Tag Archives: Hawaii

Wettest City in the U.S.

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“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

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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

Diversity of Climate Zones on Hawai’i

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The island of Hawai’i is a place of tremendous climate diversity. From tropical to sub-arctic, the Big Island has it all.

Hawai’i has eleven of the thirteen climatic zones defined in the Koppen Climate Classification System.  Developed by Wladimir Koppen in 1884, this climate system is based on average values of temperature and precipitation as well as the distribution of native vegetation. Hawai’i only lacks the extremes of cold winters and summer heat waves.

The primary reason for this wide range of climates is topography.  Two huge volcanic mountains, Mauna Kea (13,796 feet) and Mauna Loa (13,679 feet), dominate the landscape of Hawai’i. Since air temperature decreases 3.6°F per one thousand feet, it can be in the 80’s at the beach and below freezing in the summit regions on any given day.  Mauna Kea, the White Mountain, even supports a seasonal snow-pack.

These mountains also create orographic rainfall and affect the overall distribution of precipitation on the island.  When warm, moist air is forced up along the windward slopes, it condenses into clouds that produce rain. This precipitation supports the island’s lush rainforests and cascading waterfalls. The leeward side of the mountains, where the air descends, is sunnier and more arid.

The vast assortment of climate zones on Hawai’i is remarkable for an island roughly the size of Connecticut.  In many ways, Hawai’i is an island of all seasons.

U.V. Index in Hawai’i

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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

Pineapple Express

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Pineapple Express is a non-technical term used to describe a west-coast weather pattern that originates in the tropical Pacific.

When the jet stream dips south toward the Hawaiian Islands, it picks up warm, tropical moisture.  Flowing westward, it brings heavy rain to California, Oregon, and Washington. Flooding and mudslides are a serious threat during these events.  In addition, the system’s warm air tends to melt the snow in the lower Sierra and Cascade mountains, increasing the flood danger.

This weather pattern is named after the tropical Pacific’s popular fruit.