great storms, anticyclones are the cause of bright, dry weather. An anticyclone is a
center of high pressure and is the pressure analog to a topographic hill. On weather
maps, the highest pressure in an anticyclone is labeled with an “H” and it is
common for publicly presented maps to dispose of the isobars to concentrate on
the “H.” There is no quantitative rule as to how high, absolutely or relatively, the
pressure must be for an anticyclone to exist. It is sufficient that there are closed
isobars that engender a circulation. Although anticyclones may be circular, they
can take on all sorts of irregular shapes.
Air nearEarth’ssurface rearranges itself from the center of high pressure to the
lower pressure on the fringes, but not directly. The Coriolis Effect and force of
friction modify surface air from moving in the direction of the pressure gradient
force. In the Northern Hemisphere, the flow is from the high to low pressure side
of the isobars at an acute angle so as to set up a clockwise circulation. In the
Southern Hemisphere, the opposite Coriolis Effect causes counterclockwise circu-
lation. The air that leaves the anticyclone center lowers the center’s pressure and
helps air to descend from the upper troposphere.
In the middle and upper levels of the troposphere, there is no friction, so anticy-
clonic winds are the result of pressure gradient and Coriolis Effect together. At
altitude, anticyclonic winds are parallel to the isobars and flow clockwise.
Anticyclones are stable as compared with their unstable cyclone counterparts.
That is, anticyclonic systems invariably resist the rise of air from Earth’s surface
so as to inhibit cloud development and production ofprecipitation. Precipitation
can occur when the air is stable, but the precipitation will tend to be light because
of the relative lack of uplift and the lack of moisture.
The strongest incarnations of anticyclones are those that build up in the Arctic,
Antarctic, and Subarctic air mass source regions in the winter time and become
Arctic, Antarctic, and Polarair masses. Over a course of weeks these air masses
are generated and portions of the air masses break loose under the influence of
the polar front jet stream to visit the middle latitudes and, occasionally, subtropical
latitudes. Unlike cyclones that are associated with the mixing of two or more air
masses, anticyclones are composed of single air masses. Their leading edges are
cold fronts that usher in changes in wind direction, lower humidity, and colder
temperatures. However, behind cold fronts are loosely spaced isobars of anticy-
clones making for light and variable winds. The calm and stable air near anticy-
clonic centers is quite dry,usually cloudless, and optimal for the nighttime loss
of longwave radiation to space. The result is the coldest air experienced in the
middle latitudes. Indeed, record cold middle latitude temperatures are usually
associated with anticyclones. It might be thought that anticyclones are types of
blizzards, but this is not so; the cold air at the front edge of the anticyclone helps
to cause lift that might be associated with a blizzard by interacting with middle14 Anticyclones