The case of the missing clouds
January 18, 2017 6:00 PM   Subscribe

So I have been blessed with proximity to a large window at work, and given that the days have gotten short enough that the sun sets during the work day, I thought that a nice silver lining (or red-orange lining) to the short, cold days would be watching the sunset right before the workday ends. The DC area is not exactly known for its spectacular sunsets, but I have been noticing the strangest recurring sunset fizzle.

No matter how numerous and fluffy and glorious the clouds are from 9 am to 4:30 pm, by 5:00, when the sun is setting, they will have done one of three things:

1) Vanished, leaving behind only one or two ghostly scraps.
2) All consolidated into one long, thin, sunset-blocking cloud at the bottom of the horizon
3) If it was a really cloudy day to begin with, the whole sky will remain covered by a single haze of cloud.

For a successful sunset, you need a few clouds reasonably high above the horizon for the light to turn them pink, but this just isn't happening because the clouds are either too low or nowhere to be found. And this cloud departure is so specific to the sunset timeframe that it feels almost like the clouds themselves work 9-5 and are clocking out and heading home right at sunset time.

Is this cloud pattern and timing a Thing? Like is there any known phenomenon related to clouds thinning out or lowering relative to the horizon at the end of the day? Could it be specific to the direction I'm facing (north or southwest, I think) or the geographic locale?
posted by space snail to Science & Nature (3 answers total) 2 users marked this as a favorite
It reminds me of doing sunrise on Haleakala on Maui. It was a crystal clear pre-dawn, could see stars down to the horizon. As the sun came up, you could see half the island and the ocean but then the clouds started rolling in and by the time the sun was up we could only see the red dirt slopes of the mountain with everything else in cloud cover. So, yeah, I think they clock in. :)
posted by amanda at 6:22 PM on January 18, 2017

Yes to the pattern and timing, not necessarily to the direction or locale parts. So:

Air forms clouds, or doesn't, according to its humidity and temperature and pressure situation. You get scenic "partly cloudy" situations when different bits of air with different conditions are being mixed around among each other, and only some of those bits form clouds.

That mixing can happen when strong winds blow a stable air mass over a mountain, or when waves on the boundary between two air masses push small parts of one up into the other. But you often see it when the sun's heating effect warms blobs of air near the ground, making them rise up.

So toward the evening, as the sun loses its influence, the energy source for that vertical mixing goes away and whatever air masses you've had around then spread themselves horizontally and average themselves out (I think). If the local average conditions say "cloud", you get a hazy uniform stratus cloud. If not, you get a clear uniform region of sky.

The only remaining part of the mystery is where you're located relative to those uniform-ish patches of stable air: if you're right under a cloudy one, you see cloudy sky, although probably with a clear strip toward the horizon I surmise? If you're under a clear one, you see a clear sky, except for a cloudy strip toward the horizon as you say. That's the next patch over, it only looks thin because you're seeing it from a long way away and edge-on. Some days, there wasn't enough humidity in the air to stay cloudy hardly at all after the vertical mixing went away. And I predict, if you watch carefully, some days you'll see multiple strips near the horizon, as you catch multiple clear/cloudy patches in the act.

Bear in mind this all comes from just a lot of sky watching and conjecture. But I've seen what you describe, and this is how it seems to work.
posted by traveler_ at 9:43 PM on January 18, 2017 [1 favorite]

Clouds fizzling near sunset is expected behavior for certain types of clouds. The sun heats the ground which heats the air above. Warm air rises, as it rises the air expands and cools. If the sun is still heating the ground the air continues to rise from below, forcing the cooling air ever upwards.

The maximum amount of water vapor that can be in the air, the saturation vapor pressure (svp), depends solely on the air temperature. That relationship is exponential, a small change in temperature results in a large change in the saturation vapor pressure. So, as the air rises and cools its saturation vapor pressure decreases. If the air cools enough that the svp equals the actual vapor pressure any further rising (thus cooling) will force the water vapor to condense out of the air. That is, a cloud forms. This is what is happening when you see puffy white fair weather cumulus clouds. As the day gets closer to sunset the amount of solar heating diminishes, the air stops rising and the clouds start dissipating.

Clouds don't actually consolidate near the horizon. I think what you're seeing there is a different phenomenon. Toward sunset the clouds still dissipate but as the sun gets closer and closer to the horizon sunlight has to pass a much greater distance through the atmosphere to reach your eye. Because it is passing through a greater distance the sunlight has a much better chance of being scattered by gunk in the atmosphere. There are two types of scattering. Particles smaller than the wavelength of light will selectively scatter light by wavelength. This is known as Rayleigh scattering and is what gives us blue skies in the middle of the day and red/orange skies at sunrise/set. Scattering by particles at the same size or larger than the wavelengths of light is known as Mie scattering and that scatters light indiscriminately. That's why fog and clouds look white or gray and what causes us to see the gray gunk along the horizon at sunset.

For sunset forecasts I highly recommend SunsetWx.
posted by plastic_animals at 9:31 AM on January 19, 2017 [1 favorite]

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