Why are warm summer nights less common here than in the eastern U.S.?
One element to consider is that Seattle is farther north and therefore the sunlight is less directly overhead...and less intense than points at latitudes farther south. For example, Seattle is at 47 degrees latitude north while Boston is at 42 degrees north and New York City 40 degrees north. A less recognized, but very important element is that our air holds less water vapor which in turn retains less warmth. This is measured by what we call the dew point. On a recent warm evening in the Puget Sound area, the dew points were in the mid to upper 50's. Those along the east coast, by contrast, were in the upper 60's to low 70's. Therefore the air over the east cold held considerably more water vapor... which acted to reduce overnight cooling. By the way, that's because the air over the eastern U.S. is often dominated by the presence of the Gulf Stream along the Atlantic coast, and by the Gulf of Mexico to the south.