The gloom of June: Seattle’s early summer, cloudy skies

With summer just around the corner, temperatures are heating up and sunny skies are dominating the forecast.

But for much of June, the Seattle area is blanketed by early morning clouds before the sun peeks out later in the day. A lot of the time, these clouds may not break at all, making early summer skies look more like the middle of winter.

This cloud cover is so typical that Seattleites even coined the term “June gloom.”

With the summer sun high in the sky and warm temperatures become more frequent, one may think that clouds should become a rarity this time of year, but this gloom can be explained by the differences in heat capacity between the ocean and land.

Temperatures of the waters off the Washington coast tend to hover near 50 degrees in June while areas inland often warm to 70 degrees during the day. This warm air from the land’s surface rises and is replaced by the cooler air coming off the Pacific Ocean, leading to the formation of clouds near the coastline.

If winds are directed from the Pacific Ocean towards the metro area, these clouds may make it all the way into the Seattle area, especially during the overnight hours. These morning clouds prevent temperatures from getting too warm in Seattle during much of June and can even contribute to early morning drizzle.

“If you have the general onshore flow you take that marine air and bring it in right over Seattle,” atmospheric sciences professor Lynn McMurdie said. “It just sits on us, and then it will finally break out later in the day because the sun is still pretty strong but it has to break those clouds up, which is hard to do.”

McMurdie is familiar with this concept and even contributed to a paper about temperatures and radiation, which was published earlier this year. In the study, McMurdie’s team found that this June gloom causes Seattle’s peak annual temperatures to occur nearly six weeks after the summer solstice.

These clouds can be found all the way down the West Coast, towards California, where they have a different name. The fog that engulfs the Golden Gate Bridge is known as the “marine layer” and is the same phenomenon that causes Seattle’s “June gloom.” In San Francisco, these clouds are even more stubborn, as the warmest temperatures of the year occur two months after the summer solstice.

But later in the summer, these marine clouds become less frequent, allowing for sunshine to break out earlier in the day.

“Over time that changes,” McMurdie said. “So like in July. A typical day in July will be cloudy in the morning and sunny and beautiful in the afternoon. So that marine layer is really shallow. If it's shallow and doesn't take a lot of solar heating to warm it up and burn it up.”

As the ocean temperatures warm and the jet stream cools, the clouds become less thick and almost nonexistent later in the summer, allowing for July temperatures to take a dramatic spike compared to June. In fact, the average July day in Seattle is nearly six degrees warmer than that of a day in June.

So while the forecast may display a sun icon and a 70 degree high over the next few weeks, that doesn’t mean that the typical Seattle clouds will be gone just yet.

Reach reporter Anthony Edwards at science@dailyuw.com. Twitter: @edwardsanthonyb

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