Actually, if you look at the National Weather Service wind chill chart and explanation, wind chills are not calculated for temperatures below 50 degrees Fahrenheit. And if it was, given that at 40 degrees with a wind of 20 mph, the wind chill is 30 (a drop of 10 degrees as compared to the 20 you came up with at a higher temperature), well whatever calculator you used - I think it wasn't a very good one.
Here's a link to the National Weather Service wind chill chart, explanation, and calculator: http://www.weather.gov/os/windchill/index.shtml




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