25 Jun 2013

Early start for noctilucent clouds

Each summer, cloud spotters eagerly stare up at the skies, hoping to catch a glimpse of rare noctilucent clouds.

They are the highest clouds in the earth’s atmosphere, found in the mesosphere – around 50 miles above the ground.

Formed from tiny ice crystals, they are characterised by their thin, streaky cirrus-like appearance, and are usually bluish or silvery in colour.

The reason that they are so rare is because they form in a part of the atmosphere that is extremely dry and has very little moisture present.

Also, there are very few cloud condensation nuclei, such as volcanic dust or debris from meteors, for water vapour to stick to – making their presence an even greater rarity.

In addition, they are only found at latitudes of around 50 to 70 degrees north or south of the equator and require the sun to be below the horizon.

Each year, the first sightings of noctilucent clouds are documented by the Nasa’s Aeronomy of Ice in the Mesosphere (AIM) spacecraft.

This year has proven interesting, because not only have the rare clouds appeared in the northern hemisphere a week earlier than last year, but also earlier than has been observed in any other season.

What is puzzling scientists even more is that this early start to the season is coinciding with a solar maximum. Previous research has found that noctilucent clouds tend to peak during solar minima.

Cora Randall, of the Laboratory for Atmospheric and Space Physics at the University of Colorado said: “If anything, we would have expected a later start this year because the solar cycle is near its maximum.”

It has been suggested that the earlier start for noctilucent clouds in 2013 may be the result of a change in atmospheric teleconnections – the way changes in one part of the atmosphere affect another.

If you manage to capture any of these rare clouds on camera, I’d be interested to see your pictures. The easiest way to get them to me is on Twitter – @liamdutton



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