Monsoon - (n.)

The word monsoon comes from an Arabic word meaning “season.” Monsoon winds are giant sea and land breezes produced by the seasons. Summer is analogous to daytime, and winter corresponds to nighttime. Summer monsoons, in essence a giant sea breeze, bring moisture and rainfall. Winter monsoons, in contrast, tend to bring drought. The strongest monsoonal patterns are in the tropics, with the Indian monsoon being the outstanding example. The seasonal winds within this system extend over East Africa, Arabia, India, and the Arabian Sea. Their existence has been known for a long time: an Arab navigator used the summer monsoon to speed Vasco da Gama from East Africa to India, in 1498. The summer monsoon in India is a product of the low pressure zone which begins to form early in summer, over the Asian highlands. Following the Sun's apparent path to the north, the southwesterly winds appear first in Ceylon, at the end of May, and arrive at the foot of the Himalayas in July. Here they drop an enormous amount of rain, up to several meters in three months. The winter monsoon in the region is essentially indistinguishable from the easterly trade wind pattern that is normal for this region. The West African monsoon also is well known. Summer winds bring rain to the forests and savannas south of the Sahara (Sahel), from the Gulf of Guinea. Winter produces the dusty, dry Harmattan which blows from east-northeast and brings red dust into the Atlantic (along with the occasional locust). Monsoonal winds also occur in North America, in the Gulf region. In summer, low pressure regimes commonly develop over the heated land, and humid marine air may then be pulled in from the southeast, across Texas. During winter, northeastern winds are quite frequent, generated by the high atmospheric pressure over the cold interior. These monsoonal influences, however, are commonly masked by the complexities of mid-latitude air circulation. Here the air flow is dominated by the interaction between polar and tropical air masses (polar front) producing cyclones. See also "cyclones.”