Calms, Regions of, are places in the atmosphere which usually have little or no wind. Several areas of the earth’s surface are known as regions of frequent calms. These regions of calms include the northern and southern horse latitudes, the northern and southern subpolar regions, and the equatorial doldrums. All of these calms regions may be disrupted by sudden storms.
The horse latitudes
are regions of calms that lie at about 30° north latitude and 30° south latitude, between the oceanic belts of winds called the prevailing westerlies and the trade winds (see Prevailing westerly ; Trade wind ). In these areas, the air is gently sinking from high altitudes toward the earth’s surface. The sinking of the air results in areas of high barometric pressure and weak winds. These high-pressure areas shift farther north when it is summer in the Northern Hemisphere. They shift farther south when it is summer in the Southern Hemisphere.
The horse latitudes coincide with the subtropical maritime anticyclones, or subtropical highs. Subtropical highs are large areas of high pressure that occur over the oceans. Changes in their size, shape, position, or direction influence weather conditions in many parts of the world.
Subpolar regions.
The general circulation of air causes cold air over the North and South poles to sink, forming high-pressure areas known as the polar anticyclones. But since this cold region has frequent storms, periods of calms are not so persistent as in warmer regions of calms.
The doldrums
are ocean regions centered slightly north of the equator. One area of doldrums lies in the Pacific Ocean extending westward from Central America and South America to the Philippines. The other area of equatorial doldrums lies in the Atlantic Ocean between South America and Africa. The trade winds that blow from both the northeast and the southeast toward these areas of doldrums bring masses of air into the regions. The air masses are forced upward after being heated in the tropics. This causes a belt of low pressure to form. Tropical storms may occur at the edges of this belt. This belt moves north and south with the sun as the horse latitudes do.
During the days of sailing ships, sea captains avoided the doldrums areas because of the uncertain winds and heavy downpours. Sailing ships might lie becalmed in these regions for weeks.
