|MadSci Network: Earth Sciences|
Tropical storm activity hasn't actually increased if one looks at the
records from the past 50 years or so. In fact, the most active decade for
topical storms hitting the United States was the 1940's
There is currently an intense
scientific debate on the cause of this increased activity. Some scientists
hypothesize Global Warming is making intense tropical storms more common.
Others disagree, noting a historical cycle in tropical storm intensity and
number that appears connected to many variables such as ocean currents,
solar activity, El Nino, and long term droughts.
Here is one article that supports a positive connection between global
warming and increased tropical storms. See:
And here is one that disagrees
Measuring winds is actually very difficult. It is hard to get scientists to agree on what the average wind is. In the United States, we measure the wind every 5 seconds, and then average 24 measurements (2 minutes worth) together to get the wind speed. Wind gusts are reported as the highest 5- second average wind speed. Other countries use different time averaging techniques. Note that no one uses the 'instant wind speed' as this fluctuates greatly on a time scale less than a second.
Tropical storm winds are measured in several different ways. If the storm passes over a wind sensor (like an ocean buoy or a ship), we can measure it directly.
Aircraft fly into tropical storms to take measurements, including wind speed. They measure winds at about 5,000 feet or 10,000 feet above sea level and, along with temperature and pressure measurements, calculate a surface wind speed. The aircraft also drop devices into the storms to directly measure the wind speeds. (http://www.hurricanehunters.com/)
Another method is a satellite technique called the Dvorak Method. Different types of satellite images are collected for the same storm at the same time, and a calculation of wind speed is done.
None of these methods is 100% accurate, and different methods used on the same storm will realize different values. The wind speed and maximum gusts reported for any tropical storm are just estimates.
The main point is there is still a debate on whether tropical storms are becoming more numerous and/or stronger. We just don't know for sure, and the debate continues.
Try the links in the MadSci Library for more information on Earth Sciences.