windchill is always NA for San Francisco, CA

Hi:

I am working on a program which reads Weather from wunderground API for San Francisco. I used this url: http://api.wunderground.com/api//cond..., and it always give me a NA on windchill_string, windchill_f and windchill_c. And windchill is the highest temperature of the current day, right?

Please help. Thanks!