Gallery: Rare 'night shining' clouds over the North West

Noctilucent clouds were spotted once again over the weekend, on Friday - the 21st, the longest day! - shortly after sunset.

Noctilucent clouds are extremely rare very high clouds seen in the night sky, usually on clear, summer nights. They become visible about the same time as the brightest stars and are usually bluish or silvery.

Height of base: 200,000 feet.Shape: They often closely resemble thin streaky cirrus, though other shapes are often seen. They are usually blue or silver in colour, but sometimes orange or red.Latin: Noctilucent = “night-shining”Precipitation: None

WHAT ARE NOCTILUCENT CLOUDS?

Noctilucent clouds are extremely rare collections of ice crystals, occasionally appearing in late clear summer evenings after sunset, but before it gets completely dark.

They become visible about the same time as the brightest stars appear and often stay visible after dark because they are still reflecting sunlight due to their great height.

They are higher up than any other clouds, occupying the layer of atmosphere known as the Mesosphere, and are only seen at latitudes between 45°N and 80°N in the Northern Hemisphere, and equivalent latitudes in the southern hemisphere.

HOW DO NOCTILUCENT CLOUDS FORM?

Like many clouds, noctilucent clouds need water vapour, dust, and very low temperatures to form.

Low temperatures are easily attainable in the Mesosphere, but water vapour and dust are in short supply. The dust may well come from tiny meteors from space, although dust from volcanoes or man-made pollutants may add to these. Scientists believe that the moisture comes through gaps in the tropopause, or perhaps forms from the chemical reaction of methane and other chemicals.