April 17, 2025

The Day of No Weather and SuperBlue Skies

When I get up in the morning, one of the first things I do is check the latest satellite image.

Today's imagery (below) was stunning: not a single cloud in the sky


Nearly all my blogs are about some interesting weather feature...the more severe the better.  

But what to talk about on the most boring weather day in years?

When I got to the UW this morning, I took pictures of the sky ..the most perfect blue skies I could remember.


Viewed from the Seattle PanoCam this am, the light from the blue sky on Puget Sound gave the water a super-blue tint.


So why was the sky so clear?

The upper-level (500 hPa, about 18,000 ft) weather map at 11 AM  explaines a lot.   There was a strong ridge or high offshore, and such features have powerful downward motion on their eastern side...which prevents cloud formation.


Sinking air causes warming by compression and relative humidity to drop (since warm air can "hold" more water vapor than cooler air).  This prevents cloud formation.

The predicted relative humidity at around 10,000 ft (700 hPa pressure) was extraordinarily dry, with dark blue colors indicating relative humidities below 30%.


The solar radiation today was unimpeded, resulting in the variation over the day being almost a perfect cosine shape (the values at Snohomish are shown below).

Why is the sky blue on clear days?

Light from the sun has all wavelengths of visible light.   Atmospheric molecules scatter short wavelengths (like blue) more than longer wavelengths (like orange and red).  As shown in the image below, this preferential scattering of blue light makes the sky look blue.


Water droplets and particles (such as from wildfire smoke) scatter all wavelengths similarly, giving the sky a whitest cast.


As you can imagine, small particle concentrations were very low today, as shown by the PM2.5 numbers (concentrations of particles of 2.5 microns or less) around Seattle (below).  With clear skies, low relative humidity, and clean air, no wonder our skies were amazingly blue.


Enjoy the blue skies....clouds move in over the weekend.





15 comments:

  1. I wish I would have been more aware of it. Only tonight was I wondering why the dewpoint was a low 35 degrees. I need to do less sleepwalking through the nice days :)

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  2. So often in the spring the best weather seems to come during the week leaving the weekends wet. My imagination?

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  3. I glanced at the radar and it's lit up just after dark. Time to write about birds again!

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    1. A dark-eyed junco has built a nest in the wreath on our front door, and there are now four eggs in it. Never had this happen before, and it's quite exciting. Will be fun to see the fledglings when they hatch! We are trying to be very quiet when we come and go, as it's the only entrance/exit to our flat. Mama seems not to be be disturbed, however. What a lovely start to spring!

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    2. We had that happen two years ago in the raised planting bed, it was amazing. Sadly, the nest seems to have been raided by a raccoon or something, and there were no hatchlings.

      But my comment on the birds was about the migratory kind. This time of the year, they take flight after dark and continue their migration north, which shows up on the radar as false rain!

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    3. That is amazing! Don’t forget to take photos.

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    4. We have been, including a few that show mama on the nest. She'll fly away if she sees us, but hovers nearby and then returns. Other than a larger bird, no predators are likely (we hope!) to disturb the nest because of where it is. We see juncos coming and going all winter, usually in low bushes, but now one's decided our wreath is the perfect nest for her brood. So charming to watch this all unfold!

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    5. Andamhsoir- I had a very similar experience with a dark-eyed junco building a nest in a flowerpot hanging by my front door last spring. I tiptoed by the nest every time I went out so as not to freak them out. But three days after the babies hatched, I happened to see the pot swinging wildly and a while later, when I looked, the babies were gone. Since it was only 2-3 days after they hatched, I suspect they were eaten by crows, which come around often. Watch out! I don't know why they did not choose the birdhouse I offered: It would have been more protected.

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  4. We exist and gauge our surroundings by reflected light, a tiny slice of the electro magnetic frequency spectrum called “visible light” We are walking blind really The grass is green We call a certain frequency in the spectrum “green” The developmental phase of our biological robot purposely left us under throttled by limiting the sensor range. You’re dealing with an incredible design capacity, the developer didn’t patent his work. In fact he has camouflaged his existence

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  5. Good post. We've certainly been enjoying this current break from rain (just five days with zero to report), and the longer days have been great. Yards are being tidied-up, trees and bushes are leafing-out, daffodils and other bulbs are in full bloom. Even though it's not raining, it's been breezy in some spots and there are plenty of other phenomena to observe, so yes - "weather" is never dull. Good forecasts help us make the best of every day.

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  6. "The bluest skies you've ever seen are in Seattle"

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  7. The waters off the Pacific Coast seem to be cooling and the Eastern Pacific warming. Perhaps this will keep our summer cooler. If these waters don't cool as much as forecast we may see higher pressure and warmer temps region wide. This might set us up for a neutral winter next year?

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  8. cc: Perry Como

    https://www.youtube.com/watch?v=8NUzNtqu3zQ

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  9. Thank you for making complex concepts so clear for those of us who didn’t understand scientific concepts so well when we were younger but are still trying to figure them out in later years…

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  10. Thank you for explaining the science behind the exceptionally clear skies we've had this week. It is always fascinating to learn what causes various weather phenomeon, both good and bad.

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Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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