The Clouds Keep Coming!


Clouds aren’t usually the astronomer’s friend, but at this time of year they become the objects of our attention. For the past few nights, my Alberta prairie skies have been beautifully clear and filled with the clouds of solstice.

Last night, July 6, began at twilight with the best display of noctilucent clouds so far this season — and we’re now at peak season for this northern sky phenomenon. This was the scene at about 11:30 pm local time, with the wispy high-altitude clouds at their most extensive and fully lit by sunlight. Over the next hour or so, as the Sun set further below the horizon, the display disappeared as darkness came to the high atmosphere and the Sun no longer illuminated these clouds suspended over the Arctic. Here’s a diagram of the geometry of how they get lit up. If you are curious to learn more, check this NASA page.

I took a time-lapse movie of the fall of darkness on the clouds, which you can view here at my SmugMug gallery at AmazingSky.ca. The video shows how the clouds begin the night fully illuminated by the Sun but over the hour duration of the video they disappear from top to bottom. You can see a curtain of darkness moving down the clouds, caused by the Sun dropping farther below the horizon. As it does so its illumination seems to drop toward the horizon as night falls, leaving only clouds closest to the horizon (and farthest north) still illuminated. The video also shows that the edge of the illumination appears reddish — that’s because clouds on the edge of the descending dark shadow are being lit only by a low red Sun setting below the limb of the Earth. Pretty neat, and something I’ve not seen before in any image or movie. (The movie is HD quality and will take a while to load, sorry!)

However, tonight normal, everyday weather clouds moves in, curtailing any late night NLC watch. Tomorrow, I head to the Rockies to do some time-lapse nightscape shooting in the mountains. Yes, I know I’ll miss seeing Duke and Duchess Will and Kate start off the Calgary Stampede Parade. Royalty will just have to get on without me.

— Alan, July 7, 2011 / Image © 2011 Alan Dyer

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