July 25, 2008 -- We've seen the image so many times: the satellite picture of clouds in the Atlantic or the Caribbean, gradually taking the familiar spiral shape of a hurricane.
Most of us think of the storms as beginning over the steamy Atlantic waters, which provide fuel for hurricanes as they strengthen and threaten landfall.
But it turns out that many storms begin much farther away -- all the way over on the far side of Africa.
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They can start as thunderstorms in Ethiopia, picking up heat and energy as they drift westward across the Sahara desert.
"It's a remote part of the world, but it really does have a lot of implications for folks living downstream," said Jason Dunion, a scientist at the Hurricane Research Division of NOAA, the National Oceanic and Atmospheric Administration.
For years, Dunion and fellow researchers have been studying hurricanes to determine their true origins. That led them to take interest in tropical disturbances from as far away as the Red Sea.
"Only about one in 10 or so of these tropical 'waves' actually forms into a named storm," said Dunion, "but what's really interesting is they account for over half the hurricanes and tropical storms that we see in the entire Atlantic."
Hurricane Bertha, earlier this month, became a named storm just a few hundred miles off the African coast. It was also one of the longest-lasting hurricanes on record, though it did not make landfall.
Dolly, which hit south Texas on Wednesday, did not fit the out-of-Africa pattern. It formed in the Caribbean.
What determines if a particular disturbance will become a major hurricane, or fade to nothing as it crosses Africa? Among other things, scientists are curious about sand from the Sahara, which can be picked up by the wind and sometimes travel for thousands of miles.
Saharan sand -- along with small amounts of microbial life unique to Africa -- has been detected as far away as southern Florida.
Scientists believe the sand may suppress hurricane formation, though the theory is heavily debated. If there are more sand particles in the upper layers of the atmosphere, they theorize, raindrops may coalesce around them, and a storm may lose power before it can actually become a hurricane.
So forecasters look far beyond the tropical Atlantic for the ingredients of a tropical storm. By the time a storm is strong enough to be named, meteorologists may have been watching it for more than a week.
If they can understand a hurricane's true beginnings, they say, perhaps they can give more precise forecasts so that people have more time to get out of harm's way.