The study, published recently in the journal Geophysical Research Letters, looked at relative humidity data from the Atmospheric Infrared Sounder (AIRS) instrument on NASA's Aqua spacecraft. Data from close to 200 North Atlantic hurricanes between 2002 and 2010 were analyzed and compared to all the other data available from the National Oceanic and Atmospheric Administration's National Hurricane Center.
Researchers found that hurricanes that rapidly intensified also tended to have higher relative-humidity levels than storms that weakened or stayed the same. In other words, rapidly intensifying hurricanes tend to have a more moist large-scale environment.
"Our results show relative humidity and its variations within a hurricane's large-scale environment may be useful predictors in improving intensity forecast models," said Longtao Wu, lead author of the study and a reasearcher at the University of California Los Angeles-Jet Propulsion Labratory Joint Institute for Regional Earth System Science and Engineering. "This is the first satellite analysis to quantify this small but statistically significant correlation."
Since the early 90s, forecasts of hurricane paths have gotten progressively better. Forecasts of hurricane strength, however, have not improved nearly as much. This is due to the fact that hurricane intensity is sensitive to a variety of factors within the storm and its environment. In general, relative humidity decreases the further from the storm's center it is measured.
"We speculate that decreasing relative humidity levels farther from a storm's center may be an important factor in a cyclone's rapid intensification," said Hui Su, study co-author. "A drier environment farther from a storm's center limits the development of its outer rain bands and favors the growth of its inner core. Conversely, a wet environment farther from a storm's center can weaken a cyclone by making it easier for rain bands to form outside the storm's core, which compete with the inner core's growth."
(Image courtesy NASA GSFC/LANCE MODIS Rapid Response Team)