Monsoon

any of a type of major wind system that seasonally reverses its direction—e.g., one that blows for approximately six months from the northeast and six months from the southwest. The most prominent examples of such seasonal winds occur in southern Asia and in Africa. Monsoonal tendencies also are apparent along the Gulf Coast of the United States and in central Europe, as well as in various other areas.

The primary cause of monsoons lies in the difference of the annual temperature trends over land and sea, though other factors may be involved as well. Seasonal changes in temperature are large over land but small over ocean waters. A monsoon blows from cold toward warm regions: from sea toward land in summer and from land toward sea in winter. Atmospheric pressure is high in cold regions and low in warm ones, permitting the movement of air to occur.

At the poleward limit of a monsoon system, the winds shift sharply. In India, for example, the monsoon blows from the southwest in July–August, while north of India the winds are from the east. Over northern Australia the monsoon comes from the northwest in January–February, and at the southern limit the winds again become easterly.

Most summer monsoons have a dominant westerly component and a strong tendency to ascend and produce copious amounts of rain (because of the condensation of water vapour in the rising air). The intensity and duration, however, are not uniform from year to year. Winter monsoons, by contrast, have a dominant easterly component and a strong tendency to diverge, subside, and cause drought.