Precipitation during the wet season is primarily caused by air mass thunderstorms and the easterly flow out of the subtropical high (Bermuda High). Intense daytime heating of the ground causes the warm moist tropical air to rise, creating the afternoon thundershowers typical of tropical climates. 2:00 pm is the mean time of daily thundershowers across South Florida and the Everglades. Late in the wet season (August and September), precipitation levels reach their highest levels as tropical depressions and lows add to daily rainfall. Occasionally, tropical lows can become severe tropical cyclones and cause significant damage when they make landfall across south Florida. Tropical storms average one a year, and major hurricanes about once every ten years. Between 1871 and 1981, 138 tropical cyclones struck directly over or close to the Everglades.[27] Strong winds from these storms disperse plant seeds and replenish mangrove forests, coral reefs, and other ecosystems. Dramatic fluctuations in precipitation are characteristic of the South Florida climate. Droughts, floods, and tropical cyclones are part of the natural water system in the Everglades.[30]