It's not in the HTML that I can find. I believe it's *mostly* dictated by the weather service, plus the refresh time set in the widget. If you have a widget that only refreshes the icon every hour, for example (or two, I've seen some set for that), even if the weather service changes it at 9, you may not see it until 10. (times suggested are arbitrary, LOL, I have no idea what the exact timing is)
On the other hand, if you set the widget to refresh more often, you'll catch the "flash" as it changes more often when you're looking at your springboard. THAT drives me batty. I had a widget I adjusted the other day that was pinging for weather every 15 minutes, and I was sick to death of seeing the "dunno" png, even briefly. So it's a trade off.
I *really* need to get a hold of that weather icon set. Did it come with Noki, or some other set? I don't think I've seen a widget download that came with it, but I probably wasn't paying enough attention at the time. LOL