Francis Galton named the anticyclone, the weather system behind clear skies
The big H on a weather map marks sinking air. As it descends it warms and dries, clearing away cloud, which is why highs usually bring sunny days, cold nights and sometimes smog or fog. Victorian polymath Francis Galton coined the word anticyclone for them in 1877.
A high-pressure area, or anticyclone, is a region where surface pressure exceeds that of its surroundings. Air spills outward from the centre toward lower pressure, but Earth's rotation bends its path, so highs spin clockwise in the Northern Hemisphere and anticlockwise in the Southern. Friction with the ground slows the outflow and pushes it further outward than it would otherwise travel. The bigger the pressure gap between a high and a neighbouring low, the stronger the wind. The opposite term, cyclone, was coined earlier by Henry Piddington of the East India Company after a devastating storm at Coringa, India, in December 1789.
Two main recipes produce highs. The strongest come from dense, cold air spreading out of polar regions; these weaken once they cross warmer seas, which modify the air. Weaker but more common highs form where air aloft sinks through the troposphere, as happens beneath the western side of upper-level troughs. Sinking air is compressed and heated, drying the air mass. With no clouds, daytime sun heats the ground strongly while night-time heat escapes freely to space, so temperatures swing widely. Light winds under a high can trap urban pollution as haze, and if overnight humidity nears 100 percent, fog forms.
Some highs are permanent features. Around 30 degrees latitude, in the horse latitudes, air that rose over the equator descends to form the subtropical ridge, a warm-core high that strengthens with height and lies behind many of the world's deserts. The Azores or Bermuda High brings fair Atlantic weather and summer heat to western Europe; in 2003 an unusually strong ridge pushed heat waves as far north as Scandinavia. By contrast the Siberian High is cold-core, weakening with height, and can sit almost motionless for more than a month in deep winter, driving a winter monsoon along the Pacific coast.
Siberia's neighbour holds the pressure record. The highest barometric reading ever taken on Earth, 1,085.7 hectopascals, was measured at Tosontsengel in Zavkhan, Mongolia, on 19 December 2001.
Source: High-pressure area
