Video: noctilucent clouds

Clouds on the outer edge of the Earth’s atmosphere are operating as an “exquisite” temperature gauge monitoring climate change.

Images from a Nasa mission into space have shown for the first time that noctilucent clouds, which are only created at certain temperatures, appear to be increasing in frequency and extent.

The "night-shining" clouds can be viewed from Earth, but the new satellite images show a global overview of the clouds enabling scientists to measure them.

Build-up of the greenhouse gases carbon dioxide and methane in the upper atmosphere may be responsible for the cloud changes, scientists said.

Increased carbon dioxide cools the upper atmosphere and makes it easier for ice crystals to form the "polar mesospheric" clouds.

"The clouds are an exquisite thermometer," said Scott Bailey, from Virginia Polytechnic Institute and State University.

"The magnitude of the temperature changes is only about five degrees Fahrenheit (3C). So, a very small change in temperature leads to a dramatic change in cloud behaviour. We conclude from that that these clouds are a very sensitive measure of temperature change."

Nasa's Aeronomy of Ice in the Mesosphere (AIM) project was launched in April to study the clouds, made of ice crystals formed around dust particles more than 50 miles above Earth's surface. Sitting 600km above the Earth, the spacecraft is able to build a global picture of the clouds.

"It won't affect people, but we're causing the outer part of the atmosphere to change, which means we are changing the entire atmosphere, which is important to know," said James Russell III, from Hampton University in Virginia.

Dr Russell told the American Geophysical Union Fall Meeting on Monday that AIM's three instruments had given a completely new perspective on the clouds.

"We'd never seen a picture of the whole polar region before, to see these clouds on a daily basis. That in itself is a revelation - it shows how variable they are from day to day, from orbit to orbit," he added.

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