August 26, 2009

Humidity Creates Nighttime Heat Waves In California

Scripps climate researchers have issued a new study showing an increase in the number of hot, humid days in California.

Led by Alexander Gershunov, of the University of California's Scripps Institution of Oceanography, the team examined a heat wave that hit the southwestern United States from mid July to early August 2006.

They noted that temperature records were broken during this time and also noted a link between high humidity levels and the deaths of more than 600 people, 25,000 cattle and 70,000 poultry in California.

The high humidity levels allowed the heat to stick around throughout the nights, which is a shift from the cooler summer nights that are normal for California.

"Hot, sticky nights in turn lead to hot humid days, helping to perpetuate the heat wave, "and the cycle feeds on itself until the winds change," said Gershunov.

Scientists noted that there are heat waves that fall into either of two types. One is the typical daytime heat wave event, which involves dry daytime heat and "rejuvenating" nighttime cooling. The other type includes less-common nighttime heat waves, which occur when there are high humidity levels.

"Water vapor is the main greenhouse gas. During the night in humid environments, air doesn't cool nearly as much as it does in dry conditions," said Gershunov.

"Elevated humidity also causes heat waves to last longer. Hotter nights pre-condition hotter days and the cycle feeds on itself until the winds change."

"The weather pattern that traditionally causes heat waves in California is tending to bring with it more humidity, changing the character of heat waves from the dry daytime heat and cool nights typical for this region, to the muggy heat around the clock that locals are simply not accustomed to," added Gershunov.

Researchers found that the 2006 pattern of high humidity is part of a trend of increasing nighttime heat wave activity that has been speeding up since the 1980s.

The nighttime heat waves of 2001, 2003 and 2006 were each unprecedented on record when they occurred, researchers wrote.

"We care about them because people and animals and the environment here are just not used to it," said Gershunov.

"And it impacts health, agriculture and energy use in a huge way."


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