Atmosphere And Weather

How do Hurricanes Form



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Hurricanes are one of the most fascinating yet most destructive of meteorological phenomena.  Though there is generally several days warning time when a hurricane is approaching, mankind is still powerless to stop it.  There is little to do but evacuate or take cover, and then to watch in awe as nature unleashes its fury.

While they still don’t know how to stop or control hurricanes, in recent years scientists have gained a far greater understanding of how hurricanes work, including how they form.

There are six known factors that contribute to the formation of a hurricane, some more necessary than others.

1.  Hurricanes need some form of atmospheric disturbance in the area.

The seed that has the potential to become a hurricane is an atmospheric disturbance that develops into a low pressure area.  Winds then naturally flow from surrounding high pressure areas toward the low pressure area, which can become the center (called the "eye") of a hurricane.


2.  Hurricanes need warm water.

Hurricanes form over water, but not any water.  The regional water temperature must be 80 F or more.  Thus hurricanes are a tropical phenomenon, and not a polar one.


3.  Hurricanes need warmth underwater.

Not only must the surface of the water be at least 80 F, so must the water underneath the surface.  Evidence indicates that warm water of this temperature must go down at least 150 feet.  Warm water that goes at least that deep creates enough instability in the atmosphere to form thunderstorms and convection activity.  As the warm air from the warm water rises and forms clouds, it cools and releases energy.  This energy is what feeds a hurricane.


4.  Hurricanes need humidity.

High humidity enables continued cloud formation, which sustains this cycle that gives the hurricane its energy.


5.  Hurricanes need to form at least five degrees from the equator.

In order for thunderstorms to form into the circular pattern of a hurricane, they need to come under what is known as the Coriolis force.  The Coriolis force is a principle of physics whereby moving objects are deflected relative to a rotating reference frame.  Because the earth rotates, it causes the air in a hurricane to circle in a counterclockwise manner north of the equator, and clockwise south of the equator.  The closer to the middle of the rotating sphere - the Earth - the weaker is the Coriolis force.  Starting about 5 degrees latitude from the equator, the force is strong enough to get the hurricane air circling.


6.  Hurricanes need there to not be a high level of wind shear.

Hurricanes extend up to 40,000 feet vertically into the troposphere.  High wind shear at these upper levels can disrupt the hurricane.  By “blowing the top off” the hurricane, high wind shear can release a lot of the heat and moisture sustaining the hurricane.  Or at the very least it can alter the shape of the hurricane and distort the vortex, which tends to destabilize and weaken the storm.


These factors explain not only how a hurricane forms, but also how it is sustained, or ceases to be sustained.  For example, as a hurricane travels farther from the equator over water of a lower temperature, or when it travels over land, one or more of the above factors becomes absent.  The hurricane is no longer being fed by new warm, moist, rising air from the tropical ocean.  Without this fuel, it loses energy.


Sources:

“How & Where Hurricanes Form”

“How Hurricanes Form”

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