From the Old Faithful Area in Yellowstone National Park, head north for about 4 miles to the Grand Prismatic Spring parking area. Walk about 1/2 miles on to the boardwalk.
The water in the thermal spring is nearly always hotter than the surrounding air therefore there's usually steam rising from the spring. I had to wait 10 minutes for the mist to clear and snap off a couple shots before the mist closed in again. And the mist has a pungent sulfur smell to it. Not only does it smell bad but you want to minimize your camera's exposure to it.
Grand Prismatic Spring from the boardwalk.