In orographic precipitation, winds move there up and over a mountain barrier. The forced descent of warm moist air over a mountain barrier produces precipitation. Moist oceanic air arrives at the coast. The air cools as it is forced up the slope. It cools because atmospheric pressure decreases with elevation. When cooling is sufficient, water vapor begins to condense.
Farther up the slope, precipitation occurs. The temperature cools but at a slower rate because of latent heat release. It is now hot because of the added latent heat release and dries since its moisture has been removed. When you see orographic or relief, you want to think of mountains.
There are major mountain belts, for example, on the west side of North America, where you’ll find the Rockies or the Himalayas over in Asia. Planetary winds are curved due to the Coriolis effect, the earth rotating. Water has a high specific heat, so it takes a long time to heat up and cool down. Also, the land has a low specific heat. So it heats up and cools down fast.
What Is The Orographic Effect?
The orographic effect is the charm used to describe changes to airflow. When the land’s topography air upward, these changes can cause disturbances in the weather system. As the air mass gains altitude, it cools down, raising the relative humidity to 100%. It also creates orographic clouds, which serve as a source of precipitation. The air will be saturated if you can drop the air temperature to the dew point temperature. At that point, a cloud is going to begin to form.
The big mountain has two sides: the windward and leeward sides. Windward winds travel up the side of the hill and bring moist air up the mountainside. On the leeward side, winter is dry as they descend the mountains. Winds are significant as they are the driving function of the orographic process.
It is another key component of the orographic effect. The orographic lift can cause orographic precipitation along the windward side. On the other side of the mountain, we see the rain shadow effect, mainly in the desert area.
Windward side
The windward side of a mountain is the side that faces the prevailing winds. Prevailing means the way the winds usually blow. As a result, the windward side is the west side. Because the air rises along the windward side and with cooling air, its relative humidity rises.
Therefore the windward side tends to be the side that gets clouds. Clouds produce precipitation. Additionally, the windward side also tends to be the vegetated side. So everything on this side of the mountain will be cool and wet.
Leeward side
Now on the other side of the mountain, the leeward side. The air begins to sink the mountain, and when cool air sinks, it becomes compressed. Also, it warms and evaporates. The backside of the mountain is called the leeward side. The air descends on the leeward side.
As the air is falling, it’s warming. Warming air means dropping relative humidity. The leeward side is a side that’s sheltered from the prevailing winds. Because of this, the leeward side tends to be the dry side. In many places, the leeward side tends to be a desert.
In California, the leeward side is also referred to sometimes as the rain shadow. Or the side of the mountain in the rain shadow, indicating that it does not get a lot of precipitation. Again leeward and rain shadow both indicate dry or a desert. So the leeward side becomes a desert climate with drier climates.
Warm air comes from the ocean and rises upward. Expanding warm air creates a rain shadow. The warm air cools and condenses so that this mountainside will be wet and cold or cool. On the other side of the mountain, there will be a desert. It changes the climate and the weather. It is what we call the Orographic effect.
According to the National Oceanic and Atmospheric Administration, the dry lapse rate is when the temperature of a parcel of dry air decreases as the parcel is lifted into the atmosphere. The dry adiabatic lapse rate is 5.5 degrees Fahrenheit for 1000 feet or 9.8 degrees Celsius per kilometer.
Moisturing rate is when the temperature of a part of saturated air decreases. As the partial is lifted in the atmosphere, this rate is not constant like the dry lapse rate. But it is dependent on the partial temperature and pressure.
Why is California green and lush while Nevada is dry or arid?
- Coastal cities receive moisture from the ocean. There’s more humidity in the air, and they can get more rain.
- The warm, wet air from the Pacific rises over California, Seattle, and Washington and releases rain on them.
- It then heads eastward towards Nevada.
References:
Stull, Roland. Practical Meteorology: An Algebra-based Survey of Atmospheric Science. University of British Columbia.
Whiteman, C. David. Mountain Meteorology: Fundamentals and Applications. Oxford University Press.
Rain Shadows by Don White. Australian Weather News.
