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Tropical Wind Belts

In unit 3, we discussed world winds and mentioned the wind belts affecting the tropics: the doldrums, horse latitudes, and trades. Just as the entire tropical belt shifts north and south with the Sun, so do these wind belts. Precipitation amounts can vary significantly near the boundaries of these wind belts.

THE DOLDRUMS. Doldrum is a nautical  

Formed by the converging trade winds of the Northern and Southern hemispheres, the con-vergence produces thick clouds and abundant rainfall. Where the doldrum belt narrows, the ITCZ is well defined by the increased convective activity (thunderstorms) and precipitation. Where it widens, the ITCZ is an ill-defined feature and difficult to locate. Geographically, the doldrums are at or near the equator. In January, the belt lies mainly, but not entirely, in the Southern Hemisphere. By July, it shifts into the Northern Hemisphere. Its average yearly position is a few degrees into the Northern Hemisphere.

THE HORSE LATITUDES. These belts are found just poleward of the tropics of Cancer and Capricorn, and lie roughly between the 25th and 35th parallels. The dominant feature of these belts is the subtropical high-pressure centers. They separate the tropical easterly winds from the midlatitude westerlies. The winds of the horse latitudes are light and variable, much like the doldrums, but the skies are cloudless or nearly so beneath the subtropical highs.

THE TRADE WIND BELTS. These are the predominant wind belts of the tropics. The winds are very uniform in direction and speed. In the Northern Hemisphere the winds are northeasterly, while in the Southern Hemisphere they are southeasterly. Wind speeds average 10 to 16 knots, but they are strongest and most steady in the center of these belts. The steadiness decreases over the western portions of the oceans and in the vicinity of the doldrums. Precipitation is also greater in these locations, because the air is warmer, more moist, and less stable than that experienced over other areas of the trades. The weather in the trade wind belt is generally fair, but migratory weather disturbances such as polar fronts and tropical low-pressure systems do move through the belt. The latter move from east to west, as easterly waves or tropical lows. We will cover these disturbances later in the unit. A common phenomena of this belt is the presence of trade wind cumulus. These fair-weather clouds cover from one-third to one-half of the tropical oceanic area. Another feature of the trades is a low-level temperature inversion, known as the trade inversion. Generally located below 10,000 feet, it is more predominant over the eastern portions of the oceans. Its height has a significant effect on the weather. The higher the inversion, the greater the depth of the moist layer; the clouds build to greater heights; cloud cover is more extensive; and rain is more likely. When the base of this inversion is low, the chance of rain is less likely, and the weather is generally better, except for periods of reduced visibility due to haze.

Maritime Tropical Weather

Typical weather over the tropical oceans is characterized by cumuliform clouds, although all forms of clouds are observed. Scattered rain showers are common, and risibilities are good except in the showers. Cumulonimbus clouds are not that common and are usually restricted to areas affected by synoptic disturbances. The mean air temperature is near 80F (26C) throughout the year and is generally within a few degrees of the sea-surface temperature.

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