far past the Himalayas, leading to an arid climate on the leeward (north) side of the mountain range and the desertification of the Tarim Basin (1).
Image showing how Rain Shadows are formed
How rain shadows a re formed:
Winds laden with moisture coming from the water bodies like oceans get obstructed by high mountains like Himalayas and form rain shadow regions on the other side.
The air carries moisture laden winds flow to the landmasses and causes rainfall on one side of the lofty reliefs like mountains, plateaus, etc. after getting obstructed.
If the landforms are tall and wide enough, most of the humidity will be lost to precipitation over the windward side (also known as the rainward side) before ever making it past the top. As the air goes up it cools to the point that the air reaches its adiabatic dew point. At the adiabatic dew point, moisture condenses onto the mountain and it precipitates on the top and windward sides of the mountain.
As the air descends the leeward side of the landforms, it is compressed and heated, producing foehn winds that absorb moisture downslope and cast a broad "shadow" of dry climate region behind the mountain crests. This climate typically takes the form of shrub–steppe, xeric shrublands or even deserts. (1)
So the other side of the mountains don’t receive rainfall or get very little rain. This barren region is called rain shadow region.
Footnotes:
Q: (After reading my reply the same person sent me another Q on it):
A detailed answer Dr. Krishna, Thank You! Well, on a large scale landmass that explains the geographical rain shadows that have been established and those areas are very old I believe. But I was more curious about local, small ,compact rainshadows. For eg. the town where I currently live has peculiar characteristics like some parts do not receive rains but some part becomes completely drenched. There are few rainshadow localities scattered around here, (although the monsoon rains doesn't count as all the areas receive rainfall equally, but during light rains or pre-monsoon scenario these localities either receive scantly rainfalls or remain completely dry compared to other localities 1–2 kms away). So what can be the reason for such inequality?
When the clouds are sparse and scattered , the areas that have clouds get rain while the areas that are not covered by clouds don’t get it.
The amount of water nearby too is responsible to some extent. If there is more water for the sun to evaporate, there will usually be more clouds and more rain. This water could come from the sea, lochs, rivers and even garden ponds! A place with more water bodies gets more rain.
Also the direction of wind too dictate things. For instance, most of the wind in Scotland come from the west, known as westerly winds or westerlies. Westerlies bring warm moist air from over the North Atlantic Ocean. Warm wet air is perfect for forming clouds - meaning more rain! That's why the west coast of Scotland is often rainier than the east coast.
Uneven land like elevations like mountains too can cause this. For clouds to form, the air temperature needs to be cold enough for water to condense from water vapour into tiny liquid water droplets. Mountains are great at cooling air down. Westerly winds blow warm wet air from the sea onto the land and the mountains force the air upwards. As the air rises, it cools down. The vapour turns back into drops of water, forming clouds. Clouds bring rain. This is why places up high tend to be wetter frequently.
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