While the 2012 Atlantic hurricane season is winding down and experts expect another hurricane or two before it's over.
The really busy part of the season — typically August and September — is over, but this week marks the second "peak" of the hurricane season.
"On average, cyclones are more likely to form this week than any time in the last three weeks of the season," said Brian McNoldy, a hurricane researcher at the University of Miami.
The official end to hurricane season is November 30.
In the summer leading up to the true peak of the hurricane season on Sept. 10, cyclones are most likely to form in the eastern and mid-Atlantic Ocean, often in the tropics or subtropics. But conditions there are becoming less hospitable to hurricane formation, mainly due to an increase in wind shear, which is an imbalance between winds at the surface and higher up in the atmosphere.
Wind shear is caused by differences in atmospheric pressure between neighboring regions and reduces the strength of cyclones by separating their warm core from the system of circulation above it. Cyclones are basically large heat engines that are driven by a temperature difference between the warm ocean surface and the cool atmosphere above; wind shear disrupts this pattern of circulation and weakens storms.
Now, hurricanes are more likely to form farther west, in the Caribbean and Gulf of Mexico. In those areas, there is an abundance of warm water ready to fuel cyclones. On average, surface temperatures need to be at least 78 degrees Fahrenheit (26 degrees Celsius) to fuel hurricanes, according to the National Oceanographic and Atmospheric Administration (NOAA).
At this point in the year, weather systems are more likely to reach farther south than earlier in the season, McNoldy said. That's because there is more cold air over North America, making cold fronts stronger and better able to penetrate further south, he said. These cold fronts often produce low pressure systems that pull in moisture-rich air from nearby, which tends to rise and create thunderstorms in a swirling pattern. Given the right conditions and enough time to grow over the warm waters of the Gulf and Caribbean, these systems can become tropical cyclones, he said. (Tropical cyclone is a generic term for hurricanes, tropical storms and typhoons.)
This hurricane season has been slightly more active than average, McNoldy said, with nine hurricanes and 17 named storms.