Although summers are hot and humid, the high temperatures are tempered by frequent thunderstorms. The summer wet season extends from about mid-May to November, with temperatures ranging into the low 90s (30 to 35 degrees Celsius). The majority of the year's average annual rainfall (nearly 60 inches, or 152 cm) falls during the wet season. Continental cold fronts generally cannot penetrate as far south as south Florida during the summer, so warmth and humidity build. Thunderstorms arise when warm, moist air masses combine with an unstable atmosphere. Thunderstorms can be as brief as a few minutes or as long as several hours. As cooler air glides inland from the Gulf of Mexico and Atlantic Ocean, thunderstorms can erupt along sea-breeze fronts. At times, these thunderstorms can be an almost daily occurrence. Sometimes these storms move very slowly, enabling them to drop several inches of rain. The largest amounts of rainfall typically are associated with tropical storms and hurricanes.