meteorologyscience

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.

a small-diameter column of rapidly swirling air in contact with a water surface. Waterspouts are almost always produced by a swiftly growing cumulus cloud. They may assume many shapes and often occur in a series, called a waterspout family, produced by the same upward-moving air current. Waterspouts are closely related to other atmospheric phenomena such as tornadoes, whirlwinds, and fire storms.

It is only in recent years that some of the workings of waterspouts have been unraveled, though waterspouts have been known and remarked upon since ancient times. For much of history, they have been subjects of mystery, speculation, and fear. A few intense waterspouts have caused deaths when they moved inland over populated areas, and they certainly constitute a threat to small craft; however, there are few authentic cases of large ships being destroyed by a spout. The superstition that firing a cannonball or other projectile into a spout can “break it up” has no scientific foundation. Contrary to popular opinion, a waterspout does not “suck up” water to great heights, though it may lift the water level a metre or so at its point of contact with the surface. It is suspected, but remains unproven, that waterspouts sometimes draw fish and frogs into its vortex and then drop them onto land, thus accounting for the reported falls of such objects.

Modern scientific interest in waterspouts began with the appearance of a particularly large and persistent spout on August 19, 1896, off the coast of Massachusetts, where thousands of vacationers and several scientists observed it. Its height was estimated to be 1,095 metres (3,593 feet, or almost 0.7 mile) and its width, 256 metres (840 feet) at the crest, 43 metres (141...

grapha small-diameter column of violently rotating air developed within a convective cloud and in contact with the ground. Tornadoes occur most often in association with thunderstorms during the spring and summer in the mid-latitudes of both the Northern and Southern Hemispheres. These whirling atmospheric vortices can generate the strongest winds known on Earth: wind speeds in the range of 500 km (300 miles) per hour have been estimated. When winds of this magnitude strike a populated area, they can cause fantastic destruction and great loss of life, mainly through injuries from flying debris and collapsing structures. Most tornadoes, however, are comparatively weak events that occur in sparsely populated areas and cause minor damage.

This article describes tornado occurrence and formation as products of instability within the Earth’s air masses and wind systems. Wind speeds and destructiveness are discussed with special reference to the Fujita Scale of tornado intensity. For short, descriptive entries on closely related phenomena not covered in this article, see waterspout, whirlwind, and fire storm.