Many people live near mountain chains, either on the windward side or the leeward side of the mountains. People who live on each side of the mountain can experience different weather phenomena because of a method called orographic lift.
Orographic lift is a term that defines what is happening when air rises over a land barrier, such as mountains or hills. Orographic lift is an adiabatic process, which means that all of the changes that happen within an air parcel occurs only in the parcel through changes in temperature and how much moisture can be condensed within the parcel. A parcel of air resembles a beach ball or a hot air balloon, but this is a theoretical construct where gases and particles within that parcel cannot escape it.
Air is less dense than land, so the air is forced to rise over land. As a result, the air starts to rise above the land barrier in question. As the parcel rises, the parcel cools down and condenses. The air will condense and start to form clouds when the temperature of the air equals that of the dew point. The height where the temperature and dew point are equal to each other is called the Lifted Condensation Level. People can see this when they see the base of the cloud. The air will not condense before this point and with mountain ranges, the clouds and precipitation fall on the windward side of the mountain. The windward side is the side of the mountain that the wind encounters. The land on the windward side of a mountain can be lush and green as a result of this precipitation.
On the other side of the mountain, the leeward side, the air rapidly descends and becomes warmer once more. The parcel now lacks some of the moisture that it contained on the windward side of the mountain because some of it precipitated out. As a result, the dewpoint and the temperature increased throughout its descent. The land on the leeward side of the mountain by forming an area called a ‘rain-shadow’ desert. As a rain-shadow desert implies, the land is dry due to the lack of precipitation and the higher temperatures on that side of the mountain.
So what does this mean for the people that live on each side of the mountain? Well, the climates of the windward and leeward side can vary due to the amount of precipitation in each place. This can be observed when examining the landscape of the windward and leeward side of a mountain chain. The windward side will have more vegetation because of the amount of precipitation that it receives, while the leeward side will lack that lush, green vegetation. Some rain shadow deserts can be defined as a desert-like climate as well.
This orographic lift can also be observed via satellites. Check out this Tweet from the National Weather Service in the Bay Area that depicts a rain shadow due to downsloping winds that occurred on February 26th!
The dark area in the middle of the satellite image is a ‘rain-shadow.’ The clouds are evaporating there because of the lack of moisture in that part of the region, but more clouds reform once more when it encounters the next land barrier. So even in the middle of a frontal system, orographic lift can still control which areas see precipitation.
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©2019 Weather Forecaster Shannon Sullivan