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atmospheric pressure distribution in which there is a low central pressure relative to the surrounding pressure. The resulting pressure gradient, combined with the Coriolis effectCoriolis effect
[for G.-G. de Coriolis, a French mathematician], tendency for any moving body on or above the earth's surface, e.g., an ocean current or an artillery round, to drift sideways from its course because of the earth's rotation.
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, causes air to circulate about the core of lowest pressure in a counterclockwise direction in the Northern Hemisphere and in a clockwise direction in the Southern Hemisphere. Near the surface of the earth, the frictional drag on the air moving over land or water causes it to spiral gradually inward toward lower pressures. This inward movement of air is compensated for by rising currents near the center, which are cooled by expansion when they reach the lower pressures of higher altitudes. The cooling, in turn, greatly increases the relative humidity of the air, so that "lows" are generally characterized by cloudiness and high humidity; they are thus often referred to simply as storms.

According to the theory first proposed by the Norwegian physicist Vilhelm BjerknesBjerknes, Vilhelm Frimann Koren
, 1862–1951, Norwegian physicist and pioneer in modern meteorology. He worked on applying hydrodynamic and thermodynamic theories to atmospheric and hydrospheric conditions in order to predict future weather conditions.
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, the extratropical, or middle-latitude, cyclone originates as a wave, or perturbation, in the polar frontpolar front,
zone of transition between polar and tropical air masses. Its average position during the winter is at about 30° lat. and during the summer at about 60° lat.
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 separating the cold polar easterly winds from the warmer prevailing winds farther toward the equator. This wave, once induced by the opposing air currents, is accentuated by the rotational sense of the circulation, which pumps warm, moist air toward the pole around the eastern side of the cyclone center and cold, dry air toward the equator to the west of the center. Such wave cyclones often intensify, expanding the radius of the affected area to 500 mi (805 km) or more, while reducing atmospheric pressure, especially toward the center.

Tropical cyclones, formed over warm tropical oceans, are not associated with frontsfront,
in meteorology, zone of transition between adjacent air masses. If a cold air mass is advancing to replace a warmer one, their mutual boundary is termed a cold front; if the reverse, then the boundary is termed a warm front, whereas a stationary front indicates that no
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, as are the middle-latitude wave cyclones, nor are they as large as the latter. A tropical cyclone that has matured to a severe intensity is called a hurricanehurricane,
tropical cyclone in which winds attain speeds greater than 74 mi (119 km) per hr. Wind speeds gust over 200 mi (320 km) per hr in some hurricanes. The term is often restricted to those storms occurring over the N Atlantic Ocean; the identical phenomenon occurring over
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 when it occurs in the Atlantic Ocean or adjacent seas, a typhoon when it occurs in the Pacific Ocean or adjacent seas, or simply a cyclone or tropical cyclone when it occurs in the Indian Ocean region.

Cyclones in middle latitudes move generally from west to east along with the prevailing winds and cover 500 to 1,000 mi (800–1,610 km) each day; tropical cyclones usually move toward the west with the flow of the trade winds during their formative stages, then curve toward the poles around subtropical anticyclonesanticyclone,
region of high atmospheric pressure; anticyclones are commonly referred to as "highs." The pressure gradient, or change between the core of the anticyclone and its surroundings, combined with the Coriolis effect, causes air to circulate about the core in a clockwise
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See D. Longshore, Encyclopedia of Hurricanes, Typhoons, and Cyclones (1998).



in industry, a device for removing suspended solid particles or droplets from air or a gas by means of centrifugal force (Figure 1).

A dust-laden gas stream usually enters the upper cylinder of a cyclone through an inlet at high speed; the inlet may be situated at a tangent to the cylindrical surface of the cyclone or may be involute. As a result, the gas acquires rotational motion and spirals

Figure 1. A cyclone: (a) general view, (b) schematic diagram; (1) lower cone, (2) upper cylinder, which forms an annular space, (3) outlet pipe, (4) metal canopy

from the top to the bottom, forming an outside vortex. As the gas spirals downward, the suspended particles are flung toward the walls of the cyclone by the inertial force. The particles spiral downward with the gas stream to the bottom of the cyclone and are then removed through a dust outlet. The cleaned gas spirals upward through an outlet pipe, forming an inside vortex, and leaves the device.

Cyclones in which the gas enters axially are also used. In such devices, rotational motion is imparted to the gas stream by a guide vane, which may be a screw or an impeller with canted blades.

The extent to which dust can be removed from a gas by a cyclone depends on the geometric dimensions and the shape of the cyclone, the properties of the dust, the velocity of the gas stream, and certain other factors. The collection of particles in a cyclone may be improved by increasing the velocity of the gas stream (the most effective velocities range from 20 to 25 m/sec) or by reducing the diameter of the cyclone. Therefore, an arrangement of several cyclones in parallel is used to obtain a high collection efficiency when a large amount of gas is to be cleaned. In cyclones of the most advanced design, particles with a diameter of 5 micrometers or larger may be collected with a sufficiently high efficiency. (See alsoLIQUID CYCLONE.)



an atmospheric disturbance characterized by low pressure at the center and vortical movements of air. A distinction is made between extratropical and tropical cyclones. The latter have special properties and occur less frequently (see).

The barometric pressure in a cyclone is lowest at its center and increases toward the periphery; that is, the horizontal pressure gradients are directed from outside the cyclone inward. In a mature cyclone, the pressure at the center at sea level may drop to 960–950 millibars (1 bar = 105 newtons/m2) and occasionally down to 930–920 millibars; the average pressure at sea level is about 1,012 millibars. Closed isobars (lines of equal pressure), shaped like an irregular oval, bound the region of low pressure, which may be anywhere from a few hundred to 2,000–3,000 km in diameter and in which the air is moving in a vortex. In the free atmosphere, above the boundary layer, which is approximately the first 1,000 m, the air moves approximately along the isobars, deflecting from the pressure gradient at an angle close to 90° to the right in the northern hemisphere and to the left in the southern hemisphere; this occurs as a result of the Coriolis force and the centrifugal force that arises during movement along curved trajectories. In the boundary layer, the force of gravity causes the wind to deviate to some extent, depending on elevation, from the isobars toward the pressure gradient. At the surface of the earth, the wind and the pressure gradient form an angle of about 60°; that is, the flow of air toward the center of the cyclone is joined to the vortical movement of the air. The lines of flow resemble spirals, converging at the center of the cyclone. The wind velocities in a cyclone are greater than in adjacent regions of the atmosphere, sometimes reaching more than 20 m/sec (storm) and even 30 m/sec (hurricane).

Cloudy weather predominates in a cyclone because of the ascending components of air movement, particularly near the fronts. Cyclones are responsible for most of the precipitation occurring in the extratropical latitudes. As a result of the vortical air movements, air masses of different temperatures from different latitudes of the earth are drawn into the cyclone region. The temperate differences within a cyclone are related to this. This applies particularly to traveling cyclones that occur along the main fronts of the troposphere (the arctic, antarctic, and polar fronts). Weak cyclones are also observed above the warm parts of the earth (deserts and inland seas). Called thermal lows, such cyclones are relatively immobile and have a fairly even temperature distribution.

At higher elevations, the isobars of a cyclone gradually lose their closed form. This occurs in different ways, depending on the stage of development of the cyclone and the temperature distribution within it. In the first stage of development, a traveling (frontal) cyclone engulfs only the lower part of the troposphere, while at its stage of greatest development, it may spread through the entire troposphere and even extend into the lower stratosphere. Thermal lows are always restricted to the lower troposphere.

Traveling cyclones generally move from west to east. The direction of movement of a given cyclone is determined by the direction of the general transfer of air in the upper troposphere. Movement in the opposite direction, that is, from east to west, is rare. The average velocity of a cyclone’s movement is 30–45 km/hr, but there are cyclones that move faster, up to 100 km/hr, especially in the initial stages of development. In the concluding stage, a cyclone may remain immobile for a long time. The movement of a cyclone through a certain region causes sharp and significant local changes not only in atmospheric pressure and wind but also in air temperature, humidity, cloudiness, and precipitation.

Traveling cyclones usually develop along the main fronts of the troposphere, which have originated earlier; they are wave-like disturbances as the air flows along both sides of the front. Unstable frontal waves grow into cyclonic vortices. Moving along the front, which is usually extended latitudinally, the cyclone itself alters the front, giving rise to meridional components of the wind and thus promoting the transfer of the warm air in the forward (eastern) part of the cyclone toward the higher latitudes and the transfer of the cold air in the rear (western) part toward the lower latitudes. In the southern part of the cyclone, a warm sector, which is bounded by a warm front and a cold front, is created in the lower layers (the stage of a young cyclone). Then, when the cold and warm fronts merge (occlusion of the cyclone), the warm air is forced away from the earth’s surface by the cold air into the higher layers. The warm sector is eliminated, and a more even temperature distribution is established in the cyclone (the stage of an occluded cyclone). The reserve of energy capable of being converted into kinetic energy runs out in the cyclone, and thus the cyclone dissipates or merges with another cyclone.

A series, or family, of cyclones, consisting of several cyclones moving one after another, usually develops along a main front. At the end of the series’ development, individual cyclones that have not yet dissipated merge together into a vast, relatively immobile, deep, and high central cyclone consisting of cold air throughout its height. This cyclone also gradually dissipates. Anticyclones, with high pressure at the center, develop between cyclones at the same time as the cyclones form. The entire process of a particular cyclone’s development takes several days; a cyclone series and the main cyclone may exist for one or two weeks. At any given moment, there are several main fronts and the cyclone series associated with them in each hemisphere; the total number of cyclones in a given year runs into several hundred over each hemisphere.

There are certain latitudes and regions in which the formation of main fronts and frontal disturbances occurs quite regularly. As a result, there are certain geographic regularities in the frequency of occurrence and movement of individual cyclones and anticyclones and cyclone and anticyclone series; that is, there exist patterns of cyclonic activity. However, the effects of land and sea, topography, orography, and other geographical factors on the formation and movement of cyclones and anticyclones and their interaction make the general picture of cyclonic activity quite complex and fast-changing. Cyclonic activity leads to interlatitudinal exchange of air, movement, heat, and moisture, which makes it important in the atmospheric general circulation.


Palmén, E., and C. Newton. Tsirkuliatsionnye sistemy atmosfery. Leningrad, 1973. (Translated from English.)
Petterssen, S. Analiz i prognoz pogody. Leningrad, 1961. (Translated from English.)
Khromov, S. P. Osnovy sinopticheskoi meteorologii. Leningrad, 1948.
Zverev, A. S. Sinopticheskaia meteorologiia i osnovy predvychisleniia pogody. Leningrad, 1968.
Pogosian, Kh. P. Tsiklony. Leningrad, 1976.



(chemical engineering)
A static reaction vessel in which fluids under pressure form a vortex.
(mechanical engineering)
Any cone-shaped air-cleaning apparatus operated by centrifugal separation that is used in particle collecting and fine grinding operations.
A low-pressure region of the earth's atmosphere with roundish to elongated-oval ground plan, in-moving air currents, centrally upward air movement, and generally outward movement at various higher elevations in the troposphere.


cycloneclick for a larger image
A severe weather system with low central pressure. In the fully matured stage, it develops an eye at the center where there is no weather. The eye is surrounded by an inner circle of intense winds and rotating cloud mass. Around the inner circle is the outer circle, which has less fierce weather. The diameter of a cyclone can extend up to 320 miles (500 km). These are known as cyclones if they form in the Bay of Bengal and the Arabian Sea, where the wind speed is between 34 and 64 knots. The same name applies to the storms in the South Pacific region but when the speed exceeds 65 knots. All cyclones are given names by the World Meteorological Organization (WMO).


1. another name for depression
2. a violent tropical storm; hurricane
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