Sacramento has a hot-summer Mediterranean climate (Köppen "Csa"), characterized by very hot, dry summers and mild to cool winters with occasional rainfall. The wet season is generally October through April, though there may be a day or two of light rainfall in June or September. The normal annual mean temperature is , with the monthly daily average temperature ranging from in December to in July. Summer heat is sometimes moderated by a sea breeze known as the "delta breeze" which comes through the Sacramento–San Joaquin River Delta from the San Francisco Bay, and temperatures cool down sharply at night.