I used to think measuring in rain was a bad idea because it would often prove inaccurate when checked under dry conditions. But I think the inaccuracy is not as bad as I (or we, many of us thought this way) thought. Of course it's important to get a calibration ride under similar conditions as when you measured.
So, since I don't really like measuring in the rain, I've had to come up with other excuses -- er, I mean good reasons. One was that your notebook will get too wet and you either can't write in it or what you write will get washed away. But then I discovered, from other track officials, that you can purchase rain-safe notebooks, pencils, and pens-- so there goes reason #2!
I'm just left with this: when conditions are too wet you cannot apply paint marks on the road, or get tape to stick on it. There it is, you can't mark the course . . . until some smart-aleck points out that you could drive in a nail and washer. AND you can often give a precise location by referring to landmarks . . . What can I say? It's possible, I've done it, I don't like it, but I seem to be running out of excuses!