Every year around this time, mysterious electric blue clouds appear over the North and South pole. They are called noctilucent clouds and they can only be seen in deep twilight, when the Sun is below the horizon. According to NASA, "their origin is still largely a mystery":
Various theories associate them with meteoric dust, rocket exhaust, global warming — or some mixture of the three.
They are the highest clouds, located almost on the edge of space at 54 miles from the Earth's surface, in the mesosphere. They are very difficult to observe, but they appear as white and blue tendrils when they are illuminated by the Sun and the rest of the atmosphere is in our planet's shadow.
These were photographed by Brian Whittaker at 11,000m, on a flight from Ottawa to Newfoundland. [NASA]