tropical cyclones

Tropical Cyclones

Formation

Cyclones are formed from simple thunderstorms. However, these thunderstorms can only grow to cyclone strength with cooperation from both the ocean and the atmosphere. First of all, the ocean water itself must be warmer than 26.5 degrees Celsius (81°F). The heat and moisture from this warm water is ultimately the source of energy for cyclones. Cyclones will weaken rapidly when they travel over land or colder ocean waters -- locations where their heat and/or moisture sources do not exist.

Related to having warm ocean water, high relative humidities in the lower and middle troposphere are also required for cyclone development. These high humidities reduce the amount of evaporation in clouds and maximizes the latent heat released because there is more precipitation.

The vertical wind shear in a tropical cyclone's environment is also important. Wind shear is defined as the amount of change in the wind's direction or speed with increasing altitude.

When the wind shear is weak, the storms that are part of the cyclone grow vertically, and the latent heat from condensation is released into the air directly above the storm, aiding in development. When there is stronger wind shear, this means that the storms become more slanted and the latent heat release is dispersed over a much larger area.

Stages of Development

Stages of Development from tropical depression to cyclone

Cyclones evolve through a life cycle of stages from birth to death. A tropical disturbance in time can grow to a more intense stage by attaining a specified sustained wind speed.

Cyclones can often live for a long period of time -- as much as two to three weeks. They may initiate as a cluster of thunderstorms over the tropical ocean waters. Once a disturbance has become a tropical depression, the amount of time it takes to achieve the next stage, tropical storm, can take as little as half a day up to a couple of days. It may not happen at all. The same may occur for the amount of time a tropical storm needs to intensify into a cyclone. Atmospheric and oceanic conditions play the major role in determining these events.

Below, in this satellite image from 1995, we can see different tropical disturbances in each stage are evident. At the far left, Tropical storm Jerry is over Florida, while Hurricanes Iris and Humberto are further east, amongst a couple of tropical depressions.

Movement of Cyclones

Movement of Cyclones steered by the global winds

The global wind pattern is also known as the "general circulation" and the surface winds of each hemisphere are divided into three wind belts:

Polar Easterlies: From 60-90 degrees latitude.

Prevailing Westerlies: From 30-60 degrees latitude (aka Westerlies).

Tropical Easterlies: From 0-30 degrees latitude (aka Trade Winds).

The easterly trade winds of both hemispheres converge at an area near the equator called the "Intertropical Convergence Zone (ITCZ)", producing a narrow band of clouds and thunderstorms that encircle portions of the globe.

The path of a cyclone greatly depends upon the wind belt in which it is located. A cyclone originating in the eastern tropical Pacific, for example, is driven westward by easterly trade winds in the tropics.

Eventually, these storms turn northwestward around the subtropical high and migrate into higher latitudes.

In time, cyclones move into the middle latitudes and are driven northeastward by the westerlies, occasionally merging with midlatitude frontal systems.

Cyclones draw their energy from the warm surface water of the tropics and latent heat of condensation, which explains why cyclones dissipate rapidly once they move over cold water

Occluded Front

when a cold front overtakes a warm front

A developing cyclone typically has a preceding warm front (the leading edge of a warm moist air mass) and a faster moving cold front (the leading edge of a colder drier air mass wrapping around the storm). North of the warm front is a mass of cooler air that was in place before the storm even entered the region.

As the storm intensifies, the cold front rotates around the storm and catches the warm front. This forms an occluded front, which is the boundary that separates the new cold air mass (to the west) from the older cool air mass already in place north of the warm front. Symbolically, an occluded front is represented by a solid line with alternating triangles and circles pointing the direction the front is moving.

tropical cyclones