With the warm waters of late summer, low pressure systems, and other conditions in place, hurricanes arrive like clockwork during "hurricane season."
Hurricanes start out as low pressure weather systems over Africa and emerge as tropical disturbances over the warm waters of the Atlantic Ocean during the late summer and early fall. The moisture and warmth energize these low pressure disturbances, creating thunderstorms. Some thunderstorms dissipate and that's the end of the story. However, many pickup strength and wind speed. When these storm systems continue developing and winds have reached 40 miles per hour, they are then designated as tropical storms and given a name from the National Hurricane Center. Once a storm is named, it isn't necessary destined to become a full-blown hurricane. In fact, many tropical storms fizzle out. Others intensify, continuing to drawn energy from the warm, moist waters below while venting cooler, dry air out above. Energy and heat is released and strong winds form. Meanwhile the center of the storm heats up due to the release of energy and water vapor, causing an air pressure drop and stronger, more intense winds in the core. This creates a cycle of more heat, continued low pressure, and stronger winds.
Finally, if the winds reach 74 miles per hour, the named tropical storm becomes a Category One hurricane. Hurricanes can vary in strength, moving up and down the Saffir-Simpson scale as they lose or gain intensity. For example, when hurricanes reach land, they quickly weaken because they are no longer fueled by warm waters. However, many move over the land, back to open waters where they quickly regain their lost strength.
Category One hurricanes are the least destructive with winds ranging from 74-95 miles per hour. Downed trees, damaged bushes, and damaged mobile homes are typical victims of Category One hurricanes.
Category Two hurricanes have winds ranging in speed from 96-110 miles per hour and typically bring storm surges of 6-8 feet above normal. Category Three hurricanes feature winds from 111-130 miles per hour and 9-12 foot storm surges. Category Four hurricanes are characterized by wind speeds of 13-155 miles per hour and storm surges of 13-18 feet above normal. Finally, Category Five hurricanes have winds in excess of 156 miles per hour and storm surges above 18 feet.
Obviously, as hurricanes work their way up the scale with stronger winds and larger storm surges, the potential for damage increases dramatically. Only a few category five hurricanes have made landfall in the U.S. However, lower rated hurricanes can wreak havoc when conditions are right. Hurricane Katrina, the most devastating hurricane to hit the United States, made landfall as a Category Three hurricane.
While hurricanes are destructive as far as man and civilization are concerned, they do have beneficial qualities. One, they provide up to a quarter of the annual rainfall in the southern states and may even play a vital role in maintaining the Earth's heat balance - moving heat from the tropical regions to the poles. Hurricanes have a long term positive effect on marshes and coastal wetlands as well with storm surges adding critical nutrients and sediment to these fragile lands. Under the sea, coral benefits from hurricanes as well. For example, cooler waters move to the surface, counteracting the "bleaching" effect of warm waters. In addition, some species of coral, such as elkhorn coral, break off of reefs and reattach elsewhere, forming a new reef. Though capable of extreme destruction, hurricanes help build up barrier islands which in turn provide the first line of defense against the hurricane's fury.
Warm ocean waters coupled with tropical disturbances and low pressure systems each summer and fall provide the perfect conditions for a hurricane to form. Whether a tropical depression progresses to a tropical storm or a hurricane depends on the conditions at the time. As summer approaches fall, these conditions are perfect for a storm.