How Do Hurricanes Form?

A hurricane is a tropical storm that has winds of over 74mph.  “Hurricane” is a word meaning “evil spirit and big wind” in the Caribbean Indian language.  Hurricanes need three things to form: warm water, moist air, and converging winds.  A hurricane forms when winds meet over warm water (at least 80 degrees Fahrenheit) and push each other in a circular pattern.  Most Atlantic hurricanes begin on the west coast of Africa and are pushed across the ocean to the east coast of the USA.

The majority of hurricanes begin near the equatorial belt.  This is because at that point, the Coriolis effect (gravitational spin of the earth) converges there with leftward-circulating winds from the south meeting rightward-circulating winds from the north.

As the winds spin, they pull warm air from the water’s surface and push it upwards, creating a funnel.  This funnel is called the eye of the hurricane and is, surprisingly, very calm.  The wall of the eye (or Eye Wall), however, is where the wind is most turbulent as it compresses into a relatively tight space with the spin of the cloud formations.  The clouds are formed from the moisture pulled up from the water’s surface as the air rises from it.

From the center of the hurricane’s eye to the Eye Wall is calm.  From the Eye Wall outward it is turbulent, but becomes decreasingly so as the area of the storm gets larger.  The cirulating wind and cloud formations around the eye are the rain bands where rain falls, but is often evaporated back into the storm before touching down.

As it spins in this way (left if headed south, right if headed north), the hurricane will feed on itself, using the heat of the water below as its fuel source.  Hurricanes grow and recirculate much of their energy as they move, creating a snowball effect of larger clouds and heavier winds.  Because the hurricane requires warm water to fuel it, however, it usually loses most of its force upon landfall or when moving into cooler climates where the water isn’t as warm.

Many storms, for instance, will become hurricanes on the weather map only to move into a micro-climate of cooler water (perhaps a current of cooler water originating at the pole) and then lose their force, dying off.

A hurricane’s life span is usually measured in a few short days.  The storm will begin as a tropical depression, or a low pressure zone in the tropics with relatively low wind speeds.  As the winds feed on each other and the storm moves north or south of the equator, the spin will increase and wind speeds will get up near 40mph or more.  At this point, the baby hurricane has become a tropical storm.

Eventually, if conditions are right, the tropical storm will continue moving north or southwards and will reach wind speeds of over 74mph.  It is then a hurricane.  In general, the less spread out a hurricane is, the more potential damage it can do during landfall.  Think of it as a ball thrown at your nose.  The bigger the ball is, the more spread out the impact area will be and the less damage your nose will incur.    Smaller balls like baseballs or golf balls, however, will hit your nose directly and put all of their force in a smaller area, causing more damage.

Every year, on average, about 100 tropical storms develop.  Of those, about half become hurricanes and of those, barely 5-10% actual make landfall in an inhabited area.  Most hurricanes never make the shore, instead running themselves out over open water.