This blog provides commentary on interesting geological events occurring around the world in the context of my own work. This work is, broadly, geological fluid dynamics. The events that I highlight here are those that resonate with my professional life and ideas, and my goal is to interpret them in the context of ideas I've developed in my research. The blog does not represent any particular research agenda. It is written on a personal basis and does not seek to represent the University of Illinois, where I am a professor of geology and physics. Enjoy Geology in Motion! I would be glad to be alerted to geologic events of interest to post here! I hope that this blog can provide current event materials that will make geology come alive.

Banner image is by Ludie Cochrane..

Susan Kieffer can be contacted at s1kieffer at gmail.com

Friday, September 20, 2013

Super Typhoon Usagi nears Philippines. What is a super typhoon?

Cyclone Usagi on Thursday.
Credit: Colorado State University
Hurricanes Ingrid and Manuel pummel Mexico; Hurricane Humberto in the Atlantic has become a ghost of its former self, and now Super Typhoon Usagi is targeting the Philippines and Taiwan.  It is expected to become merely a severe typhoon when it hits China, right at Hong Kong. Usagi has a diameter of more than 1000 kilometers. It has eclipsed Super Typhoon Utor, which was the strongest storm of the year so far. On Thursday evening, its minimum pressure was very low, 882 mb, which makes it the "deepest and most intense storm to exist on Earth since 1984 (tied with Wilma in 2005)" (quote from The Washington Post). As of Friday at 1:30 p.m. Pacific Time, the eyewall had undergone a replacement and winds have peaked in intensity, but the cyclone is still a very dangerous storm. It will weaken as it hits Taiwan, but is still expected to have winds near 100 mph when it hits Hong Kong (likely on Sunday, right when a major autumn festival is going on).

The past two decades, cyclone activity in the South China Sea has been relatively calm, and there are some forecasters worried that the population may "wrongly think that the typhoon risk in Hong Kong has declined." In 1906 and 1937 typhoons killed 15,000 and 11,000 people respectively, but with better warning systems, there have been less than two dozen casualties since the late 70's.

What is a super typhoon? It is equivalent to a Category 5 hurricane. Different ocean basins have different rating scales for their storms. Remember that in the Atlantic, big storms that rotate and have an eye are called "hurricanes." In the Pacific, they are called "cyclones." In most ocean basins, hurricanes are rated in "categories," with values assigned from 1-5. Here is a NOAA table of categories:
A "super typhoon" is defined as a storm that reaches maximum sustained winds of at least 65 m/s (234 km/hour), so it is equivalent to a Category 4-5 cyclone. The term "major hurricane" is also used, and it is used for storms that have maximum sustained 1-minute surface winds of at least 50 m/s or 180 km/hour, so applies to Category 3 storms and stronger.

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