A cyclone is a storm or system of winds that rotates around a center of low atmospheric pressure. An anticyclone is a system of winds that rotates around a center of high atmospheric pressure. Distinctive weather patterns tend to be associated with both cyclones and anticyclones. Cyclones (commonly known as lows) generally are indicators of rain, clouds, and other forms of bad weather. Anticyclones (commonly known as highs) are predictors of fair weather.
Winds in a cyclone blow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Winds in an anticyclone blow just the opposite. Vertical air movements are associated with both cyclones and anticyclones. In cyclones, air close to the ground is forced inward toward the center of the cyclone, where pressure is lowest. It then begins to rise upward, expanding and cooling in the process. This cooling increases the humidity of the rising air, which results in cloudiness and high humidity in the cyclone.
Common to both cyclones and anticyclones are the characteristic circulation patterns. The geostrophic-wind and gradient-wind models dictate that, in the Northern Hemisphere, flow around a cyclone—cyclonic circulation—is counterclockwise, and flow around an anticyclone—anticyclonic circulation—is clockwise. Circulation directions are reversed in the Southern Hemisphere (see above the diagramsof mean sea-level pressure). In the presence of friction, the superimposed component of motion toward lower pressure produces a “spiraling” effect toward the low-pressure centre and away from the high-pressure centre.
The cyclones that form outside the equatorial belt, known as extratropical cyclones, may be regarded as large eddies in the broad air currents that flow in the general direction from west to east around the middle and higher latitudes of both hemispheres (see below). They are an essential part of the mechanism by which the excess heat received from the Sun in Earth’s equatorial belt is conveyed toward higher latitudes. These higher latitudes radiate more heat to space than they receive from the Sun, and heat must reach them by winds from the lower latitudes if their temperature is to be continually cool rather than cold. If there were no cyclones and anticyclones, the north-south movements of the air would be much more limited, and there would be little opportunity for heat to be carried poleward by winds of subtropical origin. Under such circumstances the temperature of the lower latitudes would increase, and the polar regions would cool; the temperature gradient between them would intensify.
Strong horizontal gradients of temperature are particularly favourable for the formation and development of cyclones. The temperature difference between polar regions and the Equatorbuilds up until it becomes sufficiently intense to generate new cyclones. As their associated cold fronts sweep equatorward and their warm fronts move poleward, the new cyclones reduce the temperature difference. Thus, the wind circulation on Earth represents a balance between the heating effects of solar radiation occurring in the polar regions and at the Equator. Wind circulation, through the effect of cyclones, anticyclones, and other wind systems, also periodically destroys this temperature contrast.
Cyclones of a somewhat different character occur closer to the Equator, generally forming in latitudes between 10° to 30° N and S over the oceans. They generally are known as tropical cyclones when their winds equal or exceed 74 miles (119 km) per hour. They are also known as hurricanes if they occur in the Atlantic Ocean and the Caribbean Sea, as typhoons in the western Pacific Ocean and the China Sea, and as cyclones off the coasts of Australia. These storms are of smaller diameter than the extratropical cyclones, ranging from 100 to 500 km (60 to 300 miles) in diameter, and are accompanied by winds that sometimes reach extreme violence. These storms are more fully described in the article tropical cyclone.
The life cycle of such an event is typically several days, during which the cyclone may travel from several hundred to a few thousand kilometres. In its path and wake occur dramatic weather changes. A typical sequence of weather possibly resulting from the approach and passage of a cyclone and its fronts through an area is depicted in the diagram. Shown in the occluded-front stage of the cyclogenesis diagram is a cross section of the clouds and precipitation that usually occur along line ab. Warm frontal weather is most frequently characterized by stratiform clouds, which ascend as the front approaches and potentially yield rain or snow. The passing of a warm front brings a rise in air temperature and clearing skies. The warmer air, however, may also harbour the ingredients for rain shower or thunderstorm formation, a condition that is enhanced as the cold front approaches.
In anticyclones, the situation is reversed. Air at the center of an anticyclone is forced away from the high pressure that occurs there. That air is replaced in the center by a downward draft of air from higher altitudes. As this air moves downward, it is compressed and warmed.
While cyclones are typically regions of inclement weather, anticyclones are usually meteorologically quiet regions. Generally larger than cyclones, anticyclones exhibit persistent downward motions and yield dry stable air that may extend horizontally many hundreds of kilometres.
In most cases, an actively developing anticyclone forms over a ground location in the region of cold air behind a cyclone as it moves away. This anticyclone forms before the next cyclone advances into the area. Such an anticyclone is known as a cold anticyclone. A result of the downward air motion in an anticyclone, however, is compression of the descending air. As a consequence of this compression, the air is warmed. Thus, after a few days, the air composing the anticyclone at levels 2 to 5 km (1 to 3 miles) above the ground tends to increase in temperature, and the anticyclone is transformed into a warm anticyclone.
Warm anticyclones move slowly, and cyclones are diverted around their periphery. During their transformation from cold to warm status, anticyclones usually move out of the main belt followed by cyclones in middle latitudes and often amalgamate with the quasi-permanent bands of relatively high pressure found in both hemispheres around latitude 20° to 30°—the so-called subtropical anticyclones. On some occasions the warm anticyclones remain in the belt normally occupied by the mid-latitude westerly winds. The normal cyclone tracks are then considerably modified; atmospheric depressions (areas of low pressure) are either blocked in their eastward progress or diverted to the north or south of the anticyclone. Anticyclones that interrupt the normal circulation of the westerly wind in this way are called blocking anticyclones, or blocking highs. They frequently persist for a week or more, and the occurrence of a few such blocking anticyclones may dominate the character of a season. Blocking anticyclones are particularly common over Europe, the eastern Atlantic, and the Alaskan area.
The descent and warming of the air in an anticyclone might be expected to lead to the dissolution of clouds and the absence of rain. Near the centre of the anticyclone, the winds are light and the air can become stagnant. Air pollution can build up as a result. The city of Los Angeles, for example, often has poor air quality because it is frequently under a stationary anticyclone. In winter the ground cools, and the lower layers of the atmosphere also become cold. Fog may be formed as the air is cooled to its dew point in the stagnant air. Under other circumstances, the air trapped in the first kilometre above Earth’s surface may pick up moisture from the sea or other moist surfaces, and layers of cloud may form in areas near the ground up to a height of about 1 km (0.6 mile). Such layers of cloud can be persistent in anticyclones (except over the continents in summer), but they rarely grow thick enough to produce rain. If precipitation occurs, it is usually drizzle or light snow.
Anticyclones are often regions of clear skies and sunny weather in summer; at other times of the year, cloudy and foggy weather—especially over wet ground, snow cover, and the ocean—may be more typical. Winter anticyclones produce colder than average temperatures at the surface, particularly if the skies remain clear. Anticyclones are responsible for periods of little or no rain, and such periods may be prolonged in association with blocking highs.
Cyclone and anticyclone climatology
Migrating cyclones and anticyclones tend to be distributed around certain preferred regions, known as tracks, that emanate from preferred cyclogenetic and anticyclogenetic regions. The contrast between the winter and summer mean sea-level pressure diagrams also indicates the typical cyclone tracks for both January and July. Favoured cyclogenetic regions in the Northern Hemisphere are found on the lee side of mountains and off the east coasts of continents. Cyclones then track east or southeast before eventually turning toward the northeast and decaying. The tracks are displaced farther northward in July, reflecting the more northward position of the polar front in summer. Continental cyclones usually intensify at a rate of 0.5 mb (0.05 kPa) per hour or less, although more dramatic examples can be found. Marine cyclones, on the other hand, often experience explosive development in excess of 1 mb (0.1 kPa) per hour, particularly in winter.
Anticyclones tend to migrate equatorward out of the cold air mass regions and then eastward before decaying or merging with a warm anticyclone. Like cyclones, warm anticyclones also slowly migrate poleward with the warm season.
In the Southern Hemisphere, where most of Earth’s surface is covered by oceans, the cyclones are distributed fairly uniformly through the various longitudes. Typically, cyclones form initially in latitudes 30° to 40° S and move in a generally southeastward direction, reaching maturity in latitudes near 60° S. Thus, the Antarctic continent is usually ringed by a number of mature or decaying cyclones. The belt of ocean from 40° to 60° S is a region of persistent, strong westerly winds that form part of the circulation to the north of the main cyclone centres; These are the “roaring forties,” where the westerly winds are interrupted only at intervals by the passage southeastward of developing cyclones.