The climates of the Pacific Ocean

The climates of the Pacific Ocean area are responsible for many of the differences in crops, products exported, and manner of life of the people on the various islands and coasts. The climates are primarily those to be expected for the latitudinal spread of the Pacific, with modifications mainly resulting from the distribution of land and water and the altitude of the islands. The huge size and currents are other factors affecting climates of the Pacific.


The doldrums or belt of equatorial calms is near the equator, followed in order by the trade winds, horse latitudes, and prevailing westerlies in both hemispheres going away from the equator. The whole system of wind belts shifts north in the northern summer and south in the southern summer (northern winter). In the western Pacific towards southeast Asia the monsoon winds are in control, and the normal types of wind for the latitudes appear only for a short time during the spring and fall months.


The doldrums coincide with the heat equator that receives the maximum average insolation (heat from the sun). This results in an expansion of the air and the development of a low pressure belt in the doldrums, where the winds are usually light and variable. Winds blow towards the equatorial calms from regions of higher pressure in both the north and the south Pacific, and by the effect of the earth's rotation are deflected to the right in the northern hemisphere to become the northeast trades, and to the left in the southern hemisphere to become the southeast trades. The expansion of the rising air in the doldrums causes condensation of moisture typically associated with local thunderstorms, which in the western Pacific supply abundant rainfall. Mean annual temperatures are close to 80° F, with slight variation between the coldest and warmest month and with only about 10° F range between the temperatures of day and of night. This everlasting sameness of weather--frequent showers and even temperatures--is a chief characteristic of doldrums climate.

The doldrums in the eastern Pacific are nearly always north of the equator, with the southeast trade wind blowing beyond the equator the entire year but to a less extent in winter than in summer of the northern hemisphere. The southeast trades lie between about latitude 25° south and a few degrees north. In contrast the northeast trades usually remain the entire year in the northern hemisphere, on an average between the latitudes of 5° and 25° north. In the eastern Pacific the belt of calms, or doldrums, is only 200 to 300 miles wide, and the trade winds commonly blow parallel to each other from the east. There is a minimum expansion and rising of the air, and conditions for rainfall are therefore unfavorable, so that some islands in this belt receive only 20 to 30 inches of rainfall annually. This light rainfall has permitted the accumulation of guano there, which would have been washed away if the customary heavy rainfall characteristic of most of the doldrums had occurred. Some of the dry, equatorial islands are situated in the southeast trades for all the year, or nearly all of it. This is also unfavorable for much rainfall. Another possible factor promoting continued dryness is that dry islands have few plants, and the bare sand and rock radiate much heat, causing the relative humidity of the lower part of the air column above the islands to be reduced and lessening the chance for rain. The air expands from the heat and as a consequence is cooled, but a cumulus cloud forming high over the island rather than rain is the usual result.

In the western Pacific, as compared to the eastern Pacific, the zone of the doldrums widens considerably because of the greater area of land in the tropics to the west, which promotes more heating of the atmosphere and development of local storms and condensation of water vapor. In the Australian area the doldrums shift south of the equator during the southern summer (December to February).

The trade winds, whether northeast or southeast, are not constant in physical characteristics but may vary in temperature, velocity, humidity, and density. Then instead of the two trade winds blending together to form the doldrums, the trade wind having the colder and denser mass underrides the warmer, lighter, and usually more humid mass of trade-wind air and forces it to rise, with resulting cooling and condensation. Frequently weak lows or cyclones develop that may furnish heavy precipitation. Under these conditions the doldrums are absent. The junction of the trade winds, whether or not there are doldrums present, is called the intertropical front. In the summer the intertropical front may be latitude 10v or 12° north in the western Pacific.

The southeast trades are strongest in winter in the southern hemisphere and tend to be light and variable in summer from the Tuamotus westward to Australia.

In both the trade-wind belts temperatures show small range throughout the year and are usually between 70° and 80° F. The rainfall is light (under 30 inches annually) and variable over the oceans and on low islands but is very heavy on the windward side of high islands, declining to a small amount on the leeward side of mountain barriers.

High-pressure areas or anticyclones are features of the eastern Pacific between approximately the latitudes of 20° to 40°, and these areas or cells of dense, descending air help control the weather and circulation of the winds. The generally fair weather in these highs, sometimes called the horse latitudes, is favorable for the operation of aircraft as is also the steady wind spiraling outwards from the anticyclones. The trade winds blow from these subtropical highs towards the intertropical front.

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