Putting on rain pants just before a rain, or before hiking into long, wet grass, is obviously ideal, but not always possible. Putting them on too soon may cause you to overheat and sweat. If you put them on after rain starts, you’re already a little bit wet, but the pants will keep you from getting any wetter, and will help you stay warm by adding another layer to trap body heat. It’s a judgment call based on the conditions, but it’s best to always keep rain pants easily accessible so you can put them on quickly.