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.