February 18, 2020

A Midwinter Subtlety of Puget Sound Weather

On the surface, today was a gloriously boring weather day over western Washington.

Bright blue skies, light winds, agreeable, but cool temperatures.  A day you would think meteorologists would be taking a siesta.

But like a fine wine, Northwest weather is often best appreciated in its subtleties....and today was no different.

With clear skies and weak winds, the ground could radiate efficiently to space, and much of the region had its coldest morning in a month.  As shown by the low temperatures this (Tuesday) morning (below), temperatures ranged from the low to mid-30s near the Sound, to mid-20s in the eastern Seattle suburbs to even the single digits to a few sites  near Mt. Rainier.  Frost was found all over the region, with fog in river valleys and low spots.

This morning on the way downtown for a breakfast lecture of the wonderful CleanTech Alliance, I saw a band of clouds down the center of Lake Washington.    And reaching the 49th floor of 1201 3rd Avenue Building, I saw a line of clouds extending north-south down Puget Sound.

And I knew why is was there.....and like an experienced meteorological sommelier I was able to savior the moment.   Here is the cloud line as seen from the Space Needle PanoCam---not as dramatic a view as I had enjoyed while listening to a lecture about battery technologies, but perhaps you can see it (click on image to enlarge).

The reason for the line is that there were intersecting land breezes from both sides of the Sound.  But let me explain.

Everyone knows about sea breezes--onshore winds that occur when land gets warmer than water.  But when the opposite occurs, when the land is cooler than water, there is a rush of air from land to water called the land breeze. And such land breezes are best seen when the general winds are light...like last night.

This morning there were land breezes on both sides of the Sound...both headed towards the center of the body of water.   This can be illustrated by a plot of the winds around 8 AM near Edmonds.
Circles indicate calm winds and air temperatures are also plotted (the observations over the water were from a Washington State ferry).  The Sound is roughly 45F this time of the year...roughly 10-15F warmer than the adjacent land.  Enough to form a weak land breeze.

As shown in the schematic below, as the land breezes convergence over the center of the Sound, air is forced to rise, forming a band of clouds.  This is what happened to today over Puget Sound and to a lesser extent over Lake Washington.  Take a careful look tomorrow morning...it may well happen again.


  1. Cliff, thank you. You so help me appreciate where we live. The drama, and the subtleties. Cheers

  2. Cliff - is this the same phenomenon that causes "lake effect snow" in the Great Lakes region?

  3. When I sail in summer, I can on occasion take advantage of the land breeze until about 11 AM. Then, unfortunately, there is often a dead period from 11 AM until as late as 3 to 4 PM before the sea breeze finally gets going, and it may last until almost midnight. It always puzzled me why it starts and ends so late. On the East coast the sea breeze starts at 10 to 11 AM, and dies at sunset.

  4. deliriou5 - No, lake effect snow is caused by the relatively moist air over a lake moving over the colder adjacent land mass. This cools the moist air and creates some precipitation in the form of snow. But the air is not moving thanks to a small, local temperature difference as is explained here. It is moving due to the much larger, normal circulation between air masses (wind).

  5. The minimum temperature at my location in NW Bellingham this morning was 26.2F, the coldest reading of the month so far and the coldest reading since 1/17/19.

  6. Today was also the second warmest day of the month so far at my location in NW Bellingham at 50.8F.

  7. Clean Tech Alliance? Battery technology? The clouds are interesting but the lecture too.

  8. Thanks for a very interesting explanation! Was the hail and thunder that occured Monday night in Issaquah a convergence event? It was a very narrow band, 3 miles wide or so.

  9. I have an elder friend who always says "it's good to be reminded of what you already know". I think your reminders of our local weather subtlety fits with this sentiment. Keep reminding us of the marvel of PNW and our good fortune to be here.


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