News Source: 
UW Today
July 19, 2013

Nighttime heat waves are becoming more frequent in western Washington and Oregon.

And if you don’t sleep well in hot weather, this might be a good time to buy a fan, since records show that on average heat waves tend to strike around the last week of July.

University of Washington research shows that the region west of the Cascades saw only three nighttime heat waves between 1901 and 1980, but that number quadrupled to 12 nighttime heat waves in the three decades after 1980, according to a paper published in the July issue of the Journal of Applied Meteorology and Climatology.

Nighttime heat waves are when the daily low is in the top 1 percent of the temperatures on record – in Seattle above around 61.5 F – for at least three nights in a row.

“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.”