Edward Lorenz; Pioneer in Creation of Chaos Theory

Using 12 equations, such as the relationship between air pressure and wind speed, he ran the model and found exactly what he sought. But taking a shortcut on the next run, he found that a tiny decimal point change led to a significant error.

Rather than ignore the response, which peers had considered an anomaly, Dr. Lorenz realized measurement is not perfect. If temperature, pressure or humidity measurements were off by a hundredth of a percent, the rainfall he expected in Las Vegas on Thursday could show up as a snowstorm in Beijing a week later. A computerized model of how the "butterfly effect" works can be found at the Exploratorium's Web site.

"He had the ability to make important connections between atmospheric phenomena and simple theoretical models," said Isaac Held, senior research scientist in the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory. "He taught us that complexity can follow from very simple underlying rules. His study of the limits to the predictability of weather initiated an entire new field of chaotic dynamics."

Dr. Lorenz was elected to the National Academy of Sciences in 1975, and in 1983, he and oceanographer Henry Stommel were jointly awarded the $50,000 Crafoord Prize by the Royal Swedish Academy of Sciences, an honor established to recognize fields not eligible for Nobel Prizes.

Dr. Lorenz was known as shy and humble. He enjoyed hiking and cross-country skiing and sought out mountain trails near every scientific meeting he attended, one of his daughters said.

His wife, Jane Logan Lorenz, died in 2001.

Survivors include three children, Nancy Lorenz of Roslindale, Mass., Edward Lorenz of Grasse, France, and Cheryl Lorenz of Eugene, Ore.; and four grandchildren.

Chaos theory helped shape Dr. Lorenz's conclusions as he worked to calculate long-term forecasts. Given the problems of input, the meteorologist determined that it's impossible to accurately predict weather beyond two or three weeks with a reasonable degree of accuracy.