To be fair, even the expensive weather tracking systems they use get their fair share of the calls wrong and they employ watchers at different parts of the trac

To be fair, even the expensive weather tracking systems they use get their fair share of the calls wrong and they employ watchers at different parts of the track (down the back straights, at the corners of the track, themselves at the pit straight, etc) to get a better idea. And that's over a relatively small area of space too.