We spent the weekend working at a carnival where the weather was hot and humid. Our food is stored in a refrigerated truck and it was always a thrill to step out of the truck and into the humid air where your glasses would immediately fog up. Any boxes or equipment that we took outside after being in the nice cool air conditioning would quickly be covered with water droplets.
Condensation like this can be an inconvenience, but for photographers, it can be a real problem. A couple years ago I was on a cruise down the Yangtze River in China and one morning got up and stepped out on the balcony to photograph the sun rising over a bridge. My camera was quite cool after being in the air conditioned cabin all night.
I grabbed a few shots of the bridge in the fog and then realized that my camera and lens were quickly becoming covered with condensation. Camera equipment doesn’t warm up nearly as fast as your glasses. So, any condensation that forms on the lens is going to stay there for a while. You can try and wipe it off with a lens cloth, but as long as the lens stays significantly cooler than the air, condensation will continue to form.
If you know that you will be in this situation, you really need to move your camera equipment into the warmer environment and give it time to warm up. Keep a good lens cloth with you. I really like the small chamois type lens cloth. They work really well with condensation and in misty environments like near waterfalls and in clouds and heavy fog.
One word of caution though is that you should never change lenses when your camera is fogging up. You can always wipe the condensation off of the lens but if you open the camera, condensation can form on the digital sensor and internal controls and that can be just the beginning of a lot of headaches.