CONCEPT – TROPICAL CYCLONES

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CONCEPT – TROPICAL CYCLONES

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• What is it? A cyclone is a large scale air mass that rotates around a strong center of low atmospheric pressure. Tropical cyclones form when the energy released by the condensation of moisture in rising air causes a positive feedback loop over warm ocean waters. The cyclones that occur between Tropics of Cancer and Capricorn are known as Tropical Cyclones. Extratropical cyclones begin as waves along weather fronts before occluding later in their life cycle as cold-core systems. However, some intense extratropical cyclones can become warm-core systems when a warm seclusion occurs.
• Formation: Cyclogenesis is the process of cyclone formation and intensification.

1. Extratropical cyclones begin as waves in large regions of enhanced mid-latitude temperature contrasts called baroclinic zones. These zones contract and form weather fronts as the cyclonic circulation closes and intensifies.
2. Later, extratropical cyclones occlude as cold air masses undercut the warmer air and become cold core systems.
3. A cyclone's track is guided over the course of its 2 to 6 day life cycle by the steering flow of the subtropical jet stream.

• Common properties:

1. A cyclone is a low-pressure area.
2. A cyclone's center (the eye) is the area of lowest atmospheric pressure in the region.
3. Near the center, the pressure gradient force (from the pressure in the center of the cyclone compared to the pressure outside the cyclone) and the force from the Coriolis effect must be in an approximate balance, or the cyclone would collapse on itself as a result of the difference in pressure.

• Weather fronts and cyclones: Weather fronts mark the boundary between two masses of air of different temperature, humidity, and densities, and are associated with the most prominent meteorological phenomena.

1. Strong cold fronts feature narrow bands of thunderstorms and severe weather, and may be preceded by squall lines or dry lines. Such fronts form west of the circulation center and generally move from west to east.
2. Warm fronts form east of the cyclone center and are usually preceded by stratiform precipitation and fog.
3. Warm fronts move poleward ahead of the cyclone path.
4. Occluded fronts form late in the cyclone life cycle near the center of the cyclone and often wrap around the storm center.

• Physical properties and types: Cyclones have inward spiraling winds that rotate about a zone of low pressure.

1. The largest low-pressure systems are polar vortices and extratropical cyclones of the largest scale (the synoptic scale).
2. Warm-core cyclones such as tropical cyclones and subtropical cyclones also lie within the synoptic scale.
3. Mesocyclones, tornadoes and dust devils lie within the smaller mesoscale.
4. Upper level cyclones can exist without the presence of a surface low, and can pinch off from the base of the tropical upper tropospheric trough during the summer months in the Northern Hemisphere.

• Other planets: Cyclones exist on extraterrestrial planets, such as Mars and Neptune.

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