Cyclone Gelane
Cyclones are an area of closed fluid motion that is characterized by inwardly spiraling winds that rotate counter clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. In general, most large cyclones are centered on low pressure systems. Cyclones are not singular to Earth. They have been spotted on Mars and Neptune.
The largest low-pressure systems are cold-core polar cyclones and extratropical cyclones. Warm-core cyclones such as tropical cyclones, mesocyclones, and polar lows are usually the smallest cyclones. Subtropical cyclones are of a medium size. Upper level cyclones can exist without the presence of a surface low pressure system.
There are a number of common characteristics in all cyclones. Their center is the area of lowest atmospheric pressure in the region, often known as the eye. The pressure gradient force (pressure in the center compared to the pressure outside) and the Coriolis force must be in balance or the cyclone would collapse on itself.
Cyclogenesis is a general term for several processes that result in the development of a cyclone. It can occur at various scales, from the microscale to the synoptic scale(largest cyclones). Extratropical cyclones form as waves along frontal systems before becoming cold core cyclones. Tropical cyclones form due to latent heat driven by significant thunderstorm activity. Mesocyclones form as warm core cyclones over land and can lead to tornado or waterspout formation.
The surface low has a variety of ways of forming. Small scale convective systems can spawn surface lows which are initially warm core. The disturbance can grow into a wave-like formation along the front and the low will be at the crest. Air flow will become cyclonic. The rotational flow will push polar air towards the equator with the trailing cold front. The warm front will push warm air towards the pole. The cold front will move faster than the warm front due to the slow erosion of the higher density air mass located in front of the cyclone and the higher density air mass sweeping in behind. Eventually, the warm air mass is pushed upwards into a trough of warm air known as a trowal.
Tropical cyclogenesis occurs a bit differently than mid-latitude cyclogenesis. Tropical cyclogenesis creates a warm core cyclone because of marked convection in favorable atmospheric conditions. There are six main requirements for tropical cyclogenesis to occur: sufficiently warm sea surface temperatures, atmospheric instability, high humidity in the lower to middle levels of the troposphere, enough Coriolis force to develop a low pressure center, a preexisting low level disturbance(storm), and very little vertical wind shear.
These are some of the most dangerous storms on Earth. They cause billions of dollars in damage each year.
We have written many articles about cyclones for Universe Today. Here’s an article about the matching cyclone found at Saturn’s North Pole, and here are some satellite views of deadly cyclone in Myanmar.
If you’d like more info on the cyclone, check out Visible Earth Homepage. And here’s a link to NASA’s Earth Observatory.
We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.
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