Monday, October 28, 2013

Daytime Quarter Moon with Plane Contrail Alignment

Today I learned all about contrails! Contrail is short for condensation trail. These white streaks form behind jets in the sky when water vapor from the engines freeze into tiny ice crystals suspended in the air. They don't just look like thin clouds, they are actually man-made clouds, and made from the same stuff as regular clouds.

I always love a good alignment! The tiny moon and thin contrails from a jet in the same place at the same time, from exactly my perspective on the ground, in the entire sky, just for a split second - it feels like a special moment. 

jet contrails over moon
Jet contrail alignment with quarter moon. It looks like the contrail is holding the moon up, or maybe the moon is sliding down a cloud slide!

I learned that the contrails in my photo of a plane are "exhaust contrails" - but not exhaust as in smoke or pollution, merely that the water vapor is in the air coming out the back end of the engine's turbines. Contrails can also form from a temporary drop in air pressure as the air runs over the wings. These are known as aerodynamic contrails.

plane contrails
Icy exhaust contrails through my 300mm lens, cropped in Photoshop. This is the same plane that made the contrails in the photo above! The plane didn't cross the moon, but the contrails blew into position.

Notice the gap between the engines and the contrails. The water vapor coming directly out of the engine is super hot and invisible. You can't see the trails until they condense into water droplets and then freeze. Visible steam is actually billions of tiny water droplets suspended in the air. Water vapor itself is completely invisible. I learned something!

Is there a blog dedicated to contrail science? Why yes there is:

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