St. Helena is in the heart of the Napa Valley, nestled between?Calistoga?to the north and?Rutherford?on its southern border. On its western side, the Mayacamas Mountains guard it from the cooling effects of the Pacific Ocean; to its east stand the Vaca Mountains. In conjunction, these mountain ranges serve to lock in summer daytime heat. But in the evening, cool air from the San Pablo Bay funnels up through the valley, creating very chilly nights. It isn?t uncommon for temperatures to drop 50 degrees, a shift that promotes a balance of sugar ripeness and acidity in wine grapes.