Inventor of chaos theory; Born May 23, 1917; Died April 16, 2008.

EDWARD Lorenz, who has died aged 90, was the father of chaos theory that claims small effects lead to big changes.

He was a professor at the Massachusetts Institute of Technology when he came up with the scientific concept, the so-called butterfly effect, in which he explained how something as minuscule as a butterfly flapping its wings in Brazil changes the constantly moving atmosphere in ways that could later trigger tornadoes in Texas.

His discovery of "deterministic chaos" brought about "one of the most dramatic changes in mankind's view of nature since Sir Isaac Newton", said the committee that awarded Lorenz the 1991 Kyoto Prize for basic sciences. It was one of many scientific awards that Lorenz won. There is no Nobel Prize for his specific field of expertise, meteorology.

Jerry Mahlman, a longtime friend, noted that the man who pioneered chaos theory was "the most organised person I ever knew".

Lorenz came up with the chaos theory concept in the 1960s through his own meticulous work habits when he inadvertently ran what seemed like the same calculations through a creaky computer twice and came up with vastly different answers.

When he tried to figure out what happened, he noticed a slight decimal point change - less than 0.0001 - wound up leading to significant error. That error became a seminal scientific paper, presented in 1972, about the butterfly effect.

Lorenz was "the quiet geek" who turned the old concept of "wiggle room" into hard numbers and scientific theory and meteorologists today base their forecasts on his techniques.

His 1967 book The Nature and Theory of the General Circulation of the Atmosphere is considered a classic textbook in meteorology. The concept of small changes turning into big effects also influenced many basic sciences, and other fields probably benefited more than meteorology.

Lorenz also was incredibly quiet. Getting him to talk was painfully difficult, his colleagues said, except around his late wife, Jane. He rarely wrote papers with others.

He was born in West Hartford, Connecticut, and had degrees from Dartmouth College and Harvard University as well as MIT, where he joined the meteorology staff in 1948. He later became department head and retired in 1987.

In his personal life, Lorenz loved the outdoors and was an avid hiker and climber. He is survived by three children.

SETH BORENSTEIN