Cyclones and Anticyclones

Cyclones and anticyclones are large-scale weather systems with opposite properties. A cyclone is characterized by a central region of low atmospheric pressure, and a central region of high atmospheric pressure characterizes an anticyclone. Because cyclones are a significant cause of stormy weather and anticyclones typically bring good weather, accurate meteorological predictions are greatly informed by understanding how these weather systems originate and develop.

Atmospheric Pressure and Air Circulation

Atmospheric pressure describes the physical force exerted by the weight of the air above a given area on the Earth’s surface. Meteorologists map pressure distributions with isobars, which appear as a series of curved lines connecting points with the same atmospheric pressure. This pressure is measured in units called millibars.

Under average conditions, the sea-level atmospheric pressure is approximately 1,013.2 millibars. Values greater than the average sea-level pressure are considered high, and those lower than the average sea-level pressure are considered low. Pressure gradients are horizontal or vertical differences in atmospheric pressure. Pressure gradients create winds because air is constantly moving from areas of high pressure to areas of low pressure, seeking to create equilibrium. In other words, pressure gradients cause air to move perpendicular to isobars.

Pressure gradients, however, are not the only force at work. A phenomenon known as the Coriolis effect also affects the way air circulates in the atmosphere. The Coriolis effect is an apparent force that acts on moving objects, such as air masses, in a rotating system, such as the rotating Earth. The result is that the moving object shifts perpendicular to the axis of this rotation.

To understand the Coriolis effect, one can imagine trying to throw a ball in a straight line from the North Pole to the equator. Because the Earth is wider at the equator than at the poles, points at the equator must travel a greater distance than points at the poles in the same period. A ball thrown from the North Pole to the equator would thus appear to bend to the right. Similarly, when a mass of air is moving in the Northern Hemisphere, the Coriolis force appears to deflect that mass toward the right. When a mass of air is moving in the Southern Hemisphere, the Coriolis force appears to deflect that mass toward the left.

The force arising from pressure gradients and the force associated with the Coriolis effect are roughly equal in magnitude, and at the upper levels of the atmosphere, they balance each other to create winds that travel more or less parallel to isobars. Friction, or air resistance, reduces the effects of the Coriolis force at the earth’s surface.

Cyclones and Their Formation

Cyclones and anticyclones are large-scale weather systems shaped by atmospheric pressure gradients, the Coriolis effect, and surface friction. A cyclone has a central region of low atmospheric pressure with winds circulating around that center. On a weather chart, a cyclone appears as a series of roughly circular or oval isobars; the area inside the innermost isobar is the region of lowest pressure. Isobars that take this particular configuration are known as troughs.

The direction in which a cyclone’s winds circulate depends on the hemisphere in which the weather system forms. In the Northern Hemisphere, a cyclone has winds that move counterclockwise. The reverse is true in the Southern Hemisphere. Because cyclones cause severe stormy weather, including blizzards, nor'easters, and floods, meteorologists creating detailed and accurate weather forecasts pay close attention to how these atmospheric systems originate and develop.

The atmospheric events that occur as a cyclone form are known collectively as cyclogenesis. Mid-latitude cyclones occur between 30 and 60 degrees north and south of the equator and are about 1,000 kilometers (621 miles) or more in diameter. These typically form at fronts, or the boundaries or transition zones between two air masses with different temperatures and densities. At this first stage of cyclogenesis, the heavy cold air and lighter warm air are simply pushing against each other. Because the air masses are not moving, the place they meet is known as a stationary front.

At the next stage, the cold air—denser and heavier—begins to sink below the light warm air. In turn, the light, warm air is forced upward and then over the cold and heavy air mass. Instead of a single stationary front, two fronts are formed: one consisting of the advancing edge of the cold air and one consisting of the advancing edge of the warm air. Because of the Coriolis force, these masses of cold and warm air do not simply exchange places vertically but begin to revolve around each other, turning inward toward the area of low pressure in the center of the rotation. This pattern of winds causes warm air to “pile up” in the cyclone's center, near the surface. This phenomenon is known as convergence.

When air converges low to the ground, it has nowhere to move but up and out. As the warm-air front rises and expands (diverges), it carries water vapor that cools and condenses into clouds and rain. Different characteristic weather patterns are seen along each front. Brief, intense thunderstorms tend to form along the cold front, and slow, steady rains tend to fall along the warm front. When the cyclone nears its end, the cold front pushes on the warm front so much so that the mass of warm air is entirely separated from the low-pressure center. This is known as an occluded front and is usually associated with more rainy weather.

Anticyclones

As its name suggests, an anticyclone has properties opposite to a cyclone's. A cyclone consists of winds circulating around a center of low atmospheric pressure, while an anticyclone has a center of high atmospheric pressure, with winds circulating around that center. In the Northern Hemisphere, an anticyclone has winds that move clockwise; the reverse is true in the Southern Hemisphere.

Like a cyclone, an anticyclone appears on a weather chart as a series of roughly circular or oval isobars; however, in an anticyclone, the area bounded by the innermost isobar is the region of highest pressure. When isobars take this configuration, they are known as ridges.

An anticyclone forms when dense cold-air masses in the upper atmosphere converge, or pile up. When this convergence reaches a high enough level, the air begins to sink to the earth’s surface. As it descends, the air is compressed by increasing pressure and becomes warmer and dryer. Anticyclones are generally associated with clear weather.

Tropical Cyclones

Tropical cyclones, known as typhoons in the western Pacific Ocean and as hurricanes in the Atlantic and eastern Pacific Oceans, form in the tropics (between 23.5 degrees north and south of the equator). They are typically smaller than mid-latitude cyclones but are characterized by extremely high winds, usually exceeding speeds of 119 km (74 mi) per hour.

Because of these winds and the intense thunderstorms, occasional flash floods, and storm surges associated with them, tropical cyclones can cause great damage to life and property in coastal areas. Storm surges refer to seawater pushed inland by strong winds.

Tropical cyclones differ from mid-latitude cyclones because they are not associated with a front. Instead, this type of cyclone arises when each of a set of specific environmental conditions is present. The ocean waters above which a potential cyclone would form must be at least 26 degrees Celsius (79 degrees Fahrenheit) up to a depth of 46 meters (about 150 feet) or more. This condition makes it natural that tropical cyclones tend to originate relatively near the equator.

The air near the middle of the troposphere, the lowest region of the Earth’s atmosphere, must be moist. There must be relatively low vertical wind shear between the ocean’s surface and the upper levels of the troposphere, meaning that wind speeds must not change radically as they ascend. Air must be able to cool relatively quickly as it ascends. Finally, some kind of atmospheric disturbance, or “seedling,” must exist, such as a trough or an elongated area of low pressure.

If all these conditions are present, a tropical cyclone may begin to form, first with the establishment of a pattern of convection over the ocean. Warm and moist air rises into the atmosphere, cooling, and condensing. As the air releases heat, it becomes even lighter, thus powering its own ascent. The water vapor that is released forms the clouds and thunderstorms commonly associated with tropical cyclones. As air moves up and away from the surface, more air rushes in to take its place, creating high winds at the surface. This process creates a self-sustaining cycle that can cause the cyclone to grow and intensify as long as it remains over the water. Usually, however, tropical cyclones begin to dissipate as soon as they move inland because the cycle is broken when the storm system no longer has access to the warm, moist air that moves over the ocean.

Global surface temperatures, including the sea surface temperatures, have been rising on average, primarily because humans are introducing higher concentrations of carbon dioxide, methane, and other heat-trapping greenhouse gases into the atmosphere. Because of this, larger areas of water have reached the minimum temperature for cyclone formation, resulting in tropical cyclones forming increasingly closer to the poles between the early 1980s and early 2010s. Researchers predict greater evaporation from warmer sea surface water, making for moister, warm air over the oceans. At the same time, the stratosphere, the middle region of the Earth's atmosphere, has been cooling. Thus, tropical cyclones will likely be more intense as they gather more moisture from the air and water, resulting in more precipitation and higher wind speeds from the larger pressure gradient. Mid-latitude storms may behave similarly, with nor'easters producing more snow, for instance. Rising sea levels will also likely increase storm surges when cyclones approach land.

As global climate change continues to be a dire environmental issue in the twenty-first century, its effects on the intensity, frequency, and behavior of cyclones and anticyclones are becoming increasingly evident. As anthropogenic climate change proliferates, warming temperatures produce increased rain and stronger winds. Storms move more slowly, causing a stalling process related to warming temperatures. Areas experiencing hurricanes have more intense storms in categories 4 and 5. Rising sea levels related to global climate change only intensify cyclones and anticyclones further. The consequences of global climate change on tropical and extratropical cyclones and anticyclones have dire economic, social, and environmental effects. 

Principal Terms

convergence: a tendency of air masses to accumulate in a region where more air is flowing in than is flowing out

Coriolis effect: the illusion of deflection observed when a body moves through the atmosphere with regard to an individual situated on the moving surface of the Earth

cyclogenesis: the series of atmospheric events that occur during the formation of a cyclone weather system

divergence: a tendency of air masses to spread in a region where more air is flowing out than is flowing in

front: the boundary between two masses of air with different densities and temperatures; usually named for the mass that is advancing (for example, in a cold front, the mass that is colder is moving toward a warmer mass)

hurricane: a cyclone that is found in the tropics (between 23.5 degrees north and south of the equator) and that has winds that are equal to or exceed 64 knots, or 74 miles per hour

isobar: on a map, a line connecting two or more points that share the same atmospheric pressure, either at a particular time or, on average, in a particular period

mid-latitude cyclone: a synoptic-scale cyclone found in the mid-latitudes (between 30 and 60 degrees north and south of the equator)

synoptic scale: a scale used to describe high- and low-pressure atmospheric systems that have a horizontal span of 1,000 kilometers (621 miles) or more

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