Bert Sperling, of research firm Sperling's BestPlaces, said Seattle's mild temperatures, cool nights and low humidity helped it reach the top of the list, followed by Portland, Ore., and San Francisco.
Other cities making the summer comfort list include Denver; San Jose, Calif.; Buffalo, N.Y.; Salt Lake City; Milwaukee; Detroit; and Pittsburgh.
"These are the cities that are most likely to keep cool and comfortable during summer," Sperling said. "Every city will have some days that sizzle, but these are the places with the best combination of manageable highs, cool nights, and lower than average humidity."
Sperling said the top "Sizzling Cities" -- those with the worst heat and humidity without cool nights to balance them -- include Phoenix, Las Vegas and Dallas.