Anticyclones

First conceived by Sir Francis Galton in the 1860s, anticyclones are, in some senses, the opposite of cyclones. Where cyclones are associated with the world’s 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 near Earth’s surface 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 circulation. 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 anticyclonic 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 of precipitation. 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.