Astronomers and clouds don’t generally get on. However, during summer in the northern hemisphere some mysterious and beautiful clouds may 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).
NLC formation is restricted to the summer months when conditions in the mesosphere near the poles are sufficiently cool enough (-120°C) to allow ice to form in the low pressure environment. Conditions aiding the formation of NLCs tend to last from the start of June until the beginning of August. 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 ideally situated for NLC observers.
You can observe noctilucent clouds during the next 6-8 weeks by going out and looking towards the north after about 11pm. Taking pictures of noctilucent clouds is also easy; for example, ISO200 for 5-10 seconds should pick them up.
NLC 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. 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? The association with Krakatoa is not accepted by all scientists but it is true to say that NLC have been widely observed ever since. Aside from powerful volcanic eruptions like Krakatoa there are no plausible mechanisms for transporting dust from the lower atmosphere to the mesosphere. Some scientists have speculated that the seeds of NLCs are the meteoric dust swept up by the Earth as it orbits the Sun.
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.