Wonderful video of Noctilucent Clouds over Canada on June 21.
These clouds are the highest visible clouds in the atmosphere and take place at altitudes about 50 miles above the ground. They are formed when filaments of summertime water vapor collect around meteoroids to form tiny ice crystals.
They used to be confined to mainly Arctic latitudes, but they are now brightening and spreading farther south. They have been seen in the past few years as far south as Colorado and Kansas.