30
Nov
07

Sometimes a Depression is Just a Depression

Looks like the National Hurricane Center is drawing fire for a “rush” to name tropical depressions storms. This year we had a large number of storms, but the majority of storms were tropical storms, and critics are now wondering if some of those tropical storms even deserved an actual name.

Just to give you a bit of background, there are three types of Atlantic tropical systems (the Indian and Pacific storms have different names): depressions (wind speeds of 0-39 mph), storms (40-74 mph) and hurricanes (75+). The wind speeds are determined by hurricane hunter aircraft which drop probes into the storm to determine wind speed at the center of the low. A system is only “named” (give a proper first name) if it reaches tropical storm strength.

Four of this years storms (and possibly another two) may have never progressed past tropical depressions. While hunter aircraft determined the speeds of the winds, another telltale sign of storm strength is the internal pressure of the storm system. Surface lows like tropical cyclones cause a decrease in atmospheric pressure as they spin. (The sea-level pressure of the atmosphere is 760 mm of Hg, or 760 torr, or 1 ATM– the Atmospheric Science definition is typically “bar”, where 1 bar = 760 torr = 1000 milibar, or mb.) Hurricanes can cause dramatic decreases in pressure. Hurricane Andrew hit in August of 1992 with an internal pressure of 922 mb. Hurricane Humberto which formed this past season was only a category 1 hurricane at its peak with winds of 80 mph and an internal pressure of 985 mb at its lowest point.

Now, some storms reached 40 mph with their winds but the internal pressures never dropped below 1000 mb. Some see this as a sign that there was no clear organization and the storm was “upgraded” to pad the season to match forecasts. If that’s the case, it’s horribly irresponsible because insurance companies are calculating their rates based upon the activity of the hurricane seasons. If you’re worried that you’re not going to get close to your 15-storm prediction and you’re fudging your results, you’re increasing the insurance rates of people all along the gulf coast! Now you can see the concern.

Now, I’m not going to call the people at the NHC liars. Each of the storms mentioned is a borderline case for a tropical storm. I know how exciting these storms can be to track and monitor. But would getting concrete proof it’s a tropical storm to save a few million people billions of dollars in insurance premiums be that much of a hassle?


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My name is Doc. Welcome to my blog. If you're visiting from another blog, add me to your blogroll (and I'll happily reciprocate). I have a Ph.D. in Chemistry and live in Wisconsin. If you have any questions, feel free to email me. My email is docattheautopsy at gmail. (No linking to deflate the incredible spam monsters).

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