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Purple Rain

Why aren’t Seattle’s temperature extremes aligned with the solstices?

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Editor’s note: Purple Rain is a column that explores relevant weather topics in the Seattle region, highlighting research conducted by UW faculty.

As any Seattleite knows, “June gloom” often puts a damper on any early-summer parties.

Clear skies for Independence Day are a long shot and the hottest days of summer seem to arrive at the very end.

Is this really the case, though?

As a group of scientists at the UW discovered, you’re not crazy to think that the temperatures are misaligned from the seasons.

“We were just talking at coffee hour one day and started talking about the climatology of Seattle,” physicist Aaron Donohoe said. “And how weird it is that the winter minimum [temperature] is early January, but the seasonal maximum is August. That just defies the way we understand things work.”

This curiosity prompted Donohoe, a researcher in the Applied Physics Lab, to explore the asymmetry between the solstices and temperatures. Donohoe compiled a team of scientists to explore the topic further.

Why aren’t Seattle’s temperature extremes aligned with the solstices?

Graphics with courtesy of Brian Brettschneider. Climatologist: University of Alaska, Fairbanks


Because the heat capacity of water is greater than air, temperature extremes near oceans are delayed by many weeks in comparison with the solstices. The wind coming from the relatively warm water in the early winter keeps temperatures in Seattle much warmer than areas further away from the ocean, such as Eastern Washington. This same reason causes the early summer to be much cooler, as the cold sea winds blow into Western Washington.

What surprised researchers, though, was the difference in this delay in peak temperature between the winter and summer. In Seattle, the annual minimum temperature occurs about two weeks after the winter solstice, while the maximum is six weeks after the summer solstice.

Why aren’t Seattle’s temperature extremes aligned with the solstices? 1

Graphics with courtesy of Brian Brettschneider. Climatologist: University of Alaska, Fairbanks


"So what we found is that what is going on in Seattle is basically going on over the whole West coast,” Donohoe said. “I think San Francisco is probably the most prominent one. Their summer maximum temperature is way late, like mid-to-late August.”

After finding these differences, the team at the UW began hypothesizing reasons as to why this lag is so different between the seasons.

One potential answer lies is solar radiation.

While the solar cycle is aligned with the solstice, the amount of radiation reaching the surface is different due to cloud coverage. The team found that the maximum surface radiation is 14 days later than the summer solstice, while the winter minimum is 10 days prior to the winter solstice.

Lynn McMurdie, an atmospheric scientist and contributor to the paper, says the answer lies in the seasonal cloud coverage, especially the infamous “June gloom.”

“If you have onshore flow in the spring and summer you’re bringing in cold marine air into the interior,” McMurdie said. “And the clouds, too. That kind of suppresses the warming from the sun. The sun angle is high, but it has to get through this layer of clouds, and it delays the heating of the land.”

Along the California coast and in San Francisco, the delay in peak summer temperatures is even more prominent than in Seattle — occurring nearly two months after the solstice. 

Why aren’t Seattle’s temperature extremes aligned with the solstices 2

Aaron Donohoe Applied Physics Lab UW.

As summer goes along and the ocean warms, clouds that dominate the West coast in the early summer become less frequent, allowing for the sun to reach the surface.

This downwelling solar radiation reaching the surface is also a reason why the minimum surface solar radiation precedes the winter solstice.

“In the winter, the clouds play a role in a different way,” McMurdie said. “If you look at the coast, the rainiest months are November, December, and January, almost equally. So when you have a deep, thick cloud system, it’s almost like bringing the solstice earlier, because there’s so little solar radiation coming in so there’s no more heating.”

Although the team has found potential answers for much of the seasonal lag across the Western U.S., they plan to continue the research in the future. Donohoe’s team hopes to answer the question for additional areas, as well as explore the same lag on a diurnal cycle.

At least for now, there is an answer to why Seattleites still need jackets come the first day of summer. Maybe it will be warm enough for a celebration on the Fourth of July.

Reach reporter Anthony Edwards at Twitter: @edwardsanthonyb

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