It seems accuweather calculates this with their Real Feel algorithm. I just learned about this today, was surprised how much colder their values were compared to windchill:
"The Real Feel temperature is a patented product developed by several of us at AccuWeather. I am not familer with the general explanation. But, I can tell you that we use the following variables to calculate the value:
Sun angle for a given date
Solar intensity as defined from the solar index
percent of sky cover
precipitation and intensity
Each variable carries a weight for a given date and weather situation. These weights are determined through various factors. I cannot give you the formula since this is patented. "
source Expert: Dan Kottlowski http://en.allexperts.com/q/Accuweather-2546/Real-Feel-Temperature.htm
Any third party software get this fancy?