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Dr. Simpson, ... the first female meteorologist to earn a doctorate, developed the first scientific model of clouds, discovered what keeps hurricanes whirling forward and revealed what drives the atmospheric currents in the tropics. She later conducted unique "weather modification" experiments and ran an international satellite project that measures tropical rainfall over the oceans, enterprises that continue to have significant impacts in the field. She worked for the past 30 years at NASA Goddard Space Flight Center in Greenbelt, where she was chief scientist for meteorology.

Animated, opinionated and deeply knowledgeable about tropical meteorology, Dr. Simpson first made her mark in the 1950s when she and Herbert Riehl explained how the atmosphere moved heat and moisture away from the tropics to higher latitudes. The "hot tower" hypothesis, supported by her models of cumulus clouds, helped explain how the trade winds keep blowing and how hurricanes retain the heat that powers them.

"There is zero doubt that there has never been a more capable woman in meteorology, and she would also be in the top five of all meteorologists in history, no matter the gender," said Greg Holland, director of the Earth Systems Laboratory at the National Center for Atmospheric Research in Boulder, Colo.

Her most important professional achievement, she often said, came near the end of her long career, when NASA in 1986 asked her to lead the science study for the proposed Tropical Rainfall Measuring Mission. The joint mission with the Japanese space agency has been key in helping scientists learn how hurricanes start in the Atlantic, how dust and smoke can drastically influence rainfall, and how to estimate latent heat released by tropical cloud systems.

Dr. Simpson was also a mentor for two generations of top-level scientists, particularly women. In 1998, she was named one of the Ms. Foundation's top 10 female role models.

"Any comparison between the way it was when I started and the way it is now is like comparing the covered wagon with a jet plane," she told the Christian Science Monitor in 1989. "But this doesn't mean that women don't still have obstacles to overcome. . . . Sometimes you have to fight just to keep the opportunities you have."

She was candid about the number of obstacles she had to overcome. Born Joanne Gerould on March 23, 1923, in Boston, she became interested in clouds while learning to sail and later as a student pilot. She studied under Carl-Gustaf Rossby at the University of Chicago. After graduating and obtaining a master's degree, she and two other women sought fellowships for doctoral work in meteorology. A faculty adviser said that no woman had ever received a doctorate in meteorology, none ever would. So she began saving for tuition by teaching at the Illinois Institute of Technology, where she met German immigrant Herbert Riehl, who lectured on tropical storm research. Riehl agreed to be her adviser, and she began working on cloud research at the University of Chicago.

At the time, clouds were considered a result, not a cause, of weather, but Dr. Simpson thought them fascinating. Rossby told her no one else was very interested in the topic, so it was a good subject "for a little girl to study."

After she received her doctorate in 1949, she and Riehl wrote several landmark papers about hurricanes and tropical meteorology. In 1951, she became a research meteorologist at the Woods Hole Oceanographic Institute in Massachusetts, where with the help of a slide rule, she constructed some of the first mathematical models of clouds. To validate her work, she needed to fly into the very tall clouds near the equator. The Navy lent Woods Hole an old PBY-6A airplane, which they outfitted with scientific instruments. But Woods Hole's director said women were not allowed on its field trips. The naval officer who arranged the aircraft, however, told the director "No Joanne, no airplane." She flew.

The next few years took her to England on a Guggenheim fellowship and to the UCLA faculty.

She was also named an adviser to the National Hurricane Research Project. In 1963, she flew above the clouds during the NHRP's Project Stormfury and ejected flares that dispersed silver iodide into the clouds. The clouds behaved as she had predicted, so she wrote an article "and a furor broke loose," Dr. Simpson later said. "I was totally unaware of the level of emotion and hostility that was directed against anything that had to do with cloud seeding."