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Like the entire archipelago, O'ahu has two barely discernable seasons: summer (roughly May to October) and winter (November to April). Temperatures throughout the year vary approximately between 70°F to 80°F with higher humidity likely in the summer; during the winter months it rains more frequently. But the island has several microclimates – governed by the weather patterns of the trade winds and their interaction with the island’s two mountain ranges – that define seasonal differences for each coast and the mountainous interior.

The prevailing trade winds move across the island from the northeast, where most of the moisture is deposited, making the Windward Coast (the east side) and eastern Ko'olau mountain range lush and relatively cooler than other parts of the island. The winds then move westward across the interior of the island, bumping against the Wai'anae mountains, which exacts any remaining moisture, leaving the west side (also known as the Wai'anae or Leeward Coast) and the South Shore (Honolulu and Waikiki) dry and warm. The traditional Hawaiian home is often built with louvered windows to catch the trade winds, but air-con is a welcome invention for coastal Honolulu and Waikiki residents.

During the winter months, Kona winds reverse this trend bringing intense rain to the drier parts of the island.

The National Weather Service provides recorded weather forecasts (973-4381) and mar­ine conditions (973-4382) for all of O'ahu.