Park visitors enjoy the swimming pool facilities at the Bothe - Napa Valley State Park just north of St. Helena. June 21, 2010
3. Pools get thirsty, too When temperatures soar, crystal-blue pools are more likely to turn green. Algae starts spreading and growing under the baking sun "like a greenhouse," making pools require more chlorine, says Best Pool Supply Inc.'s Jim Veach. A small temperature burst from 84 degrees to 90 degrees will make a pool need almost double its amount of chlorine, Veach said.