Ask Tom Why / Weather / Weather Blog

A Hawaiian mountain gets 470 inches of rain a year. Why?

Posted On: May 17, 2018   |   Posted By: WGN TV

Dear Tom,
We recently returned from a trip to the Hawaiian Islands. On the island of Kauai, Mount Waialeale receives about 470 inches of rain a year. What causes such heavy rainfall?
- The Barnovich family
Dear Barnovich family,
Mount Waialeale is the wettest location in the United States, with 472 inches of rain per year. Such massive rains occur in mountainous locations situated such that prevailing winds force warm, moist air far upward. As air ascends, it cools - in dry air, the temperature drops about 5 degrees for every 1,000 feet of elevation increase. And as air cools, its ability to hold moisture diminishes sharply: The moisture condenses into clouds and precipitation. Compared to Mount Waialeale's massive rainfall, Chicago's annual precipitation of 36.89 inches seems puny.

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