Nighttime Heat Waves in Pacific Northwest Quadruple over Last 30 Years

The heat is turning up in the Pacific Northwest. Washington and Oregon have been documented undergoing a marked rise in nighttime heat waves.

Nighttime heat waves -- defined by a three-day period where the daily low temperature is in the top one percent of highest temperatures recorded -- have more than quadrupled in the last 30 years, according to researchers at the University of Washington.

According to the researchers, the region west of the Cascades saw only three nighttime heat waves between 1901 and 1980, but that number quadrupled to 12 nighttime heat wave events in the three decades after 1980.

"In general, minimum daily temperatures have been warming faster than maximum temperatures, so we're not surprised to see a trend in the minimum events," said corresponding author Karin Bumbaco, a research scientist at the Joint Institute for the Study of the Atmosphere and Ocean. "Still, we were surprised to see this significant increase in the frequency of nighttime heat waves."

Bumbaco and her colleagues' research was sparked by a 2009 heat wave in the northwest that broke temperature records and led to a local run on air conditioners in a region where air-conditioned homes are uncommon.

In studying data from the 2009 heat wave, the researchers noticed the record-setting daytime temperatures, but what really stood out, they said, was the string of abnormally warn nights. The string of eight consecutive hot nights was the longest seen in the observational record.

"It was hard to cool down at night, there wasn't much relief at all," Bumbaco said.

The researchers first explained the heat waves by the known phenomena of the cool breeze blowing in off the ocean being overcome by warm air flow from the east. But they also noticed that the nighttime heat waves also correlated with high humidity, where water vapor in the air serves as a blanket that traps the heat.

Bumbaco says the new observations may help weather forecasters more accurately differentiate between hot days and hot nights in the future.

Bumbaco and her colleagues' research is published in the Journal of Applied Meteorology and Climatology.