What is a cyclone and an anticyclone?
An anticyclone (that is, opposite to a cyclone) is a weather phenomenon defined by the United States National Weather Service's glossary as "a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere".
When the wind swirls clockwise in the northern hemisphere or counter-clockwise in the southern hemisphere, it is called anticyclonic flow. An example of cyclonic flow is the flow around a low pressure area while an example of anticyclonic flow is the flow around a high pressure area. A hurricane is a cyclone.
- The absence of fronts means winds may be very light. Consequently, high-pressure areas are often associated with settled, dry and bright conditions. In summer, anticyclones bring dry, hot weather. In winter, clear skies may bring cold nights and frost.
- Hurricanes, typhoons and cyclones are all rotating storms spawned in the tropics. As a group, they can be referred to as tropical cyclones. Because of the Coriolis effect, these storms rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
- A monsoon is a seasonal shift in the prevailing wind direction, that usually brings with it a different kind of weather. The Indian Ocean version of the hurricane, which is traditionally called a "cyclone" in the Indian Ocean, can also form and move ashore in association with the onset of the monsoon.
An anticyclonic tornado is a tornado which rotates in a clockwise direction in the Northern Hemisphere and a counterclockwise direction in the Southern Hemisphere. The term is a naming convention denoting the anomaly from normal rotation which is cyclonic in upwards of 98 percent of tornadoes.
- Tornadoes can appear from any direction. Most move from southwest to northeast, or west to east. Some tornadoes have changed direction amid path, or even backtracked. [A tornado can double back suddenly, for example, when its bottom is hit by outflow winds from a thunderstorm's core.]
- In the northern hemisphere, the low pressure systems that spawn tornadoes almost always rotate counter-clockwise because of the Coriolis effect, so that explains how a tornado's movement is indirectly affected by the Coriolis effect.
- A tornado is not necessarily visible; however, the intense low pressure caused by the high wind speeds (as described by Bernoulli's principle) and rapid rotation (due to cyclostrophic balance) usually cause water vapor in the air to condense into cloud droplets due to adiabatic cooling.
Areas of high pressure are called anticyclones, whilst low pressure areas are known as cyclones or depressions. Each brings with it different weather patterns. Anticyclones typically result in stable, fine weather, with clear skies whilst depressions are associated with cloudier, wetter, windier conditions.
- Tropical cyclone rainfall forecasting involves using scientific models and other tools to predict the precipitation expected in tropical cyclones such as hurricanes and typhoons. Knowledge of tropical cyclone rainfall climatology is helpful in the determination of a tropical cyclone rainfall forecast.
- How do cyclones and anticyclones affect the weather? The rising air of a cyclone often causes stormy weather. The sinking air of an anticyclone often causes dry, clean weather. Air masses that form over the ocean generally bring precipitation.
- As the warm air mass rises, it condenses into a broad area of clouds. A warm front brings gentle rain or light snow, followed by warmer, milder weather. Stationary front Forms when warm and cold air meet and neither air mass has the force to move the other. They remain stationary, or “standing still.”
Updated: 3rd October 2019