High-altitude clouds shine at nightfall above Edmonton, Alberta, Canada, in this 2011 image. Known as noctilucent ("night-shining") clouds, they're caused by sunlight reflecting off grains of ice high in the atmosphere. They're visible only during twilight, when the Sun is below the horizon at just the right angle to illuminate the clouds. Recent research suggests that rocket exhaust is making the clouds more common. [NASA/Dave Hughes]