Cyclones and anticyclones are the major forces that shape everyday weather. Understanding how these systems behave is critically important to accurate meteorological forecasting. The primary difference between the two types of weather systems is their atmospheric pressure. Cyclones represent areas of low pressure, while anticyclones represent areas of high pressure.
How Cyclones Work
Cyclones are areas of low pressure. Since air moves from areas of high pressure to low pressure, cyclones produce a convergence at the surface. This converging air is forced upwards into the atmosphere, creating a divergence aloft. As warm, moist air is sucked into the low and forced aloft, it produces an unstable atmosphere. This warm, moist air cools, condenses and forms storm clouds. Cyclones can be tropical in nature, such as a hurricane, or a low-pressure system over a land mass, such as the United States. Cyclones spin in a counterclockwise direction in the Northern Hemisphere and clockwise in the Southern Hemisphere.
Effects of Cyclones
In general, cyclones are associated with clouds, rain and thunderstorms. They produce steep pressure gradients, creating strong surface winds. Over the United States, cyclones will draw in warm, moist air from the Gulf of Mexico, creating a warm front. This generally produces light, steady rain to the northeast of a low, ahead of the warm front. Cyclones also draw in cold air from the north. This colder air forms a cold front, which collides with the warm, moist air to produce showers and thunderstorms to the southeast of a low, ahead of the cold front.
How Anticyclones Work
Anticyclones are areas of high pressure. The sinking air spreads out when it reaches the ground, producing a divergence at the surface. Aloft, air rushes in to fill the void, creating a convergence aloft. Anticyclones produce a stable atmosphere. Anticyclones, or highs, are also referred to as blocking highs because they tend to force areas of low pressure to travel around them. For example, a hurricane (tropical cyclone) that encounters an area of high pressure will be deflected around the cyclone. Blocking highs have spared the East Coast of the United States from many hurricane strikes, pushing them out over the Atlantic Ocean. Anticyclones spin in a clockwise direction in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.
Effects of Anticyclones
In general, anticyclones are associated with fair weather. As the air sinks, it warms and dries. This produces clear skies and increases the air's ability to transmit radiant energy. In the summer, this means high temperatures due to solar heating of the surface. During the winter, this means low temperatures due to the radiation of heat from the surface into space. Cyclones typically have low-pressure gradients, producing light, variable winds at the surface. Cyclones tend to be slow movers, providing extended periods of fair weather. During the summer and fall, a Bermuda High can establish itself off the eastern coast of the U.S. for long periods of time, producing high temperatures in the Southeast and blocking hurricanes.
- The United Kingdom Environmental Change Network; Anticyclones (High Pressure); Daniel Jackson; July 1998
- The United Kingdom Environmental Change Network; Cyclones (Low Pressure); Daniel Jackson; July 1998
- The Physical Environment; An Introduction to Physical Geography; Michael E. Ritter, Ph.D.; 2006
- University of Illinois; Online Meteorological Guide; Anticyclones
- University of Illinois; Online Meteorological Guide; Cyclones
- National Weather Service; Glossary; Bermuda High
- Photo Credit SW Productions/Brand X Pictures/Getty Images
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