What is a rain shadow?

A rain shadow is a dry region on the leeward side of a mountainous area, where less precipitation falls compared to the windward side. They develop when there is a prevailing wind direction with sufficient speed that blows perpendicular to a mountain range that has a significant elevation difference between its peaks and the adjacent lowlands. This phenomenon creates stark climatic contrasts between the wet, vegetated windward slopes and the arid, often desert-like conditions on the leeward side. 

Image: Schematic diagram of a typical rain shadow. Courtesy of Encyclopedia Britannica, Inc.

Why is it important?

Rain shadows significantly influence the climate, vegetation, and various human activities in the affected regions. Several ski resorts are in the rain shadow of major mountain ranges and suffer consequences as a result. Agriculture, population distribution, and water resources are impacted by rain shadows as well.  

Resorts

Rain shadows impact ski conditions and the ski industry in a variety of important ways. Ski resorts located on the windward side of mountain ranges typically receive more snowfall due to orographic effects. In contrast, resorts on the leeward side of a mountain range experience less snowfall and overall drier conditions. The rain shadow effect leads to reduced precipitation, which can result in thinner snowpacks and shorter ski seasons. This can impact the resort's revenue and viability on the whole.

Several ski resorts across the world suffer from lower annual snowfall as a result of being located in the rain shadow of a mountain range. We see this in the Cascades of Washington and Oregon and in the Andes of Chile and Argentina. Resorts like Mt. Baker and Stevens Pass in the Cascades receive considerably more snow than Mission Ridge, which lies east of the range. In South America, Portillo, located on the western side of the Andes receives more precipitation than Las Lenas on the east side in Argentina. 

Climate and Vegetation

Rain shadows create stark contrasts in climate and vegetation between the windward and leeward sides of mountain ranges. The windward side of the range typically experiences a wet and cool climate, supporting lush vegetation. In contrast, the leeward side has a dry, often arid climate due to the lack of precipitation. The variation in precipitation results in diverse ecosystems. Windward sides can have dense forests and rich biodiversity, while the leeward sides may feature grasslands, shrublands, or deserts with sparse vegetation. 

Regions in rain shadows may struggle with water scarcity, influencing agriculture and industry. The disparity in rainfall impacts farming practices as well. Windward sides can support water-intensive crops, while leeward sides may require irrigation and are more suited for drought-resistant crops.

Examples

There are several notable examples of rain shadows across the world. 

• Washington/Oregon, USA
  • Location: Eastern side of the Cascade Mountains, Washington and Oregon.
  • Mountain Range: Cascade Mountains
  • Rainfall: Annual precipitation is less than 10 inches in many areas.

Image: Satellite imagery of the Cascade Mountains of Washington and Oregon with rain shadows seen to the east of the mountain range.

Image: Annual precipitation in Washington showing the rainfall discrepancy between the Cascade Mountains and the lowlands to the east. The rain shadow created by the Olympic Range is visible as well. Courtesy of www.choosewashingtonstate.com.

• Death Valley, USA
  • Location: Eastern side of the Sierra Nevada Mountains, California.
  • Mountain Range: The Sierra Nevada
  • Rainfall: Annual precipitation averages less than 2 inches.
• Patagonia, Argentina
  • Location: East of the Andes Mountains.
  • Mountain Range: The Andes
  • Rainfall: The western slopes receive heavy rainfall, while the eastern plains receive as little as 4 inches annually.

Image: Satellite imagery of the Andes Mountains in Chile and Argentina with a large rain shadow seen to the east of the mountain range.

• Gobi Desert, Mongolia/China
  • Location: Northern and southern slopes of the Himalayas and Tibetan Plateau.
  • Mountain Range: The Himalayas
  • Rainfall: Annual precipitation is typically less than 7 inches.
• Great Basin, USA 
  • Location: Eastern side of the Sierra Nevada.
  • Mountain Range: The Sierra Nevada.
  • Rainfall: Many areas receive less than 10 inches of rain annually.
• Atacama Desert, Chile 
  • Location: On the leeward side of the Andes Mountains.
  • Mountain Range: The Andes.
  • Rainfall: Some areas receive less than 0.6 inches of rain annually.
• Central Otago, New Zealand 
  • Location: East of the Southern Alps.
  • Mountain Range: The Southern Alps
  • Rainfall: Annual precipitation can be as low as 16 inches.

Image: Satellite imagery of the Southern Alps in New Zealand with a rain shadow seen to the southeast of the mountain range.

• Tibetan Plateau, China
  • Location: North of the Himalayas.
  • Mountain Range: The Himalayas 
  • Rainfall: Some areas receive less than 4 inches of rain annually.

Image: Satellite imagery of the Tibetan Plateau with a rain shadow seen north of the Himalayas.

• Simpson Desert, Australia
  • Location: East of the Great Dividing Range.
  • Mountain Range: The Great Dividing Range 
  • Rainfall: Annual precipitation is less than 5 inches in many areas.

Image: Satellite imagery of the Hawaiian Islands with rain shadows seen on the western side of Hawaii, Maui, Molokai, and Oahu.

Image: Annual precipitation for the Hawaiian Islands showing the rainfall discrepancy resulting from the rain shadows. Courtesy of  The Teacher-Friendly Guide to the Earth Science of the Western US.

What is the mechanism?

A rain shadow forms when a moist air mass is forced to ascend a mountain range. As the air rises, it cools and loses its moisture as precipitation on the windward side. Once the now-dry air crosses the mountain crest and descends on the leeward side, it warms up, increasing its capacity to hold moisture, which reduces the likelihood of precipitation. This creates a dry region on the leeward side, known as the rain shadow, characterized by significantly lower rainfall compared to the windward side. This phenomenon results in distinct climatic differences, with lush, wet conditions on the windward slopes and arid, desert-like conditions on the leeward slopes.

Image: Schematic diagram of a typical rain shadow. Courtesy of Adobe Stock / Saint Images.

Rain shadows are a fascinating meteorological phenomenon that creates profound environmental, agricultural, and human impacts by causing stark contrasts in climate and vegetation between the windward and leeward sides of mountain ranges. The effect shapes diverse landscapes and influences human activities around the world.

Luke Stone
Forecaster, OpenSnow