Weather Blog

Why is late July the peak of summer, instead of June 21?

Why is late July the peak of summer, instead of June 21?

Welcome to what is statistically the driest time of the year in Seattle -- truly the peak of summer.

July 30th and August 4th are tied in the record books for least frequent amount of rain -- over the past 117 years, it's only rained on those dates 9 times.

(Yes, it's no coincidence that Torchlight Parade and the big SeaFair events are held during this last weekend in July/first weekend in August. They did their homework.)

Here is a neat graph depicting Seattle's odds of rain versus the calendar:

It's also the warmest time of the year. Normal high temperatures are now at their peak levels of 77 degrees and will hold there through early August, when we begin our gradual march down to winter's low of 45.

But you might be wondering why now and not on the summer solstice on June 21, when the days are at their longest and the Northern Hemisphere is receiving the most sunlight? Doesn't logic dictate that the end of June should be the hottest time of year?

The trick is that just like when you put a pot of water on a stove, it takes a while to boil, there is an inherent delay in the warming of the atmosphere. It takes roughly 30 days for the peak of the sun's warmth to transfer down to the lower levels of the atmosphere.

So that is why there is a lag time for heat. Around here, it coincidentally matches with when the jet stream is usually at its farthest distance from us. In the summer, the jet stream arcs well to our north into Alaska (or at least, it's supposed to, Mr. 2011 summer), then spends the late summer and early fall gradually drifting south until it roosts around here in late October and beyond.

(While August is one of our nicest months, it's already getting wetter in southern and southeastern Alaska. There, April, May and June are the drier months as the jet stream remains south, and in Anchorage, August is one of their wetter months, and then if you go further south, Juneau's wettest months are September and October, and then here in Seattle, our wettest months are November through January.)

A similar, but longer lag time comes in to play with hurricane season. Only with ocean waters, it takes about 90 days for them to heat up. That's why the peak of hurricane season is in September and October because that is when the Gulf of Mexico, Caribbean and Atlantic waters are at their peak warmth.

So get out and enjoy this weekend, knowing it's there almost every year if you need to get about the best guarantee Seattle has to offer for getting by dry. Just plan ahead way early if, say, you want to get married outdoors around this time -- the "secret" is well known to many a local wedding and event planners!