Noctilucent Clouds

Noctilucent clouds glowing on the horizon just after sunset

Veres Viktor (NASA)

Scientists refer to these types of clouds with two different names, depending on where the clouds are observed from. They are called Polar Mesospheric Clouds (PMCs) when they are viewed from space, and are called Noctilucent Clouds (NLCs) when viewed by observers on Earth, typically from location near the Earth's poles. Unlike lower clouds that are associated with weather, these clouds form very high, at about 85,300 meters above the Earth's surface, in the mesosphere. They are likely made of frozen water or ice crystals. NLCs are seen best just after sunset as they glow an electric, beautiful blue-white (noctilucent actually means "night-shining").