Forecasters for days watched Harvey, Irma, Jose, Lee and now Maria make steady marches west off Africa before they got named. About four out of five major hurricanes - those with winds of at least 178km/h - start out similarly: They form off the African coast as unstable waves or patches of storminess. The National Hurricane Centre monitors them, giving them yellow, orange or red letter Xs on forecast outlook maps.
Not all of these waves survive the trip west. They need favourable winds, warm water and moist air to get stronger. Some get strong immediately; others intensify over the Caribbean or the Gulf of Mexico.
Some even don't get their acts together until they cross over the Pacific, said Colorado State University hurricane researcher Phil Klotzbach.
The rest of the storms usually form in the warm and unstable waters of the Gulf of Mexico, popping up from mid-latitude normal storm fronts, often early or late in hurricane season, Klotzbach said.
During the peak of hurricane season - mid-August to mid-October - it's Africa that acts as the chief Atlantic storm generator.
Even those less common ones that form in the Gulf of Mexico aren't total surprises with meteorologists monitoring storm clouds clustering together a couple days before they become named storms.
Once a named storm forms, "it's hard to get rid of it" and it'll keep going until it's stopped, Klotzbach said.
Four things generally kill a hurricane: High-level winds, dry air, cold water and land. And it's pretty much just chance if they run into any of those four storm-killers, said MIT meteorology professor Kerry Emanuel.
High-level winds - called shear - are a major issue. These winds at about 3000m high can decapitate a hurricane. Maria, the latest storm, has almost no shear and so it can get more powerful, but a wall of shear hit and is killing Lee, Klotzbach said.
Warm water is a hurricane's fuel - the temperature needs to be 26C or warmer. When the water cools, the storm runs out of gas. Sometimes the storm runs into cold water and other times it makes the cold water itself by not moving much and churning it up from the depths.
When storms go over land, they lose fuel and eventually disappear. That happened to Harvey and Irma.
On average, Atlantic named storms last about six days. But there are exceptions. One almost lasted 28 days in 1899. Hurricane Ginger made it to 27 days in 1971. Five years ago, Hurricane Nadine lasted for 22 days.
Starting off from Africa, Nadine made three loops in the unpopulated central Atlantic, forming a track that looks like a long-tailed bird. It became a hurricane twice, a record 13 days apart.
Blowing in the wind
How hurricanes are categorised according to the US National Hurricane Centre:
Category 1: Sustained winds of 119-153km/h. Very dangerous winds will produce some damage
Category 2: Sustained winds of 154-177 km/h. Extremely dangerous winds will cause extensive damage
Category 3 (major hurricane): Sustained winds of 178-208km/h. Devastating damage will occur
Category 4 (major hurricane): Sustained winds of 209-251km/h. Catastrophic damage will occur
Category 5 (major hurricane): Sustained winds of 252km/h or higher. Catastrophic damage will occur
- AP
