Wow! Over 1600 hurricanes and tropical systems have been recorded since records began in 1851. That’s almost ten per year! Some, like Hurricane Sandy or Katrina, made international news, as direct landfalls led to widespread destruction and devastation. Others, however, have either grazed the coast or spun harmlessly in the ocean, leaving little to no impacts on land. Why do some become “monster” hurricanes while others are merely “fish” storms? We have the answers.
First, a definition. A tropical cyclone is typically a large, low pressure system made up of circulating thunderstorms and is usually characterized by strong winds and heavy rain. For a cluster of thunderstorms over a body of water to develop into a tropical cyclone, four key ingredients must be met: warm water (generally above 80°F), low vertical wind shear (little change in wind direction and/or speed with height), high moisture and a steep lapse rate (sharp change in temperature with height). Since these ideal conditions are mainly found in the tropics, it's no wonder most cyclones are found there, as seen in the image above. Typically, the best time of year for development in the Atlantic falls from June 1st to November 30th. For more information on how tropical cyclones work, see our blog here.
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Ever wonder why one tropical cyclone dissipates quickly while another goes on to become a major hurricane like Katrina? To answer this question, we have to examine the external environment. As long as a tropical cyclone continues to travel through areas that have the four major ingredients listed above in place, it will typically strength and move up the Saffir Simpson Scale (see the various stages here). However, many things can lead to its demise. If it moves over land or travels over colder waters, it will eventually fall apart. Why? Because it lost its source of energy, namely warm ocean waters that support convection and thunderstorm growth. Something else that may weaken a thriving tropical disturbance is African dust. Sometimes hot, dry air from Saharan Desert storms can travel hundreds of miles into the Atlantic and wrap into a tropical system, decreasing its lapse rates and “choking it off” from moisture. Lastly, if a tropical system encounters high winds, the wind shear usually “rips” the tropical system apart.
So what hurricanes top the chart for the wettest, windiest and costliest in history? See the table below.
The next time you see a tropical system create headlines, you’ll know the science behind the storm.
