|
|
CMSE
Online Front Page |
In North Carolina, public interest in tropical storms is always highest in late summer and early fall when hurricanes threaten the Carolina coast. When storms lurk offshore, the Internet is generally busy with requests for sites bringing current information.
However, tropical storms don't just happen in August and September. They are active somewhere in the world every month of the year. Tracking them on the Internet can be an interesting class project at many grade levels, combining science with mathematics and geography.
The word hurricane is from the extinct language of the Carib people who met the early Spanish explorers. Only in the Americas are tropical storms called hurricanes. In east Asia, the storms are often called typhoons, a word from the Malay language. Elsewhere, tropical storms are called tropical cyclones, and this is the term used by scientists.
A cyclone is any storm whose winds below in a circle around a low-pressure center: counterclockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere. Cyclones can be huge, like the 1000-mile wide storms common in winter, or small, like tornados. Officially, tropical cyclones are medium-sized cyclonic storms of tropical or subtropical oceans, having organized convection (thunderstorm activity) but no warm or cold fronts. If the sustained winds near the center are at least 17 meters per second (39 miles per hour), the tropical cyclone is called a tropical storm. Weaker tropical cyclones are called tropical depressions. To be labeled a hurricane or typhoon, a tropical storm's sustained winds must reach 33 meters per second (74 miles per hour).
Tropical cyclones form over all the world's tropical oceans except the South Atlantic. They draw their energy from the evaporation of warm surface water, at least 80 °F (26.5 °C). They never form within 5° latitude of the Equator, because at the Equator the earth's rotation does not impart any turning motion to storm winds. Generally they are confined to the tropics, but off the east coasts of Asia and North America warm currents (the Japan Current and the Gulf Stream) allow them to range northward far into the temperate zone.
In a typical year about 100 tropical cyclones are recorded, 70 in the Northern Hemisphere and 30 in the Southern. (The lack of storms in the South Atlantic and the great width of the tropical North Atlantic and North Pacific are the major factors in the disparity betwen hemispheres.) Because the North Pacific Ocean is so wide and so many storms occur in it, meteorologists divide it in half at longitude 180° (the International Date Line). This means there are six tropical cyclone regions or "basins": the North Atlantic, Northeast Pacific, Northwest Pacific, South Pacific, North Indian, and South Indian.
|
The number of storms in each region varies considerably from year to year, as we know from our experience with the North Atlantic, but a good year in one region is often a bad year in another. The totals for each hemisphere don't vary as much. The chart shows Northern Hemisphere storms by year and region from 1994 through 1998. The number of Atlantic storms varied from 8 to 22 (175%), but the total number of Northern Hemisphere storms only varied from 65 to 78 (20%). As the chart also reveals, the Northwest Pacific has by far the largest number of tropical cyclones. |
|
|
|
The second chart shows the number of storms beginning each month for the year beginning June 1, 1998 and ending May 31, 1999. Northern Hemisphere storm activity peaks in late summer and early fall, as we know. However, there are a few Northern Hemisphere tropical cyclones even in the winter. Most of these storms are in the Philippine Sea or South China Sea, where the water remains warm year round. The Southern Hemisphere season is a little different: it starts late in spring (December) and doesn't extend much beyond the beginning of fall (March). This leaves a gap in May, when only a few storms occur. |
First of all, three U.S. Navy meteorology sites watch tropical cyclones throughout the world. The Naval Atlantic Meteorology and Oceanography Center (NLMOC) at Norfolk, Virginia, covers the Atlantic; the Pacific Meteorology and Oceanography Center (NPMOC) at Pearl Harbor, Hawaii, covers the eastern Pacific both north and south of the Equator, and the Joint Typhoon Warning Center (JTWC) in Agana, Guam, covers the western Pacific and Indian oceans. These sites provide forecasts for all storms, accompanied by maps showing the location and predicted track of the storm. Following is the NLMOC map for Hurricane Dennis on the morning of August 30, 1999:
To use this map, students need several skills. Obviously, they need geographic skills. In addition, they need some measurement skills. The predicted positions of the storm are marked in Universal Time (Z). 2906Z is 6 hours, Universal Time, on August 29. Universal Time is four hours ahead of Eastern Daylight Time (five hours ahead of Eastern Standard), so 2906Z is 2:00 am on the 29th. Also, wind speed is shown in knots (KTS); 1 knot equals about 1.15 miles per hour, so the predicted intensity of "95KTS, gusts to 115KTS" means about 110 mph with gusts to 130 mph.
There are many other sources of tropical cyclone information around the world, including the National Hurricane Center in Miami, the Central Pacific Hurricane Center in Honolulu, the Philippines Weather Branch in Manila, the Hong Kong Observatory, the Fiji Regional Specialized Meteorology Center in the South Pacific, and the Australian Bureau of Meteorology. (Spanish-speaking students can get data from tropical storm sites in Cuba and Mexico.) All of these sites provide forecasts and warnings, and many of them provide graphical projections.
The National Hurricane Center does not provide predicted storm tracks; instead, it provides strike probability maps, like this one for Hurricane Dennis:
This map shows the strike probability to be between 10% and 20% for the whole southeast coast from Miami to Wilmington. Obviously, these maps also require students to acquire several skills for their interpretation.
CMSE Director Russ Rowlett maintains a web site with dozens of tropical cyclone information links. Listed below is his site, the sites mentioned above, and a number of others. if you have Internet capability in your classroom, there are enough links here to generate plenty of student interest!
Internet Sources
FEEDBACK: We'd be happy to have your comments and suggestions.
Copyright © 1999, Center for Mathematics and Science Education. Teachers have permission to duplicate this page for use in teaching their own classes. All other rights reserved. You are welcome to link to this page, but do not copy its contents.
Posted August 26, 1999.
CMSE Online features remain online as long as they remain current; they may be updated if new information becomes available.
|
http://www.unc.edu/depts/cmse/science/hurricanes.htm Center for Mathematics and Science
Education |
CMSE
Online Front Page |
