Noctilucent Clouds
Click on the image below to see a stunning slideshow and explanation of the origins of noctilucent clouds.
NASA sent a satellite into orbit to study these clouds and has a site with photos and details on how they are changing over time.
NOAA has a page of links to sites featuring noctilucent clouds and the Northern Lights if you want to see more.
APOD has a good picture of noctilucent clouds over Edmonton.