Hurricane Isabel (2003) seen from the International Space Station, image property of NASA.
With the peak of hurricane season quickly approaching, I think it’s time to brush up on some key terms. As some of you may be aware, hurricanes don’t just pop up out of nowhere, they go through several stages of development. Let’s break down these stages.
Tropical Disturbance/Wave – An organized area of rain and thunderstorms with no definable surface circulation. Usually, tropical waves are born when an area of thunderstorms over land moves over water and continues to sustain itself. Sometimes these disturbances fizzle out and nothing becomes of them but other times they continue to organize.
Tropical Depression – A tropical wave is designated a tropical depression once a surface circulation is found and thunderstorm activity is sufficient. Maximum sustained winds of tropical depressions are less than 39 mph. They are typically assigned a number by the National Hurricane Center with the first depression called tropical depression 1 followed by tropical depression 2 and so on. The numbers reset at the start of every season.
Tropical Storm – A tropical depression strengthens into a tropical storm once its sustained winds reach 39 mph. It also earns a name which comes from a pre-determined list for that year which repeats itself every 6 years (see this year's list here). Names are assigned in alphabetical order each season. If the season is extremely active and the end of the alphabet is reached, storms are named after Greek alphabet letters (Alpha, Beta, Gamma, etc.). This has only happened once before, back in 2005. The sustained wind range for a tropical storm is 39 mph to 73 mph. Land falling tropical storms should not be taken lightly as their heavy rains and strong winds can do some pretty serious damage.
Hurricane – Once a tropical storm’s sustained winds reach 74 mph, it is designated a hurricane. It retains the same name it had as a tropical storm. The Saffir-Simpson scale is used to break hurricanes down into categories from 1 to 5. Once a hurricane reaches category 3 intensity (sustained winds greater than 111 mph) it becomes known as a major hurricane. Category 5 hurricanes have sustained winds in excess of 156 mph. These beasts are rare and usually stay over the open waters as it requires a great deal of energy to sustain such a powerhouse storm. In the last 100 years, only 3 category 5 hurricanes have directly impacted the United States.
Image from NOAA Hurricane Research Division, contributed by Chris Landsea.
A common evolution of a hurricane is shown above in which a band of thunderstorms emerge off the west coast of Africa. These thunderstorms continue to organize through the various development stages and eventually the system strengthens into a hurricane. It is guided across the Atlantic Ocean by the Subtropical Ridge, a common summer-time feature. From there, it can be directed into the Caribbean, Gulf of Mexico, or the Bahamas, then strike the US Coast, or... turn harmlessly out to sea.
Now that you know what all these terms mean, you’ll be in much better shape staying ahead of the storm.
