Saturday, 1 August 2009

Noctiluscent Clouds

I recently did a radio interview about a rare type of cloud known as noctiluscent - rare in the sense that they are rarely seen by us because of the special nature of how they occur.

The clouds are very high up, some 75 to 85 km (50 miles) high, in our mesosphere. Because they are so high they are usually too faint to see with the naked eye, but when the sun shines up from below the horizon (on nights when there is no low cloud) it can reflect light from the bottom of these clouds and they become visible.

These clouds are, to give them their proper name, polar mesospheric clouds. They are called noctiluscent because they ‘shine at night’. As far as I know they were first photographed by a German observer in 1887 and the clouds are thought to prevalent around that time as a result of the cooling of the mesosphere caused by the Krakatoa eruption in Indonesia in 1883.

You can see a fine example on the Cloud Appreciation Society’s website at: http://cloudappreciationsociety.org/noctilucent-may’06/

If you have ever been lucky enough to see these clouds you will know that it can be an eerie but quite stunning sight. They often have a purple-blue colour in large part caused by the absorption of the light due to atmospheric ozone.

These clouds do only occur under fairly restrictive conditions. They need water vapour to form which is very unusual in the mesosphere as the water molecules are often broken down by the UV light. Therefore they are more prevalent in weaker solar cycles when the UV is weakest. Also they tend to occur when the mesosphere is at its coldest, that is close to the poles and during summer.

Interestingly the increase in greenhouse gases leads to a warming of the troposphere and a cooling of the mesosphere. Some have suggested that the increase in the frequency of sightings of these clouds in recent years might be a visible impact of global warming. So perhaps it’s true that every cloud does indeed have a silver lining of sorts.

2 comments:

Jam said...

can you please explain how increase in greenhouse gases results in cooling of the mesosphere?

Paul H said...

Hi Jam

The greenhouse gases warm the lower part of our atmosphere (the troposphere) by adsorption and reflection back to the surface the energy from the sun. This means that there is less returning back to space to warm the upper levels of the atmosphere. This means we expect to see a related cooling above the troposphere. Hope that helps.