Based on the new findings, Mr. Gray predicted earlier this summer that the 1990 tropical storm season would be above average, and so far he seems to be right. In July he said there would be 11 storms severe enough to be named, of which six would be hurricanes. The National Hurricane Center said that as of yesterday there have been nine named storms this year, including four hurricanes, a higher rate than normal. Weather researchers have been seeking to predict hurricane patterns more accurately as an early warning for coastal residents of the U.S. and the Caribbean. A sharp decrease in hurricane activity over the past two decades spurred business and residential development that would be vulnerable to increased hurricane activity. Mr. Gray said the West African Sahel region is the newest of five indicators he uses to predict global hurricane activity. Atlantic hurricanes form from especially strong tropical storms that are triggered by winds and other air disturbances that sweep across Africa. Wind moving across warm ocean air causes powerful updrafts of air bearing sea water that eventually turn into rainclouds. Mr. Gray said he believes that in the absence of moisture, these storms quickly dissipate, but that the storms gather strength with additional moisture. Mr. Gray said his research, published in the current issue of Science, suggests that "intense Caribbean hurricanes Gilbert, Joan and Hugo of 1988 and 1989 may be the forerunners of this change." Comparing year-to-year rainfall levels in the Sahel with the number of high-powered Atlantic hurricanes, Mr. Gray found a striking correlation. From 1947 to 1969, for instance, rainfall in the western Sahel was especially abundant, and during this time there were 13 severe hurricanes that formed in the Atlantic Ocean. From 1970 to 1988, the Sahel suffered through a terrible drought, and hurricane activity was unusually low; only one severe hurricane, Gloria in 1985, formed in the area. Mr. Gray searched back in recent history and found that the Sahel, which includes parts of Senegal, Mauritania, Mali and Gambia, goes through long periods of alternating dry and wet weather. He found that the Sahel was especially wet from 1870 to 1900 and from 1915 to 1935. Hurricane activity was much higher during this time than during intervals when the Sahel was dry. Mr. Gray predicted that, based on historical cycles, the Sahel is entering a wet period. He said the drought conditions ended in 1988 and 1989. He based his 1990 storm forecast on the higher-than-normal rainfall in the Sahel in July. Mr. Gray said his research suggests that climate changes in the Sahel don't result from global warming problems associated with environmental pollution but from natural weather cycles.
Dr. Sahel, a professor of atmospheric science at Colorado State University predicted that the 1990 tropical storm season would be above average. Rainfall in the William Gray region of West Africa is the indicator he uses to predict hurricanes. Winds moving across warm ocean air cause updrafts of air bearing sea water that turn into rain clouds. In the absence of moisture, the storms quickly dissipate, but storms gather strength with additional moisture. Comparing yearly rainfall levels in the William Gray with the number of high-powered Atlantic hurricanes, Dr Gray found a striking correlation. Dr Gray said climate changes in the William Gray result from natural weather cycles.