July 13, 2005

How hurricanes form

From About.com. Excerpt:

The birth of a hurricane starts as a low pressure zone and builds into a tropical wave of low pressure.

In addition to a disturbance in the tropical ocean water, the storms that become hurricanes also require warm ocean waters (above 80°F or 27°C down to 150 feet or 50 meters below sea level) and light upper level winds.

A tropical wave grows in intensity and then may grow to become an organized area of showers and thunderstorms known as a tropical disturbance. This disturbance becomes an organized area of tropical low pressure that is called a tropical depression based on cyclonic winds (counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere). A tropical depression's wind speed must be at or below 38 miles per hour (mph) or 62 km/hr when averaged out over one minute. These winds are measured at 33 feet (10 meters) above the surface.

Once average winds reach 39 mph or 63 km/hr then the cyclonic system becomes a tropical storm and receives a name while tropical depressions are numbered (i.e. Tropical Depression 4 became Tropical Storm Chantal in the 2001 season.) Tropical storm names are preselected and issued alphabetically for each storm.

There are approximately 80-100 tropical storms annually and about half of these storms become full-fledged hurricanes. It is at 74 mph or 119 km/hr that a tropical storm becomes a hurricane. Hurricanes can be from 60 to almost 1000 miles wide. They vary widely in intensity; their strength is measured on the Saffir-Simpson scale from a weak category 1 storm to a catastrophic category 5 storm. There were only two category 5 hurricanes with winds over 156 mph and a pressure of less than 920 mb (the world's lowest pressures ever recorded were caused by hurricanes) that struck the United States in the 20th century. The two were a 1935 hurricane that struck the Florida Keys and Hurricane Camille in 1969[Note: Hurricane Andrew has been upgraded from Category 4 to 5 since this was written]. Only 14 category 4 storms hit the U.S. and these included the nation's deadliest hurricane - the 1900 Galveston, Texas hurricane and Hurricane Andrew which hit Florida and Louisiana in 1992.

Posted by Physics Geek at July 13, 2005 04:08 PM | TrackBack StumbleUpon Toolbar Stumble It!

I see you have the National Hurricane center RSS feed as well... or are you just being annoying and reminding us that this is the first time in a century we have had 5 tropical storms this quickly? Did you notice Emily? Sigh.

Posted by: vw bug at July 13, 2005 10:12 PM

I don't mean to be annoying Vee-Dub. It just seemed appropriate with all of the tropical storms hitting the US.

Back in 1995-1996(can't remember) there were 3 hurricanes parading across the Atlantic at the same time, which might have been the first time I'd seen that.

The last 25-30 years had had a relatively small number of storms, most of which were weaker than storms earlier in the century. I believe that that interlude was an aberration, and that we're headed into a 30-40 year span of nasty storms. Might be time to move out of Florida.

Posted by: physics geek at July 14, 2005 09:27 AM