Air cools down with an increase in altitude under normal circumstances. It rises due to a variety of factors. Surface air rises when the ground beneath it gets heated by solar radiation. When a cold front cuts underneath a warm front, it forces the air to rise.

During orographic lifting, however, the air does not rise because of typical atmospheric conditions. Instead, it is forced upward as the terrain it travels over increases sharply in elevation.

When the air reaches the mountain or escarpment slopes, it is forced to rise with the elevation of the terrain. As it gains altitude, the temperature drops as a result of adiabatic cooling*. The air continues to cool as it keeps rising along the slopes.

(Adiabatic cooling is the reduction in temperature due to the expansion of air. As air rises into the atmosphere, barometric pressure decreases, allowing the air to expand and cool. This process takes place without any heat being added to or removed from the system.)

When air reaches dew point (the temperature at which water can no longer stay in its gaseous state) while still rising along the mountain slopes, condensation takes place.

The most significant consequence of this phenomenon is the orographic rain that is a direct result of the forced elevation with an increase in height of the terrain.

By the time the air reaches the top of a mountain, it is cold and dry as a result of adiabatic cooling and precipitation that took place. Usually, on the leeward mountainside, the terrain drops in elevation at the same rate as the windward side.

With the lowering terrain, gravity forces the dry air down the mountain slopes. As it descends, the air is compressed as a result of the increasing barometric pressure closer to the ground. The compression causes the air to warm up through adiabatic heating.

The result is warm, dry weather conditions, often accompanied by strong winds. The Chinook winds in the United States are a perfect example.

The diagram below illustrates how the orographic effect takes place, as well as the weather conditions associated with each step of the process:

The windward side of a mountain refers to the side directly facing the incoming wind. This is where moist air is forced to rise, cool, and produce precipitation. The leeward side is the opposite side of the mountain, sheltered from the prevailing wind. As the air descends this slope, it becomes warmer and drier, often resulting in rain shadow conditions.

The cold, moist air on the windward side and the warm, dry air on the leeward side of a mountain have a significant effect on vegetation.

Often, orographic lift occurs where a mountain is situated close to the ocean or a large body of water. The moist winds that blow from the shores result in a constant supply of water to the mountain slopes facing the sea or lake, leading to large-scale precipitation.

It is not surprising to find lush vegetation on the windward slopes of mountains. Often, these slopes receive rain during large parts of the year as a result of a constant source of moisture from the ocean, as well as the prevailing onshore winds.

Farmers and other businesses involved in the agricultural industry take advantage of this phenomenon by planting crops and developing plantations up the slopes where the largest percentage of rainfall takes place.

You can find some of the densest and most lush rainforest regions of the world around the tropics in South America and Africa on the windward sides of mountain slopes. It is a direct result of orographic lifting and the resulting constant precipitation.

Sometimes, only 16 – 32 kilometers (10 – 20 miles) away from the cold, rainy atmospheric conditions, the weather cannot be more different.

The cool, dry air that gets drawn down by gravity accelerates down the mountain slopes and warms up as a result of adiabatic heating. The result is warm, dry weather conditions, often accompanied by strong winds. The Chinook winds in the United States are a perfect example.

This means very little, if any, plant growth can occur in this climate. Where the windward side of a mountain may experience rainfall of 2032 – 2540 millimeters (80 – 100 inches), the leeward side of the elevation can receive as little as  254 millimeters (10 inches) or less.

As a result, large arid or semi-arid areas can be found on the leeward side of the mountain. In extreme cases, desert-like conditions can extend over vast regions.

The dry climate created on the leeward side of a mountain as a result of the orographic effect is also known as the rain shadow effect. The dramatic contrast in precipitation between the two sides of a mountain is what creates these conditions.

The orographic effect, or orographic lifting, occurs throughout the world on both large and localized scales. There are too many examples to list them all, but here are a few that illustrate what this phenomenon looks like in practice:

In the United States: The western slopes of the Sierra Nevada Mountains, California.

• • In India: The Khasi and Jaintia Hills.
  • In Australia: The Great Dividing Mountain Range in the southern and eastern region.
  • In South America: The Southern Andes Mountains facing the Pacific Ocean.
  • In Africa: The northwestern face of Table Mountain.
  • In Norway:  Mountains in the former Oppland region.

These are just a few of the many regions experiencing orographic lifting.

As this article clearly illustrates, even though the term orographic effect may sound foreign to you, the actual occurrence takes place all around the world and can literally be situated right on your doorstep.

The aim of this post was to highlight the orographic effect, define the occurrence, and explain the process through which it occurs.

You may want to have another look at your surroundings because wherever you may live, you will not find yourself far from a region experiencing the orographic effect.

Until next time, keep your eye on the weather!