Why it Rains a Lot Near the Mountains
Defining Orographic Lift and Its Effect on Clouds and Preciptation
Why does it rain so much in the mountains? The reason is something called the orographic effect.
How Clouds Form
To understand mountain rain, the key is to understand how clouds form. Clouds are made out of tiny drops of water or ice crystals that surround even smaller particles of dust. They may also contain particles like salt or smoke, and the water clings to these tiny particles. Essentially, clouds are rain waiting to fall. Clouds appear different from each other because they are made up of varying amounts of ice, water, and dust.
Clouds form when water evaporates from rivers, oceans, and lakes. Water also evaporates from sources on the land, such as the leaves of trees.
Then the sun comes out. As this wet air is heated by the sun, it rises. When the air goes higher, it cools. Cooler air has a lower saturation point. This means that water condenses more easily in cool air than in warm air. At some point, the water within the air begins to condense, and this forms a cloud.
Mountains and Cloud Formation
So what happens in mountainous regions?
Mountains are natural barriers for all sorts of natural processes. They shape the courses of rivers and impact animal migration. Although people cannot see it, they also effect the movement of air.
When air approaches a slope, it must rise. As air gains in altitude, it cools. This is called adiabatic cooling, or cooling due to a change in pressure. In adiabatic cooling, the air does not need to touch anything cold to become cooler.
Cooler air quickly becomes saturated with moisture, and soon the humidity increases to the point where clouds form. The clouds generally rain in the area just upwind from the prevailing winds on the mountain.
Examples of orographic lift are found throughout the world. For example, on the Pacific Coast of British Columbia, Washington and Oregon, the storms coming in off the Pacific Ocean meet the mountains close to shore, causing a lot of precipitation. This yields a beautiful temperate rainforest climate, with a lot of plant life and a minimum of 100 inches or 250 centimetres of rain a year. This ecosystem would not be possible without the impact of orographic lift.
There is a reverse trend to the orographic effect. When air reaches the other side of the mountain, it is on the lee side. This is also called the rain shadow, because the air lowers and warms here. Where the rain shadow exists, the land is often quite dry. The Tibetan Plateau is one example of a rain shadow. It lies between the Qunlun, Kinlian, and Himalayan mountain ranges.
Sources:
NASA, Kids Science News Network, How Clouds Form
Wikipedia, Orographic Lift
Clouds Over Mountain
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