Hurricanes exist to move heat away from the tropics. Classified as tropical cyclones, they have a warm core and rotate counter-clockwise in the Northern Hemisphere. The Atlantic hurricane season runs from June 1 through November 30. It is during these prime months that conditions become favorable enough to allow a hurricane to develop. We track them with satellites and measure their intensity by flying specially modified "Hurricane Hunter" aircraft directly into them. When a hurricane threatens to come ashore, a frenzy of preparation takes place to get ready for Nature's heat engine- the hurricane.


As powerful as hurricanes are, they are very delicate weather systems. It takes an almost exact set of weather conditions to create and sustain a hurricane. The illustrations below show a very simplified set of basic ingredients needed to spin up a hurricane.


WARM OCEAN WATER (HEATED BY THE SUN)
The Sun is our only source of heat on a global scale. Around 75% of the Earth's surface is covered in water- mostly the oceans. Since water holds heat energy better than the land, our tropical ocean areas are extremely efficient in storing energy transmitted by the Sun. So the heat generated by our Sun (and stored in the oceans) is our first major ingredient.



As the Spring turns to Summer, the Sun's rays become more intense and direct. The tropical oceans slowly heat up, day after day, until they reach temperatures of 80 degrees fahrenheit or higher. This 80 degree mark is usually considered the minimum threshold temperature needed to support the development of a hurricane.

A PRE-EXISTING AREA OF DISTURBED WEATHER (TROPICAL WAVE)
Now that we have a nice warm ocean, all we really need is something to come along and take advantage of the situation. Often times, Africa is the source of the "spark" that gets the fire going. Every few days, a tropical wave migrates westward off the coast of Africa- near the Cape Verde Islands. Nearly 100 of these concentrated areas of lower pressure traverse the Atlantic each hurricane season. Only a small portion of them, perhaps 10 a year, ever become anything more than a large thunderstorm complex over the water. But the ones that do develop keep forecasters and folks living west of 60 degrees longitude very nervous. The reason is that some of the Western Hemisphere's worst hurricanes can be traced back to a tropical wave that originated over Africa. This is not to say that every tropical wave that develops will become a historic hurricane, but these so called "Cape Verde" storms always need to be watched.

LIGHT WINDS IN THE UPPER ATMOSPHERE (LOW VERTICAL SHEAR)
One key ingredient is the lack of strong upper level winds blowing in the opposite direction that a fledgling tropical cyclone is moving. In other words, low vertical shear. A tropical storm and especially a hurricane needs to be able to stack up its thunderstorms (also called convection) neatly around the center (eye). If strong winds blow the tops of the thunderstorms off, streaming clouds for hundreds of miles across the Atlantic, then there is little chance for the system to strengthen. In fact, many a potential major hurricane has met its doom because of strong shear imposing on the system. Most of the time the shear or strong upper level winds will come racing from the southwest or west. So, a tropical storm or hurricane moving westward and head-on into the strong winds, will be easily dismantled.

HURRICANE EFFECTS

The maximum effects of a hurricane are usually felt within what is called the right-front quadrant. Here the winds are (typically) strongest, storm surge is highest, and the possibility of tornadoes is greatest. It is important to know whether or not your area will be affected by the right-front quadrant. It could mean the difference between maximum hurricane conditions or a glancing blow.





OTHER HURRICANE EFFECTS

TORNADOES- may form especially in the spiral rainbands of a hurricane as it moves onshore. The changing wind speeds with height acts like a huge twisting mechanism, thus allowing the possibility of tornado formation.

FLOODING RAIN- is a very real threat from a landfalling hurricane. (Hurricane Agnes in 1972 and hurricane Floyd in 1999 caused severe flooding across inland locations long after landfall.) This is especially true if the hurricane stalls out over a region, raining itself out. This can be a serious problem for areas hundreds of miles away from where the hurricane (or tropical storm) initially came ashore. Even if you live 'inland' it is important to keep tabs on the movement of any potentially landfalling tropical cyclone.

DAMAGING WINDS- will accompany any hurricane, no matter what category it is. A hurricane, by definition, has winds of at least 74 miles per hour. This wind speed alone is enough to cause damage to poorly constructed signage and knock over some trees and other vegetation. Obviously, the stronger the hurricane (higher winds), the more potential for wind damage exists. See Hurricane Preparation for more information about preparing for the high winds of a hurricane.

LIGHTNING AND HAIL- are less frequent occurrences during hurricanes than during other severe weather events- like thunderstorms. Lightning is more frequent during a typical afternoon thunderstorm because there are more factors present that promote lightning development. The same reason generally holds true concerning hail during a hurricane. There will be some lightning during a hurricane but some of the 'flashes' will actually be electric transformers exploding or powerlines sparking; sending an eerie glow into the sky.