Being on the west coast measuring in the rain is sometimes unavoidable and I learned early in my measuring career to always use waterproof notebooks and to carry rain-gear when rain is forecast. Riding in the rain is not so bad, but I still avoid it whenever possible.
One of the concerns I have about measuring in the rain is that a cold rain can cause a significant temperature change in your tires which disappears when the rain stops. The worst part is the extra time it takes to maintain your bike after the ride.