December to March are the hurricane months, with average temperatures of 27° C. The southeast trade winds blow from April to November and temperatures average 24° C. The 2,047 mm of annual rainfall is fairly well distributed throughout the year, with a slight peak during the hot southern summer.
There's good anchorage at Alofi, except during strong westerly winds.
In January 2004, Heta, the worst hurricane in living memory, flattened buildings and vegetation on the southwest side of Niue.
The official hurricane (or cyclone) season south of the equator is November-April, although hurricanes have also occurred in May and October. Since the ocean provides the energy, these low-pressure systems can only form over water with a surface temperature above 27° C; during years when water temperatures are high (such as during an El Niño) their frequency increases. The rotation of the earth must give the storm its initial spin, and this occurs mostly between latitudes 5 and 20 on either side of the equator.
As rainfall increases and the seas rise, the winds are drawn into a spiral that reaches its maximum speed in a ring around the center. In the South Pacific, a cyclone develops as these circular winds, rotating clockwise around a center, increase in velocity: force 8-9 winds blowing at 34-47 knots are called a gale, force 10-11 at 48-63 knots is a storm, force 12 winds revolving at 64 knots or more is a hurricane. Wind speeds can go as high as 100 knots, with gusts to 140 knots on the left side of the storm's path in the direction it's moving.
The eye of the hurricane can be 10-30 kilometers wide and surprisingly clear and calm, although at sea, contradictory wave patterns continue to wreak havoc. In the South Pacific, most hurricanes move south at speeds of 5-20 knots. As water is sucked into the low-pressure eye of the hurricane and waves reach 14 meters in height, coastlines can receive a surge of up to four meters of water, especially if the storm enters a narrowing bay or occurs at high tide.