Noctilucent clouds occur when ice crystals are high enough in the atmosphere to be visible in both day-lit and dark parts of the earth. The sun reflects off the clouds and causes an apparent glow to observers where the sun is mostly set. Check it:
![picture of shining clouds](http://www.nasa.gov/images/content/181082main1_NoctilucentWide.jpg)
The BBC has an article describing the NASA spacecraft, AIM, which is designed specifically to study such clouds.
![picture of shining clouds](http://www.nasa.gov/images/content/181082main1_NoctilucentWide.jpg)
The BBC has an article describing the NASA spacecraft, AIM, which is designed specifically to study such clouds.