All you need to know about the hurricane season

The North Atlantic hurricane season officially began on 1 June, and the US National Hurricane Center expects it to be a busy one. It forecasts a 70 per cent chance of eight to 14 storms reaching hurricane strength, and three to seven becoming dangerous "major" hurricanes of category 3 and above. Reaching the upper end of that range would make 2010 one of the most active hurricane seasons on record. What does that mean for residents of the Gulf and Atlantic coasts of the US, worried about oil spreading from the Deepwater Horizon blowout?

Wasn't 2005 "a once in a century" hurricane year? Why are a similar number of storms forecast this year?

If hurricane numbers were purely random, there would be a 1 in 100 chance of a hundred-year storm being followed the next year by a second hundred-year storm. However, the number of hurricanes is far from random.

Hurricane formation in the north Atlantic and the Caribbean is linked to a cycle called the Atlantic Multidecadal Oscillation. In a cycle lasting 20 to 40 years, sea-surface temperatures from Greenland to the equator rise and fall by about 0.5 °C. During the warm phases, about twice as many weak tropical storms grow into severe hurricanes as during the cooler phases.

The last rise in sea-surface temperatures and hurricane numbers came in the mid-1990s, so the average number of storms now is above the long-term average, and well above the relatively low numbers from the 1960s to the mid-1990s. The number of hurricanes was higher than average from the 1940s to the early 1960s. Those shifts roughly kept pace with the sea-surface temperature cycle.