Rain Shadows on the Summits of Hawaii

On Hawaii’s Big Island, prevailing Pacific trade winds from the northeast bring higher levels of rainfall to northern mountain slopes, producing dramatic changes in vegetation as you move from northeast to west across the island.

This pattern is mostly due to large mountains that block passage of precipitation systems, casting a “shadow” of dryness behind them. Ancient eruptions from the extinct 5,480-foot Kohala and the 13,796-foot dormant Mauna Kea Volcanoes shaped the mountainous northwest portion of the island, seen above in this Landsat 7 satellite image from January 2001. Clouds hover over the lush, dark green forests on the rainier eastern slopes, while drier western slopes appear mostly earthy-brown. The sharp contrast in vegetation is called the rain shadow effect.

On the Hawaiian Islands, rain shadowing occurs when coastal winds from the northeast push air up and over the mountains. As the air rises, it chills in the cool, high-altitude temperatures. Moisture condenses and falls as rain or snow on the windward slopes. The air that crests the mountaintop warms and dries as it heads down the leeward slopes, leaving them dry and making vegetation sparse.

When drought occurs on Hawaii, the most severe conditions tend to be on the rain-shadowed western slopes, away from the prevailing winds.  This was obvious in 2010, when the islands were hit by an intense drought that impacted agriculture yields and water availability.

References:
Hawaiian Islands. NASA’s Earth Observatory.

Related:
Featured Video: Drought Grips Hawaii in 2010

Image created by Hunter Allen, using Landsat data provided by the United States Geological Survey’s Global Visualization Viewer. Caption by Caitlyn Kennedy.