Tropical rain belt
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The weather in the tropics is dominated by the tropical rain belt, which oscillates from the northern to the southern tropics over the course of the year. The tropical rain belt lies in the southern hemisphere roughly from October to March, and during this time the northern tropics experience a dry season in which precipitation is very rare, and days are typically hot and sunny throughout. From April to September, the rain belt lies in the northern hemisphere, and a wet season occurs there, while the southern tropics experience their dry season.
The rain belt reaches roughly as far north as the Tropic of Cancer and as far south as the Tropic of Capricorn, although there have been in such cases where the rain belt itself has gone far into the temperate zones especially those near the Tropic of Cancer and the Tropic of Capricorn[1]. One such case is in the Texas surplus of rain during the summer of 2007 [2]. Near these latitudes, there is one wet season and one dry season annually. On the equator, there are two wet and two dry seasons as the rain belt passes over twice a year, once moving north and once moving south. Between the tropics and the equator, locations may experience a short wet and a long wet season. Local geography may substantially modify these climate patterns, however.
[edit] Relationship to Global dimming
The earth is hottest roughly where the sun shines most strongly. This occurs north or south of the equator depending on the season. The hot air rises and cools, causing tropical rainfall. The now-dry air then moves away from the equator and sinks, warming and dropping in relative humidity and creating an area of almost no rain. Where rain falls throughout the year there is tropical rain forest. Where rain falls almost never there is desert, for example the Sahara. Where rain falls only when the rainbelt is closest, there is a summer rainy season and a winter dry season, for example in the Sahel.
There is significant evidence that human activities affect the position of the rain belt. Most particulate pollution is released in the northern hemisphere, and atmospheric mixing between the hemispheres is minor. This decreases the sunlight reaching the northern hemisphere, moving the belt of maximum sunlight southward. This may explain why in recent decades the Sahel has experienced record droughts. More recently, Western Europe and North America have made great efforts to reduce particulate emissions, while the ex-Soviet bloc may have also involuntarily reduced emissions. Thus the maximum sunlight belt has moved north again, bringing unexpected flooding to the Sahel and droughts to Zimbabwe.
