Cyclone and Anticyclone
The terms cyclone and anticyclone are used to describe areas of low and high atmospheric pressure, respectively. Air flowing around one or the other of these areas is said to be moving cyclonically in the first case and anticyclonically in the second. In the northern hemisphere, cyclonic winds travel in a counterclockwise direction and anticyclonic winds, in a clockwise direction. When a cyclone or anticyclone is associated with a wave front, it is called a wave, a frontal, or a mid-latitude cyclone or anticyclone.
Vertical air movements are associated with both cyclones and anticyclones. In the former case, air close to the ground is forced inward, toward the center of a cyclone, where pressure is lowest, and then begins to rise upward. At some height, the rising air begins to diverge outward away from the cyclone center.
In an anticyclone, the situation is reversed. Air at the center of an anticyclone is forced away from the high pressure that occurs there and is replaced by a downward draft of air from higher altitudes. That air is replaced, in turn, by a convergence of air from higher altitudes moving into the upper region of the anticyclone.
Distinctive weather patterns tend to be associated with both cyclones and anticyclones. Cyclones and low pressure systems are generally harbingers of rain, clouds, and other forms of bad weather, while anticyclones and high pressure systems are predictors of fair weather.
One factor in the formation of cyclones and anticyclones may be the development of irregularities in a jet stream. When streams of air in the upper atmosphere begin to meander back and forth along an east-west axis, they mayadd to cyclonic or anticyclonic systems that already exist in the lower troposphere. As a result, relatively stable cyclones (or anticyclones) or families of cyclones (or anticyclones) may develop and travel in an easterly or northeasterly direction across the continent.
On relatively rare occasions, such storms may pick up enough energy to be destructive of property and human life. Tornadoes and possibly hurricanes are examples of such extreme conditions.