But science has an explanation -- they're called "noctilucent" clouds, and they're some of the highest clouds in Earth's atmosphere, hanging out about 50 miles above the Earth's surface -- that's over 260,000 feet! (Most of our highest clouds, like the common cirrus cloud, are usually between 30,000-35,000 feet.)
These noctilucent clouds are made of tiny ice crystals and are most commonly seen in the higher latitudes (between 50-70 degrees; Seattle is at 47ish, so not too out of the realm) during the summer months.
As for why they glow? That's simple. It's just reflecting simple sunlight. According to Wikipedia, the clouds are only visible in the few hours after sunset, when the sun is below the horizon for us on the ground and we're in the Earth's shadow, but sunlight is still shining in the far upper reaches of the atmosphere -- enough to reflect off those 50-mile-high clouds. Otherwise, the clouds are too faint to be seen.
It is a semi-recent discovery -- first noticed in the late 1800s. Scientists are still not 100% sure what creates them, but some theories say they're tied to volcanic activity, and others say their increasing frequency is a sign of global climate change.
But one study conducted by a researcher at the United States Naval Research Laboratory found that, believe it or not, one source could be from the Space Shuttle. Their study found that during a launch, water vapor from a shuttle's rocket boosters is left behind as it passes through the upper layers of the atmosphere. The water then turns into ice crystals and gets blown toward the poles by prevailing wind currents. They say the clouds have been known to pop up several days after a launch. Lo and behold, the Space Shuttle Atlantis has been in space for the past week or so and is just getting ready to come home.
So, could the mysterious cloud be a neat little reminder that the shuttle is up there? We don't know for sure. But it sure was interesting to see!
There's some great articles I found online while trying to figure out what those clouds are and they make for some good reading.
This link: www.nasa.gov will take you to the article that explains the study connecting noctilucent clouds to Space Shuttle exhaust.
More information on noctilucent clouds can be found at science.nasa.gov
And of course, Wikipedia also is a good research source for more information on these sorts of things.
Here's another photo sent in, this one by Wade B. Clark, Jr.:
And here's another photo sent in, this one by Greg Jones:
There's even more photos of this in our KOMO Weather Photo Gallery
Not the only strange cloud of the past day
Meanwhile, Wednesday morning saw another strange looking cloud, although this one is much more common. David Baker captured this rainbow-like effect in a thin cloud over Shelton:
Those clouds are made of tiny ice crystals, which will refract the sunlight much like a prism will. It's similar to how a rainbow works, only you're working with ice crystals instead of raindrops.
You can read more, and see more photos of this phenomenon at this link.