Dr Adrian Jannetta FRAS
Astronomers and clouds don’t generally get on. However, during summer in places far north or south of the equator strange, beautiful clouds often appear in the sky long after sunset. Known to astronomers as noctilucent clouds (NLC), these delicate and tenuous clouds are seen shining long after the ordinary clouds of the troposphere have darkened. NLC are the highest clouds in the atmosphere and form within the mesosphere at heights of 85km (about 52 miles) above the ground (which means you’ll hear scientists referring to them as Polar Mesospheric Clouds).
Night shining clouds of the Summer
The clouds can be seen from the ground between latitudes of 50 and 60 degrees north or south of the equator. At higher latitudes the summer twilight is too bright for the clouds to be visible. Northumberland (approximately 55°N) is in a prime location for seeing noctilucent clouds.
After sunset, during twilight, the familiar clouds of the troposphere are still catching the last of the Sun's rays. Soon, they grow dark and become silhouetted against the background sky. Noctilucent clouds are much higher above the ground than clouds of the troposphere, in a layer of the atmosphere called the mesosphere. Even though they form many hundreds of miles north of Northumberland they are visible because of their height above the ground. Being so high and so far north
How and when NLCs form
Noctilucent cloud formation is restricted to the summer months when conditions in the mesosphere around the poles are sufficiently cold enough (-120°C) to allow ice to form in the low pressure environment. In the northern hemisphere conditions aiding the formation of NLCs tend to last from the end of May until the beginning of August. NLCs form when water vapour condenses onto a dusty ‘seed’ high in the atmosphere. The precise nature of that seed has been the subject of much discussion in recent years. The earliest reports of NLC came in 1885 just two years after the eruption of Krakatoa, an event which affected the Earth’s atmosphere and weather for a decade or more. Could the seeds of NLCs be volcanic dust? Aside from powerful volcanic eruptions like Krakatoa there are no plausible mechanisms for transporting dust from the lower atmosphere to the mesosphere and for it to remain there for long periods. In recent years evidence points towards an extra-terrestrial dust source; the seeds of NLCs are meteoric dust swept up by the Earth as it orbits the Sun. These clouds are truly space weather!
Conditions in the mesosphere are affected by solar activity. There is a well known inverse correlation between solar activity and NLC intensity. In years close to solar maximum the frequency of strong NLC displays is reduced. There are more frequent intense displays during years away from solar maximum. The most recent solar maximum occurred in 2013 so we can expect displays of noctilucent clouds to improve over the next few years.
What you can expect to see
There can be an incredible wealth of structure within the clouds. Ripples, whirls, diffuse bands and more. Here's a nice illustration of the different features you might see. The gallery below shows images taken around Northumberland during the past few years.
Noctilucent clouds become easier to see as the sky becomes darker. You'll likely have to wait at least at least 90 minutes after sunset until they first appear in the northern sky in Northumberland. They seem to grow brighter as the sky becomes darker.
Taking pictures of NLCs
Taking pictures of noctilucent clouds is relatively straightforward. You'll need a DSLR and a tripod. Unlike aurora photography you'll be shooting in somewhat lighter conditions (twilight, as opposed to full darkness). This will give more flexibility in choosing exposure and ISO settings. ISO200 for 5-10 seconds should pick them up without a problem. Over exposure will result in a bright blue sky. It's also possible to pick them up on night-scene modes on smartphone cameras.
Tracers of climate change?
NLCs are becoming visible more frequently - a study of NLC frequency from the 1960s to the 90s suggested they became twice as prevalent. The traditional window of latitude through which the clouds can be seen is also increasing (NLC have recently been sighted from locations just 40° from the equator) and they are intensifying in brightness. The temperature of the mesosphere is influenced by the presence of carbon dioxide and the humidity is increased by methane. Both gases are being increasingly produced by humans and so NLCs could be regarded as a visible indication of anthropogenic climate change. The validity of the connection is still controversial - there simply isn't a vast amount of data to go on. In the past, the exhaust gases of the Space Shuttles and other rockets have also been observed to contribute to the formation of NLCs.
A NASA satellite is studying NLCs from orbit: it's called the Aeronomy of Ice in the Mesosphere (AIM) satellite. You can inspect the most recent image of the noctilucent cloud coverage over the northern hemisphere here. The primary goals of AIM are to determine the processes which form NLCs, to measure the sizes of ice crystals in the clouds and to monitor the composition of the mesosphere over a period of at least two years. AIM may eventually provide evidence to show whether or not these delicate and beautiful clouds are a manifestation of the changes brought about by human production of greenhouse gases.