Wettest City in the U.S.

“Rain rain go away, come again another day.”  With rain on fourteen of the last twenty-four days, this old nursery rhyme sums up the feelings of many New Yorkers.  For the residents of Hilo, Hawai’i, however, this sentiment is not an option.

Hilo is the wettest city in the United States.  Situated on the windward coast of the island of Hawai’i, the city is in the path of the moisture laden Trade Winds. Averaging 126 inches of rain annually, Hilo experiences every conceivable type of rainfall, from mist to downpours.  Looking at the calendar’s point of view, some form of precipitation falls there 272 days of the year on average.

As soggy as it may sound, Hilo’s amazing tropical rain forests, waterfalls, and rainbows would not be possible without this significant precipitation.

Above the Clouds on Haleakalā

While traveling among the Hawaiian Islands, I had the opportunity to visit Haleakalā National Park.  Ascending its volcanic slopes, I was struck by its summit region known as “kua mauna”, the land above the clouds.  Its unique view is made possible by an elevated temperature inversion.

In the troposphere, the weather layer of our atmosphere, air temperature usually decreases with height.  An inversion occurs when something causes that situation to reverse and allows air temperature to increase with height.

At Haleakalā , the inversion is caused by a large-scale subsidence in the Trade Winds.  Blowing from centers of high pressure across the Pacific, cool, dense air aloft is warmed by compression as it descends to lower altitudes.  In opposition, solar heating warms air near the surface allowing it to rise and cool, forming clouds.  When these cool clouds meet the warmer air above them, an inversion layer is formed.

The inversion layer acts like a cap for cloud convection.  Therefore, the summit of Haleakalā (10,023 feet), rising above the inversion altitude, stands out like an island in a sea of clouds.

View from the summit of Haleakalā, Maui, Hawai'i

Photo Credt: MF at The Weather Gamut

U.V. Index in Hawai’i

Since returning from Hawai’i, everyone has been asking me, “Why aren’t you more tan?”  My response is, “SPF 70 and a hat.”  The UV index in Hawai’i ranges from high to extreme, so sun protection is necessary to avoid serious sunburn and other long-term skin problems.

The UV index is a scale that measures the intensity of the sun’s ultraviolet radiation reaching the Earth’s surface. Readings vary from place to place as local factors affect the amount of UV light that reaches the ground. These include, the thickness of the ozone layer, latitude, season, elevation, and cloud cover. Developed in the early 1990’s by the NWS and EPA to warn the public about the risk of overexposure to the sun, it is calculated on a daily basis for every city in the U.S.

Hawai’i is located in the tropics at approximately 20°N latitude.  As a result, the sun sits higher in the sky as compared to the mainland.  When the sun’s rays are more directly overhead, they filter through less of the atmosphere and are therefore more intense when they reach the ground. Consequently, the UV index in Hawai’i is higher than any other location in the U.S.  It averages around 7 in the winter and 11+ in the summer.

Chart Source: EPA