An island under a subtropical high receives very little rainfall because of the persistent temperature inversion. Surface heating over some larger islands causes light convective showers. Cloud tops are only slightly higher than those over open water. Temperatures are mild, showing small seasonal and diurnal changes. A good example is the pleasant, balmy climate of Bermuda.

Figure 17-1 and Figure 17-2 show prevailing winds throughout the Tropics for July and January. Note that trade winds blowing out of the subtropical highs over ocean areas are predominantly northeasterly in the Northern Hemisphere and southeasterly in the Southern Hemisphere. The inversion from the subtropical highs is carried into the trade winds and is known as the trade wind inversion. As in a subtropical high, the inversion is strongest where the trade winds blow away from the west coast of a continent and weakest where they blow onto an eastern continental shore. Daily variations from these prevailing directions are small, except during tropical storms. As a result, weather at any specific location in a trade wind belt varies little from day to day.

In the trade wind belt, on average, about one-half of skies over open water are covered by clouds. Tops range from 3,000 to 8,000 ft, depending on the height of the inversion. Showers, although more common than under a subtropical high, are still light with comparatively little rainfall. Flying weather is generally quite good.

Where trade winds blow offshore along the west coasts of continents, skies are generally clear and the area is quite arid. The Baja Peninsula of Baja California is a well-known example. Where trade winds blow onshore on the east sides of continents, rainfall is generally abundant in showers and occasional thunderstorms. The eastern coast of Mexico is a good example. Rainfall may be carried a considerable distance inland where the winds are not blocked by a mountain barrier. Inland areas blocked by a mountain barrier are deserts; examples include the Sahara Desert and the arid regions of the southwestern United States. Afternoon convective currents are common over arid regions due to strong surface heating. Cumulus and cumulonimbus clouds can develop, but cloud bases are high, and rainfall is scant because of the low moisture content.

Flying weather along eastern coasts and mountains is subject to the usual hazards of showers and thunderstorms. Flying over arid regions is good most of the time but can be turbulent in afternoon convective currents; be especially aware of dust devils. Blowing sand or dust sometimes restricts visibility.

Mountainous islands have the most dramatic effect on trade wind weather. Since trade winds are consistently from approximately the same direction, they always strike the same side of the island; this side is the windward side. The opposite side is the leeward side. Winds blowing up the windward side produce copious and frequent rainfall, although cloud tops rarely exceed 10,000 ft. Thunderstorms are rare. Downslope winds on the leeward slopes dry the air, leaving relatively clear skies and far less rainfall. Many islands in the trade wind belt have lush vegetation and even rain forests on the windward side, while the leeward is semiarid. For example, the island of Oahu, HI, is about 24 mi wide in the direction of the trade winds. Annual rainfall averages from about 60 in on the windward coast to 200 in at the mountaintops, decreasing to 10 in on the leeward shore.

The greatest flying hazard near these islands is obscured mountaintops. Ceiling and visibility occasionally restrict VFR flight on the windward side in showers. IFR weather is virtually nonexistent on leeward slopes.

Islands without mountains have little effect on cloudiness and rainfall. Afternoon surface heating increases convective cloudiness slightly, but shower activity is light. However, any island in either the subtropical high-pressure belt or trade wind belt enhances cumulus development, even though tops do not reach great heights. Therefore, a cumulus top higher than the average top of surrounding cumulus usually marks the approximate location of an island. If it becomes necessary for a pilot to ditch in the ocean, the pilot should look for, and head toward, a tall cumulus. It probably marks a land surface, increasing chances of survival.

Converging winds in the ITCZ force air upward. The ITCZ appears as a band of clouds consisting of showers (with occasional thunderstorms) that encircles the globe near the Equator. The solid band of clouds may extend for many hundreds of miles and is sometimes broken into smaller line segments. It exists because of the convergence of the trade winds. In the Northern Hemisphere, the trade winds move in a southwesterly direction, while in the Southern Hemisphere, they move northwesterly. The tendency for convective storms in the tropics is to be short in duration, usually on a small scale, but they can produce intense rainfall. It is estimated that 40 percent of all tropical rainfall rates exceed one in/hour. Greatest rainfall typically occurs during midday. On the Equator, this occurs twice a year in March and September, and consequently there are two wet and two dry seasons.

Figure 17-1 and Figure 17-2 show the ITCZ and its seasonal shift. The ITCZ is well-marked over tropical oceans but is weak and ill-defined over large continental areas.

Convection in the ITCZ carries huge quantities of moisture to great heights. Showers and thunderstorms frequent the ITCZ, and tops to 40,000 ft or higher are common, as shown in Figure 17-1. Precipitation is copious. Since convection dominates the ITCZ, there is little difference in weather over islands and open sea under the ITCZ.

Flying through the ITCZ usually presents no great problem if one follows the usual practice of avoiding cumulonimbus clouds and any thunderstorms.

Since the ITCZ is ill-defined over continents, this handbook will not attempt to describe ITCZ continental weather as such. Continental weather ranges from arid to rain forests and is more closely related to the monsoon than to the ITCZ.

As shown in Figure 17-1 and Figure 17-2, over the large land mass of Asia, the subtropical high pressure breaks down completely. Asia is covered by an intense high during the winter and a well-developed low during the summer. The same occurs over Australia and central Africa, although the seasons are reversed in the Southern Hemisphere.