How would I make a rain script like in camping?

In camping 1 and 2, when it rains, your screen becomes blurry and you slowly start taking damage from the rain. But when you're not in the rain, the blur slowly goes away and the damage you took stays as your health. Also, I'm expecting this area that they walk into to be a block that when you touch it, the script starts running and when you leave it, your health starts decreasing. How would I do this?