Mid-latitude cyclone
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A mid-latitude cyclone is a weather phenomenon associated with atmospheric low pressure that takes place in the temperate region between the tropical and polar regions. Hence, they are also known as temperate cyclones. In the northern hemisphere a cyclone rotates in the counterclockwise direction, while it rotates clockwise in the southern hemisphere. The rotation is caused by the Coriolis effect.
Mid-latitude cyclones are driven by baroclinic processes, that is the temperature contrast between warm and cold air masses.
Characteristics
Warm water at high latitudes provides the energy that brings on the generation of mid-latitude cyclones that exhibit characteristics which differentiate them from extratropical cyclones.- They are tall and often exceed 30,000 feet (c. 10km) in height.
- Above the surface of the earth, the air temperature near the center of the storm is colder than the surrounding air. For that reason, the storms are called "cold-core lows." A suitable chart to examine is the 700 millibars (hectopascals) (hPa) (SI) chart promulgated by [Canada.] The information is at about 10,000 feet or 3,000 meters.
- At first, the storms move from high latitudes to lower latitudes, i.e., they travel towards the southeast over North America and Europe, then eastwards.
- As the storms move across land, the air pressure may fall rapidly to below 980 millibars (980 hectopascals) (hPa) (SI).
The reverse transition of a mid-latitude cyclone to a tropical cyclone occurs occasionally. This will often involve a subtropical storm which has characteristics of both mid-latitude and tropical cyclones.
Formation
An idealized formation model of cyclonic storms was developed by Norwegian Meteorologists during the Second World War. This model is generally referred to as the “Norwegian Model.” An idealized developing storm is shown above. As previously discussed, the storm is initiated at a stationary front between warm and cold air masses shown in Figure A. Take note that these two air masses will be moving in opposite directions, which creates a horizontal velocity shear that is necessary for instability to occur. The cyclonic flow begins around a disturbed section of the stationary front. This disturbance could have a variety of causes, including effects of the ground topography or smaller convective flow. If it is assumed that the conditions are correct for instability to occur then this disturbance will grow into a wave-like formation in the front and a low will develop at the crest. Around this low cyclonic flow will result. This rotational flow will push polar air southward west of the low via a cold front, and tropical air north to the east of the low via a warm front (B). Usually the cold front will move at a quicker pace than the warm front and will “catch up” with it (C). At this point an “occluded front” forms where the warm air mass is pushed upwards into a trough of warm air (D). It is at the time of occlusion that the storm has reached maturity and the cylconic flow is at its most intense. Afterwards the strength of the storm diminishes. It should be pointed out that SSDs rarely diminish due to frictional dissipation. Systems passing over major mountain ranges are a notable exception. Rather the spin-down of cyclones can be understood from an energetics perspective. As occlusion occurs and the warm air mass is pushed upwards by the cold air underneath, the atmosphere becomes increasingly vertically stable and the centre of gravity of the system lowers. As the occlusion process extends further down the warm front and away from the central low, more and more of the available potential energy of the system is exhausted. This potential energy sink creates a kinetic energy source which injects a final burst of energy into the storms motions. After this process occurs, the growth period of the cyclone, or cyclogenisis, ends, and the low begins to spin down. A more dynamical discussion of the occlusion process and its effects on the cyclonic intensities will be dealt with in a later section.
See also
| Cyclones and Anticyclones of the world |
|---|
| Extratropical - Meso-scale - Mid-latitude - Polar - Polar low - Subtropical - Tropical |
- redirect[[Template:Portal]]
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