Up to the range officers/marshals.
On the fullbore ranges at Bisley, there are only 3 meteorological reasons that shooting will be discontinued:
1. Can't physically see the target (rain/sleet/fog/snow/take your pick)
2. Lightning (lying in an open field clutching a couple of foot of stainless steel!)
3. Winds of such intensity that the target frame mechanisms will jam if you try to raise the targets (think sails) above the pit.
I was at the Imperial last year. The friday morning was intense rain. If you saw the articles, part of Century turned into a sizeable lake, some of the campsites flooded, etc. Several inches of rain fell in a few hours.
Shooting however, continued right up till the thunder and lightning started up before it was cancelled for the rest of the day.
At the Smallbore meeting this year one detail was postponed when an intense squall blew in for 20 minutes. The shooting was not cancelled because the weather was affecting the shooters. Merely because the targets were so wet that they were pulping on the frames. With the strong winds blowing them, they were tearing round the edges of the bull-dog clips and falling off the frames. Oh, and people were aiming at their neighbours target's because their sights ran out of clicks before they'd finished accounting for the windage
"A fear of weapons is a sign of retarded sexual and emotional maturity." Sigmund Freud
Shooting is my meditation