Cyclone
An area of closed pressure circulation with rotating and converging winds, the center of which is a relative pressure minimum. The circulation is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Also called a low pressure system and the term used for a tropical cyclone in the Indian Ocean. Other phenomena with cyclonic flow may be referred to by this term, such as dust devils, tornadoes, and tropical and extratropical systems. The opposite of an anticyclone or a high pressure system.
Pressure
The force per unit area exerted by the weight of the atmosphere above a point on or above the earth’s surface.
Convergence
Wind movement that results in a horizontal net inflow of air into a particular region. Convergent winds at lower levels are associated with upward motion.
Wind
Air that flows in relation to the earth’s surface, generally horizontally. There are four areas of wind that are measured: direction, speed, character (gusts and squalls), and shifts. Surface winds are measured by wind vanes and anemometers, while upper level winds are detected through pilot balloons or aircraft reports.
Minimum
The least value attained by a function, for example, temperature, pressure, or wind speed.
Circulation
The flow or motion of a fluid in or through a given area or volume. In meteorology, it is used to describe the flow of air as it moves around a pressure system in the atmosphere. It describes smaller patterns in semi-permanent pressure systems as well as the relatively permanent global currents of air. In oceanic terms, it is used to describe a water in current flow within a large area, usually a closed circular pattern such as in the North Atlantic.
Low Pressure System
An area of a relative pressure minimum that has converging winds and rotates in the same direction as the earth. This is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Also known as an cyclone, it is the opposite of an area of high pressure, or a anticyclone.
Tropical Cyclone
A warm core low pressure system which develops over tropical, and sometimes subtropical, waters, and has an organized circulation. Depending on sustained surface winds, the system is classified as a tropical disturbance, a tropical depression, a tropical storm, or a hurricane or typhoon.
Cyclonic Flow
Winds that blow in and around a cyclone, that is counterclockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere.
Dust Devil
A small, rapidly rotating column of wind, made visible by the dust, dirt or debris it picks up. It usually occurs in arid or semi-arid areas and is most likely to develop on clear, dry, hot afternoons in response to surface heating.
Tornado
A violently rotating column of air in contact with and extending between a convective cloud and the surface of the earth. It is the most destructive of all storm-scale atmospheric phenomena. They can occur anywhere in the world given the right conditions, but are most frequent in the United States in an area bounded by the Rockies on the west and the Appalachians in the east.
Tropical Cyclone
A warm core low pressure system which develops over tropical, and sometimes subtropical, waters, and has an organized circulation. Depending on sustained surface winds, the system is classified as a tropical disturbance, a tropical depression, a tropical storm, or a hurricane or typhoon.
Extratropical Cyclone
Any cyclone that is no longer tropical in origin. Generally considered to be a migratory frontal cyclone found in the middle and high latitudes.
Anticyclone
A relative pressure maximum. An area of pressure that has diverging winds and a rotation opposite to the earth’s rotation. This is clockwise the Northern Hemisphere and counter-clockwise in the Southern Hemisphere. It is the opposite of an area of low pressure, or a cyclone.
High Pressure System
An area of relative pressure maximum that has diverging winds and a rotation opposite to the earth’s rotation. This is clockwise the in Northern Hemisphere and counterclockwise in the Southern Hemisphere. It is the opposite of an area of low pressure or a cyclone.