Temperate cyclones, also known as extratropical cyclones, are low-pressure weather systems that form outside of the tropics, typically in the middle and higher latitudes. They are characterized by strong winds, heavy precipitation, and a low-pressure centre.
Temperate cyclones form when a cold air mass and a warm air mass meet, the warm air is forced to rise, and the cold air sinks, this creates a pressure gradient that drives the winds. These are formed in in regions of large horizontal temperature variations called frontal zones.
According to the polar-front theory, extratropical cyclones develop when a wave forms on a frontal surface separating a warm air mass from a cold air mass. As the amplitude of the wave increases, the pressure at the centre of disturbance falls, eventually intensifying to the point at which a cyclonic circulation begins. The decay of such a system result when the cold air from the north in the Northern Hemisphere, or from the south in the Southern Hemisphere, on the western side of such a cyclone sweeps under all of the warm tropical air of the system so that the entire cyclone is composed of the cold air mass. This action is known as occlusion.
TEMPERATE CYCLONES TRACK
BOMB CYCLONES
What defines a bomb cyclone is how rapidly the pressure drops in the low-pressure mass — by at least 24 millibars in 24 hours. This quickly increases the pressure difference, or gradient, between the two air masses, making the winds stronger. This process of rapid intensification has a name: bombogenesis.
They are a type of intense temperate cyclones. Bombogenesis, a term used by meteorologists, occurs when a midlatitude (the latitudes between the tropics and polar regions) cyclone rapidly intensifies, or strengthens, over a 24-hour period. This intensification is represented by a drop in millibars, a measurement of pressure used in meteorology. The intensification required to classify as “bombogenesis” varies by latitude. At 60 degrees latitude, it is a drop of at least 24 millibars (24 hectopascals) over 24 hours. At the latitude of New York City, the required pressure drop is about 17.8 millibars (17.8 hectopascals) over 24 hours.
Bombogenesis can happen when a cold air mass collides with a warm air mass, such as air over warm ocean waters. It is popularly referred to as a bomb cyclone.
As far as the similarities between the two, tropical cyclones and extratropical cyclones are both symmetrical. They also have surface areas of low pressure with winds that rotate counter clockwise. Furthermore, both produce very heavy precipitation and often results in flooding. Both tropical cyclones and mid-latitude cyclones can last for several days, and sometimes as long as a week or more. Often, a tropical cyclone will transform into an extra-tropical cyclone as it recurves poleward. Occasionally, an extra-tropical cyclone may lose its frontal features, develop convection near the centre of the cyclone and turn into a tropical cyclone.
But there are certain differences as well:
| TROPICAL CYCLONES | EXTRA-TROPICAL CYCLONES |
| Formed at Tropical areas | Formed at Mid latitudes |
| Derive their energy from the latent heat of condensation | Derive their energy from horizontal contrast in temperatures. |
| They move from East to West under trade winds | Move from West to East under Sub Tropical Westerlies. |
| Can originate only in deep warm oceans | Can originate over oceans as well as land |
| They have warmer core | They have colder core |
| They have convectional formation | They have frontal formation |
| Its diameter is typically around 150 to 500 km, but can reach 1200 km, they can attain velocity up to 250 km/hr. | Their diameter range from nearly 1,000 to 4,000 km, have avg. velocity of 30-50 km/hr. |
| No temperature variation in different parts. | Have considerable temperature variation because of different fronts (warm & cold) |
