As the temperature drops during the night, you’ll often find that any moisture in the air will condense on any gear you have out in the open. On your tripod, bag or even the outside of the camera this can be unpleasant, but not disastrous. It’s when it forms on the lens, viewfinder and rear screen that it becomes a real problem.
You can minimise the probability of condensation by giving your gear some time to gradually acclimatise, rather than taking it straight out of a warm car or house into the cold air. But on many evenings you’ll still find condensation will form, so take along plenty of cloths and cleaning kit.