Some parts of the world have a much drier climate than others. In some cases this is a product of the geography. For example, deserts commonly exist on the leeward sides of mountain ranges. Persistent high pressure centered near 30°N and 30°S latitude has created a state of unending drought in certain places. You can look at a map of the world and see that major deserts are concentrated near these latitudes, where the temperate and torrid zones converge. The Sahara in northern Africa is the most well-known example. Deserts also exist in southern Africa. Progressing eastward around the north 30th parallel, we find the deserts of Saudi Arabia, Iran, Pakistan, Tibet, the southwestern United States, and northern Mexico. In the southern hemisphere, Australia is largely covered by deserts, and the climate of west-central South America is also arid.
Not all regions near 30°N and 30°S latitude have desert climates. Florida and most of the United States Gulf Coast, eastern China, and eastern Australia get a fair amount of rainfall. These regions receive adequate rainfall because moist tropical air blows inland regularly from warm ocean waters.
Places that do not get much precipitation show more variability of precipitation from month to month and year to year than places that get a lot of precipitation. Some deserts get an entire year's rainfall in a single storm. In other places, there is a wet season and a dry season. Most of the precipitation in southern California comes during the winter months. In India and Pakistan, summer monsoons bring most of the rain for the year.
In recent decades, the world's deserts, even those long considered uninhabitable, have been visited and populated by humans. This is especially true of the southwestern United States. The desert ecosystem is fragile, and human settle ment may harm the environment. There is evidence that the world's deserts are expanding, but there is disagreement as to whether that is because of human activity, or because of natural changes in climate. The environment on our planet has changed, is changing, and will continue to change. As recently as 5000 years ago, some scientists believe, much of the Middle East was green and fertile.
Semiarid regions occasionally experience droughts that render them practically deserts. These droughts are disruptive to humankind, because people rely on semiarid places such as the Great Plains for much of the food supply. During the 1930s, a prolonged drought occurred in the central United States, resulting in near famine in some regions. Other droughts, less severe, have taken place since then.
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