10 September 2011

Today, September 10, is the statistical peak of the North Atlantic hurricane season that runs annually from June 1 to November 30. 

This year my friends and family have had more than their usual share of destruction from the few hurricanes that hit the United States.  Hurricane Irene and the remnants of Lee have been headline news for weeks and Hurricane Katia, who’s missing the U.S. entirely, is affecting friends on a cruise in Greenland.   

So I wondered… how and why do hurricanes form?  I did a little research and found that even the basic facts are fascinating.

Hurricanes are tropical cyclones, complex dangerous storms that occur around the world. In the North Atlantic and on the eastern side of the Pacific we call them hurricanes.  In the northwest Pacific they’re called typhoons. 

Tropical cyclones are not completely understood but scientists know that six ingredients are required for a hurricane to form.  The ingredients, listed at NOAA, are:

1. A warm ocean surface of at least 79.7^oF to a depth of 160 feet.   To get an idea of how warm this is, the water temperature at the Eastern Maine Shelf this morning is 54-52^o from the surface to 160 feet.
2. Rapid cooling of the air as it moves upward, causing condensation and thunderstorms which release heat to drive the storm. 
3. High humidity in the mid troposphere 3 miles (15,800 feet) above the ocean.  If you were on a trans-Atlantic airplane, you’d be flying high above it.
4. The Coriolis effect must cause the storm to spin.  There is no Coriolis effect at the equator so the storms cannot form at less than 5⁰ of latitude (345 miles) from the equator. 
5. A pre-existing disturbed weather system near the ocean surface which provides the nascent storm with something to organize around.
6. Low wind shear where the storm is forming.  Wind shear is an abrupt difference in wind speed and direction and can break up a cyclone before it gets going. 

In August and September hurricanes often form off the Cape Verde Islands near the north coast of Africa.  They are then carried by the trade winds across the Atlantic and sweep over the Caribbean islands and sometimes the U.S. or Central America.  Right now Tropical Storm Maria is heading for Puerto Rico and Tropical Storm Nate is about to hit Mexico. 

Thank heaven we’re over the hump of hurricane season for 2011.  We’ve certainly seen enough of them this year.

Learn more about hurricanes at NOAA’s Hurricane FAQ page.

(satellite image of Tropical Storm Katia from NASA’s MODIS Rapid Response System on 31 Aug 2011.  Click on the image to see the original at Wikimedia Commons)