Science Codex: A new study has found that heat waves are increasing in the western portions of the Pacific Northwest, but not the kind most people envision, with scorching hot days of temperatures reaching triple digits. These heat waves occur at night.Researchers documented 15 examples of "nighttime heat waves" from 1901 through 2009 and 10 of those have occurred since 1990. Five of them took place during a four-year period from 2006-09. And since the study was accepted for publication in the Journal of Applied Meteorology and Climatology, another nighttime heat wave took place at the end of this June, the authors point out.
"Most people are familiar with daytime heat waves, when the temperatures get into the 100s and stay there for a few days," said Kathie Dello, deputy director of the Oregon Climate Service at Oregon State University and a co-author on the study. "A nighttime heat wave relates to how high the minimum temperature remains overnight.
"Daytime events are usually influenced by downslope warming over the Cascade Mountains, while nighttime heat waves seem to be triggered by humidity," said Dello, who is in OSU's College of Earth, Ocean, and Atmospheric Sciences. "Elevated low-level moisture at night tends to trap the heat in."
In their study, Dello and co-authors Karin Bumbaco and Nicholas Bond from the University of Washington defined heat waves as three consecutive days of temperatures at the warmest 1 percentile over the past century. Using that standard criterion, they documented 13 examples of daytime heat waves during the time period from 1901 to 2009. Only two of those occurred in the last 20 years.
In contrast, nighttime heat waves have been clustered over the past two decades, with what appears to be accelerating frequency. A warming climate suggests the problem may worsen, studies suggest....
Gresham, Oregon at night in 2009, shot by City of Gresham, Wikimedia Commons, under the Creative Commons Attribution-Share Alike 3.0 Unported license
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