Aloha Hawaii Palm Trees Tropics Travel Poster by made_in_atlantis
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The Pacific Ocean largely controls the temperature of the isolated islands and windward coasts, giving to both of them the maritime type of climate with small annual and diurnal range of temperature. When invaded by continental air masses, the ocean soon modifies their low temperature and low relative humidity, so that any air masses, whether coming from the tropics or from the polar regions, become filled with moisture after crossing the ocean and supply most of the rain that falls on the continents.
The circulation of ocean water, to be described later, modifies the climate over the surface of the sea and that of adjacent coasts, especially those in a windward location. East coasts in the prevailing westerlies partake of the continental climates, having large ranges of temperature. The contrast between San Francisco, which has a 10° F range between the coldest and warmest month, and Tokyo in the same latitude with a 40° F range is striking in this respect. On an average the temperature of the ocean drops about ½° F for each degree of latitude. Mean air temperatures closely follow those of the ocean surface, and the annual range is around 10° F, which is much less than on the continents.
Most tropical Pacific islands have rather uniform warm temperatures between 70° and a little over 80° F with a daily range of 9° to 16° F and a mean annual range of 1° to 9°, the least range being close to the equator. Surface temperatures of the equatorial western Pacific Ocean are close to 82° or 83° F throughout the year; in the eastern Pacific, February temperatures along the equator are 75° to 77° F, and in August the surface temperature of the ocean at the Galapagos Islands is 66° to 68° F, increasing rapidly northward and less rapidly westward. This coolness results from the cold Peru or Humboldt Current.
Rainfall in the tropical Pacific is highly variable both daily, seasonally, and annually. The maximum amount occurs on the windward side of high mountains in the trades and monsoon winds-the leeward slopes being dry. In the tropical eastern Pacific the rainfall is more associated with the trades and in the Western Pacific with the monsoons. Because of the seasonal shifting of the intertropical front, the period of maximum rainfall in Micronesia and most other islands north of the equator is in the summer and early fall (July to October), and south of the equator between November and April, which is summertime in that hemisphere. The rainfall is light on low islands that in the trade winds the year around. Islands like Hawaii, which are located near the edge of the tropics, may have their maximum rainfall in the winter because of the precipitation accompanying cyclonic disturbances that affect their latitudes.
In the doldrums the relative humidity is nearly always high, which makes the heat there more noticeable. In the monsoon regions the relative humidity in summer is high (80 to 90 per cent) and in winter low. In the zone of the trade winds the relative humidity at sea level is usually lower than in the doldrums.
Tropical cyclones or typhoons are born in the hot tropics where air is rising by convection, but the fundamental cause that starts the winds in motion has not been definitely determined. Once well started, a hurricane is dragged along by the upper winds like a whirlpool in a stream, and continues in force over the tropical waters, but the storm disintegrates from loss of energy when it meets land or a mass of cold air.