Noctilucent clouds are the highest clouds in Earth’s atmosphere, at 50 to 86 kilometers (30 to 54 miles) above the surface. Also referred to as night shining, they are a polar mesospheric cloud. They are most often seen at high latitudes, though they have been sighted further south on rare occasions. The following picture of noctilucent clouds (the white-blue streaky ones) was taken in Whitley Bay, United Kingdom.
These clouds are seen after sunset because of their extreme height in the atmosphere. Once the sun sets, the rays are at an angle that still intersects with the cloud. This diagram from NASA helps to show the geometry required to see this type of cloud:
These clouds were first spotted in 1885 and have been increasing in recent decades. If you are interested in a little more information on their formation there’s a nice short article from NASA here.
In fact there is a scientific mission dedicated to these clouds, the Aeronomy of Ice in the Atmosphere (AIM). You can find the latest image of polar mesospheric clouds on that page on the right.