From 1973 through early 2001, the National Weather Service computed windchill based on a formula developed in the 1940s by Antarctic explorers Paul Siple and Charles Passel. Windchill values became iconic in Chicago during the brutal winters of the late 1970s and early 1980s when they frequently dipped as low as 50 to 80 degrees below zero. However, later research concluded that these derived windchill values were too low, and in fall 2001 a new formula, thought to be a more realistic assessment of the effect of wind and cold on human flesh, was implemented. Using the new formula, a temperature of 10 below with a 20 m.p.h. wind would produce a windchill of minus 35 compared with 53 below zero under the old formula.