latitude cyclones. The cores of anticyclones are locations where precipitation is
relatively unlikely.
The winter configurations ofmiddle latitude cyclonesand anticyclone are
ordinarily geographically related. Cyclones are strung along the waves of the polar
front (seeMiddle latitude cyclones). Following to the north and west of the
cyclones are anticyclones that have edges circulating air into the cyclones and
the fronts. Wintertime anticyclones can obtain immense proportions and are usu-
ally hundreds of thousands of square kilometers in size; at times they can stretch
from Canada to Mexico. The pressures in winter anticyclones can sometimes pass
1050 mb and their centers travel at rates of about 50 kph.
The Siberian Express is, perhaps, the most nefarious incarnation of a middle lat-
itude anticyclone. In the winter, the coldest air of the Northern Hemisphere—
including the north polar region—is usually over Siberia. A large ridge in the polar
front jet stream will move this air from eastern Siberia to the eastern United States
in a matter of a few days. This fast-moving anticyclonic incursion is sometimes
termed a cold wave and is responsible for several days of cold, if not low-
temperature, record-setting weather. The “Arctic Express” is a similar event when
the anticyclonic air has origins in northwest Canada or northern Alaska; this
“express” also barrels along but it does not tend to be as cold.
Winter anticyclones pass over warmer and warmer surfaces as they leave their
source regions. Over a course of days, the anticyclone will moderate from its cold,
dry origins. For instance, an anticyclone that started at− 55 °C in Siberia may
modify up to 0°C by the time it reaches the Gulf of Mexico. This is not bitterly
cold weather as when it was in Siberia, but is much colder than the winter averages
over the Gulf.
Summer anticyclones are also noted for bright, dry weather with minimal
winds. In summer, the central pressure in a strong anticyclone may only be around
1,000 mb. In the middle latitudes these set up when the polar front jet stream is far
poleward and there are no strong winds directly aloft and anticyclones languish
and can become stationary. Such a pattern becomes hotter and drier with time
and is strongly associated with heat waves and droughts.
In the subtropical regions are the semi-permanent anticyclones. These are
known as the subtropical highs within the global circulation. Air in the Hadley cir-
culation leaves the equatorial region at high altitudes in the troposphere and sinks
as it moves poleward. This descending air is profoundly important to world cli-
mates because it causes the annual and seasonal dryness experienced in tropical
and subtropical latitudes. Even though the near-surface air is very hot, its propen-
sity to rise is countered by the strong descent of air from aloft. When the subtropi-
cal high is overhead of aregionthere is usually a paucity of precipitation. Indeed,
the presence of the Sahara, Australian deserts, the Atacama, and the Kalahari share
Anticyclones 15