To keep the rain off and raise the soil temperature, cloches made of clear polyethylene work well. I used rebar and extra fence posts to weight down the plastic. Of course, many other materials would work just as well. My good friend, Ron Gilbert, of Benton City let me have 150 feet of horse fence he had just pulled out at his house. Of course, I'll be sharing some veggies in return.
Peas rot in soil that is cold and wet, so it helps to keep the rain off as well as warm the soil.
Other vegetables like spinach don't mind if it gets a little too wet once in a while. It stays on throughout the cool season serving the dual purpose of pest protection. Notably, leaf miners which otherwise can be a serious problem in spinach can't lay a finger on a single leaf. In addition to the posts I usually throw some dirt on the edges to make sure it is completely bug-tight. When laying out the row cover be sure to leave it loose enough to allow the plants to grow unimpeded.