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Here's a program from our archives.HURRICANE SEASON: CausesAlong the eastern seaboard of the US, and in the tropical Atlantic, we've come to associate September with the peak of hurricane season. But why is it that hurricanes happen mostly at this time of year? I'm Jim Metzner and this is the Pulse of the Planet.Chris Landsea is a meteorologist with the National Oceanic and Atmospheric Administration's Hurricane Research Division. Landsea: The annual cycle of hurricane activity, is due to environment in which the hurricane evolves. First of all you've got to have warm water, and that is common throughout the tropics almost for the entire year, but it is warmest throughout August, September, and October. The second key ingredient is a minimum of what's called vertical wind shear. You could think of it as knocking the top off the hurricane or disrupting the circulation of it. During the winter time, you have a lot of vertical shear, and it prevents storms from forming, or even if you do get a storm to form, it doesn't allow it to get very strong.When a tropical storm has sustained winds of 74 miles or more, it becomes a full-fledged hurricane. On average, we get about 5 or 6 hurricanes a year, less than half of which hit land. Although everyone except surfers usually regard these storms as catastrophic, hurricanes are not without benefits.Landsea: One beneficial aspect of tropical storms and hurricanes is that they do bring water, as rainfall, to a lot locations that depend on it. Another is probably the ecosystems in the tropics that are geared toward periodically having hurricanes to fulfull an environmental niche there -- periodically a removal of the forest and then growing back again is probably something that is needed in the ecosystem in the tropics.More on hurricanes in future programs. We've been listening to a program from our archives. If you want to hear more, check out our podcast.