The official 2011 Atlantic Hurricane Season is over, which brings us into a most confusing period—a time of subtropical, extratropical, and tropical storms.
These powerful storms appear, disappear and change into each other, creating very confusing weather. They produce unnamed hurricanes and the “Perfect Storm”.
Find the hidden hurricane! Within the huge Perfect Storm of 1991, there was a hidden and unnamed hurricane. SOURCE: NOAA
Basically, there are two types of storms—tropical and extratropical. Both are created by hot air crashing into cold air. In a tropical storm, the hot air rises from the ocean, hits the cold air and rains out. In an extratropical storm, a cold front hits a warm front and produces rain and wind. With a tropical storm, the stormiest area is around the central hot spot, the eye. In an extratropical storm, the stormiest area is in the “crash zone” line, where the warm and cold air masses meet.
Sounds simple, doesn’t it
It gets a bit more complicated. Tropical storms in the Atlantic always become extratropical storms. When they hit land or drift north into cold water the center cools off. The eye disappears. They become a warm front. They become an extratropical storm.
Then it starts getting weird.
Sometimes a stormy cold front drifts over the hot ocean. Its bottom heats up. Its top remains a normal extratropical front while the bottom starts to act like a tropical storm. This is a subtropical storm, a half and half. If this schizophrenic storm stays in hot water, it can become tropical or even a hurricane.
How subtropical storms are born. Source: Browning Newsletter
This happens a lot at the end of the season. Our last storm, Tropical Storm Sean, started as a subtropical storm.
Here’s where it goes crazy. Storms can change as rapidly as a model in a fashion show. For example in 1991’s Perfect Storm:
An extratropical cold front became a subtropical storm, which became Hurricane Grace. It hit another extratropical storm and joined it, creating the disastrous extratropical Halloween Storm. The Halloween storm hit hot water and formed a subtropical storm that became an unnamed hurricane. The whole mess sank several ships (including the Andrea Gail) and eventually died out as an extratropical storm.
It wasn’t a Perfect Storm—it was a perfect mess. The end of a hurricane season is always messy. At least, next week, the season will be over.
Evelyn Browning Garriss, historical climatologist, is a longtime writer for The Old Farmer's Almanac. She is also editor of The Browning World Climate Bulletin and has advised farmers, businesses, and investors worldwide on upcoming climate events and their economic and social impact for the past 21 years.