The Marine Pushes Of Summer

There’s a reason why very few people in the Pacific Northwest have air conditioning. For much of the summer, cool, marine air flows into Western Washington at night, keeping low temperatures in the 50s and preventing our residences from getting too hot. Of course, we have those periods in the summer when highs skyrocket into the 90s and even exceed 100 east of the Cascade crest, but these periods are few and far between; Seattle only averages two days over 90 a year, and even during these days, lows drop into the 60s. On extremely rare occasions, high temperatures in Western Washington can exceed 100 with lows exceeding 70, and when this happens, it is best to swim in the Pacific Ocean/Puget Sound, where water temperatures rarely exceed the 50s even in the height of summer.

The Pacific Ocean is directly responsible for the notable lack of air conditioners per capita in Western Washington. During the summer, the land heats up faster than the ocean, lowering the atmospheric pressure inland relative to the pressure on the coast and causing ocean-cooled air to flow from the coast into Western Washington in the evening and nighttime hours. This air is both cool and moist, so when we wake up in the morning, we are often greeted to a thick layer of stratus clouds with temperatures some 20 degrees cooler than they were just 12 hours ago. This phenomenon, known as “onshore flow” or, more specifically, a “marine push,” is very common throughout our area during the summer, and we’ve experienced it in spades this week.

Take a look at the visible satellite picture from 7:30 this morning. Low, marine stratus clouds throughout Western Washington. A very refreshing and cool start to the day.

At 11 a.m., there is some clearing far off our coast over the open ocean, but the clouds are still firmly in place over Western Washington.

By 2 pm, the clouds are starting to burn off.

By 6 pm, they are completely gone over Western Washington, giving us bright blue skies to wrap up the day. However, much of the coast is still shrouded in clouds.

There are several ways to approximate the strength of a marine push, but one of the best ways is to look at the pressure gradient difference between a station on the coast and a station further inland in the Western Washington lowlands. The higher the pressure gradient, the stronger the westerly flow, and the stronger the marine push.

Here are some pressure gradients in hectopascals (hPa) over the Pacific Northwest for the last 24 hours. The time is on the left and is in Coordinated Universal Time (UTC). Pacific Daylight Time is 7 hours behind UTC. For example, midnight June 30th UTC is 5 pm June 29th PDT. The main gradient we want to look at is the gradient between Hoquiam and Seattle (HQM-SEA). A positive number means that Hoqiuam has a higher pressure, and since air flows from higher pressure to lower pressure, it implies that there is onshore flow. The stronger the gradient, the stronger the onshore flow. Generally speaking, a light marine push occurs when the gradient is between +1 and +2 hPa, a moderate push occurs from +2 to +3.5 hPa, and strong pushes occur when the pressure gradient is greater than +3.5 hPa. As you can see, last night we had a moderate marine push, and although the gradients slacked off this afternoon, they picked up again after the clouds dissipated inland and the atmosphere warmed, causing the surface pressure to drop slightly. With any luck, we’ll have another marine push tonight, but this one will likely be a tad weaker than the one we just saw.

However, you need more than just a strong pressure gradient to get a marine push like the ones we see during the summer. The air also needs to be relatively stable, with either a small decrease/no change in temperature in height or even an increase in temperature with height (also known as an inversion). If the atmosphere is not stable, air can easily rise and fall, and you will tend to get cumiliform clouds that are formed by convection, the process where warm air rises due to it being less dense than its surroundings. Some of the strongest of these pressure gradients occur after an intense low pressure system has passed to the east of the area, sending storm force winds down the Strait of Juan de Fuca in its wave. However, with a relatively steep temperature decline with height in the wake of the storm (not to mention extremely strong winds), the atmosphere becomes extremely well-mixed, preventing individual cloud layers from forming and instead favoring cumulus clouds and our all-too-familiar post-frontal “showers and sunbreaks.”

Take a look at some temperature profiles taken from 4 a.m. to 10 a.m. today at Sand Point in Seattle when Western Washington was under a solid deck of stratus clouds. A clear inversion begins around 500 meters up, and from 800 meters to 1600 meters, the temperature stays relatively constant between 14 and 15 degrees. That’s a pretty stable atmosphere.

Now, let’s take a look at some air temperatures at the same spot this afternoon and evening after the clouds burned off.

The afternoon sun warmed the surface, destabilized the atmosphere, eroded the inversion, and erased our low morning clouds.

Here’s a special treat for ya’ll… check out a super-high resolution satellite image from the MODIS sensor aboard NASA’s polar-orbiting TERRA satellite. There are tons of low clouds all along the West Coast, but with the exception of Western Washington, they didn’t make it too far inland.

Thanks for reading, and expect more morning clouds and afternoon sun tomorrow!

Charlie