Noctilucent clouds

• 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. They are seen less often in the southern hemisphere as there is very little land and very few people there. Only the southern tip of Argentina and Chile, and the Antarctic are at the correct latitude.

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.

What weather is associated with Noctilucent clouds?

These clouds are far too high to have any effect on weather on the ground.

How do we categorise Noctilucent clouds?

Many different patterns and shapes have been observed so attempts to classify or categorise them are difficult.