Science & Environment

Unlocking secrets of 'eerie' clouds

Image caption Noctilucent clouds are characterised as wispy and silvery-blue

An amateur scientist has compiled a forecast to enable observers to view a mysterious type of cloud.

The project is part of a series for BBC Radio inviting members of the public to submit their ideas for a scientific research project.

The eerie, "noctilucent clouds" clouds form at the edge of space, far higher in the atmosphere than more familiar clouds.

Here, BBC News looks at plans by the amateur astronomer, John Rowlands, to observe these phenomena.

Noctilucent clouds appear as wispy, luminous, silvery-blue clouds in the dark Northern sky, often with a wavy pattern.

According to the scientists, viewing conditions will be ideal around 2300BST on Monday 19 July or before dawn on Tuesday at 0200BST.

The clouds could be visible over the UK, northern Europe and North America between the latitudes 50 and 60 degrees North.

John Rowlands, from Anglesey, is one of four amateur scientists selected by judges for on BBC Radio 4's Material World programme.

He is studying the phenomenon of noctilucent clouds with help from mentor Professor Nick Mitchell, at the Centre for Space, Atmospheric and Oceanic Science at the University of Bath, UK.

These ethereal clouds appear over the northern hemisphere every year between June and August. The team is asking anyone who spots the clouds tonight, or over the next few weeks, to send in observations and photos to help their research.

Mr Rowlands has put together a research diary on the Facebook website where people can post their observations.

Mysterious clouds

Noctilucent clouds form at the top of the mesosphere, the coldest region of the Earth's atmosphere at -150C.

Lying around 80km (50 miles) above the Earth's surface, this region is about 10 times higher than the troposphere, where weather clouds form.

They were first recorded in the late 19th Century, and may be linked to climate change. Rising methane levels from industrial agriculture and melting permafrost are thought to give rise to increased water vapour levels in the mesosphere.

Image caption John Rowlands has been using satellite and radar data to predict where the clouds might form

This, combined with long-term temperature changes in the mesosphere, seems to be making the clouds more common than before.

The high altitude of noctilucent clouds allows them to catch the Sun's rays during the brief hours of midsummer darkness, when the Sun is well below the local horizon. Normal weather clouds lie in deep shadow during this time.

Because of the great height at which they form, it is extremely difficult to directly measure what is going on there. It is too high for any aircraft or balloons to reach, but too low for direct sampling by space vehicles.

So far, only very short-lived rocket flights have directly measured and sampled the mesosphere. A modest number of rocket flights continue today from Arctic Sweden and Norway. But ground-based radars and remote observations by satellites can give vital information.

One of the University of Bath's radars, based in northern Sweden, bounces radio signals off the trails left by several thousand meteors per day. The results reveal the strength and direction of winds at the edge of space.

Satellite measurements add temperature and water vapour information, two of the key factors governing the appearance of noctilucent clouds.

As well as being the place where the clouds form, the mesosphere is where meteors burn up as they hurtle towards Earth. It is the dust left behind these meteors that provides the nuclei around which ice forms. Large volumes of this ice create the displays of noctilucent clouds we see on Earth.

Planetary waves

The prediction of when noctilucent clouds will occur is based on the so-called five-day "planetary wave". This is a pattern of winds, akin to an enormous wave that surround the earth, which alternately warms and cools the mesosphere, as the peaks and troughs pass any given point.

Together, Mr Rowlands and Professor Mitchell have been using the satellite and radar data to predict when the planetary wave will be in a cooling phase - the time at which noctilucent clouds are most likely to form.

Tonight, conditions are favourable for the clouds to be seen, and then every five days from now until mid-August. Then the mesosphere becomes too warm for the clouds to form and they will disappear until the noctilucent cloud season returns next summer.

This is the first-ever weather forecast for the mesosphere, and the results of the experiment will be presented at the British Association science festival in September, where a winner will be selected from the four amateur experiments.

Professor Mitchell said: "We hope to do some useful and interesting science, but we also want to draw attention to noctilucent clouds as a beautiful and mysterious phenomenon of the summer night sky that many people don't know about."

Material World is broadcast on BBC Radio 4 on Thursdays 1630 and repeated at 2100 on Mondays. You can listen online or download the podcast.

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