Moisture is bad for compressors, and you should always try to avoid using them in high humidity or heavy rain.  Air compressors often have rotating screws in their motors that suck in air. If the air is saturated with moisture, then it may cause these screws to rust which will spell the end of your compressor.  Make sure to store your compressor in a safe, dry area. Some units will even come with bags for keeping them in, but it isn’t safe to rely on these as not all of them are completely waterproof.