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U.S. Virgin Islands Climate

The USVI has a subtropical climate with easterly trade winds and little seasonal variation in its warm temperatures. The Caribbean hurricane season, from June to November, sometimes brings destructive storms.

Although the northernmost of the islands, St. Thomas and St. John, are below latitude 18 30' and thus well within the tropics, the entire group has the advantage of possessing a fine climate. This is due in part to the northeast trade winds, which blow briskly with great regularity from the north-northeast for nine months during the year. These winds vary in velocity from a light to a moderate breeze, and attain their greatest strength usually around 4 o'clock in both the afternoon and morning.

Owing to the relatively small size of the islands and the lack of hot land breezes, due to the ever-present northeast trade winds, the temperature is equable. The climate is delightful for a winter resort, and the trade winds also furnish an equable mean for the summer months. The mean temperature at Charlotte Amalie on St. Thomas, covering a period of three years, ranged from 78 F. in March to 84F. in October while the mean temperature at Christiansted, St. Croix, covering a period of 28 years, ranged from 76.3F. in February to 82F. in August.

According to observations of John Knox, the maximum temperature on record, 1843 to 1851, is 91.9, and the minimum 68.9, the highest being in the month of September, and the lowest in the latter part of January.

The average annual rainfall is 38.23 inches for St. Thomas and 47.56 inches for St. Croix. Even over so small an area as the islands the rainfall is not evenly distributed, and it is thought to be more abundant on the northern slopes than on the southern.

In fact, water is the greatest need of the islands, and there is a very close relation between the rainfall and the production of sugar cane. There are no streams of consequence on St. Thomas or St. John. St. Croix, however, has a number of water courses, rising generally in the mountain slopes on the northern side of the island and flowing southward. There are very few wells in the islands and drinking water is obtained by collecting rain water in cisterns.

The climate of the U.S. Virgin Islands is changing. The air and ocean are warming, heavy rainstorms are becoming more severe, sea level is rising, and the ocean is becoming more acidic. In the coming decades, these environmental changes are likely to increase threats to life and property from severe storms, reduce the availability of fresh water during the dry season, harm or destroy much of the islands coral reef ecosystems, and make air temperatures uncomfortably hot more often.

The waters around the U.S. Virgin Islands have warmed by nearly two degrees since 1901, and sea level has been rising by about an inch every ten years. As the oceans and atmosphere continue to warm, sea level is likely to rise one to three feet in the next century. Rising sea level submerges marshes, mangroves, and dry land; erodes beaches; and exacerbates coastal flooding. Although most of the territory is well above sea level, the waterfront blocks of Charlotte Amalie are generally within three or four feet of sea level.

In the next several decades, warming waters are likely to harm most coral reefs, and widespread loss of coral is likely due to warming and increasing acidity of coastal waters. Rising water temperatures can harm the algae that live inside corals and provide food for them. This loss of algae weakens corals and can eventually kill them. This process is commonly known as coral bleaching because the loss of algae also causes corals to turn white.

Increasing acidity can also damage corals. Ocean acidity has increased by about 25 percent in the past three centuries, and it is likely to increase another 40 to 50 percent by 2100. As the ocean becomes more acidic, corals are less able to remove minerals from the water to build their skeletons. Shellfish and other organisms also depend on these minerals, and acidity interferes with their ability to build protective skeletons and shells.

Warming and acidification could harm the U.S. Virgin Islands marine ecosystems and economic activities that depend on them. Coral reefs provide critical habitat for a diverse range of species, while shellfish and small shell-producing plankton are an important source of food for larger animals. Healthy reefs and fish populations support fisheries and tourism.

Tropical storms and hurricanes have become more intense sicen the turn of the 21st Century. Although warming oceans provide these storms with more potential energy, scientists are not sure whether the recent intensification reflects a long-term trend. Nevertheless, hurricane wind speeds and rainfall rates are likely to increase as the climate continues to warm. Towns, roads, and ports in the U.S. Virgin Islands are vulnerable to the impacts of both winds and water during storms. Greater wind speeds and the resulting damages can make insurance for wind damage more expensive or difficult to obtain. Coastal homes and infrastructure are likely to flood more often as sea level rises because storm surges will become higher as well. As a result, rising sea level is likely to increase flood insurance premiums for people living along the coast.

The changing climate is also likely to increase inland flooding. Rainfall during heavy storms has increased by 33 percent in neighboring Puerto Rico since 1958, and similar trends have been seen throughout the Caribbean. The trend toward increasingly heavy rainstorms is likely to continue. More intense rainstorms can increase flooding as dry guts resemble rivers more frequently, and more water accumulates in lowlying areas that drain slowly. In 2010, for example, flash flooding washed out sections of roadway in Frederiksted.

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