|Noctilucent Cloud over Merthyr|
Noctilucent Cloud isn't normal cloud. It forms so high up in the atmosphere (around 83km) that it is classed as space weather, as are aurora. The tiny ice crystals from which they are made are formed but moist air lifted up to the edge of space by summer air currents and form around particles of soot left over from the burning up of meteors.
They are so diffuse that they cannot be seen in day time, but are so high that when the sun is 6 to 16 degrees below the horizon, it can still illuminate them, causing them to glow a ghostly pale blue in the dark sky, always in the northern quarter. They only occur in the summer months and July is the best month in which to see them.