What are NLC?

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The phenomena of Noctilucent Clouds

On summer nights, from specific latitudes, an awe inspiring atmospheric phenomena can sometimes be observed. Their visibility varies from extremely faint and practically unrecognisable to vast tracts of multi coloured, delicately structured clouds covering the sky like a priceless tapestry and shining with a unique, unforgettable light. These "summer sky sailors" are known as Noctilucent Clouds or NLC. "Noctilucent" is a Latin derivative which, loosely translated, means "night shining", hence NLC are night shining clouds, a very good description. Although the latter part of the nomenclature implicitly states that the phenomena are clouds they bare little resemblance to the common weather clouds with which we are all familiar. These clouds are much, much more unique, this just being one of the many facets which make NLC such an engrossing and beautiful phenomena to observe and understand.

A very short history of Noctilucent Cloud observation

NLC were first discovered in the summer of 1884 after the eruption of Krakatoa which occurred in 1883. The volcanic eruption was at first thought to have contributed to the formation of NLC due to the ejection of huge quantities of dust into the upper atmosphere, however it has been proven that the eruption was not responsible for their formation. The actual "discoverer" is debated, however recent work has shown that the first conclusive observations were performed by T.W. Backhouse from Sunderland, U.K. Gadsden and Schroder (1989) question why Noctilucent Clouds were not discovered prior to 1885 and suggest that the eruption of Krakatoa was significant in that it caused observers to take more of a careful note of the twilight skies. This is a very difficult hypothesis to comprehend as it suggests that no observers ever bothered to take note of the twilight sky. This is a highly unlikely scenario as it is would seem very improbable that astronomers and meteorologists who frequently observed would miss such a highly conspicuous phenomena. Only historical research could answer this question. NLC have been observed ever since, generally with increasing frequency that has been attributed to increasing anthropogenic pollution.

Visibility

NLC are visible during the summer months of each hemisphere, mid-May to mid-August in the Northern Hemisphere and mid-November to mid-February in the Southern Hemisphere but only from a specific latitudinal zone. This zone is 50-65 North or South, with 57 being the best latitude to observe NLC from. June and July and December and January are the best months to observe NLC for each respective hemisphere. These zones are a compromise. South of 50 and the sky is too dark, due to the sun being more than 12 below the horizon (astronomical twilight) and north of 65 and the sky is too light as the sun is less than 6 below the horizon (civil twilight). NLC are so optically thin that they scatter less than 1 part in 1000 of light incident upon them (Gadsden and Schroder, 1989). Hence the sky is too bright to observe them during civil twilight. At the end of civil twilight the clouds become visible as sky brightness has decreased by a factor of several hundred. In the specific latitudinal zone the sun stays between 6-12 below the horizon most of the night (nautical twilight). Consequently the sky is dark enough to observe NLC. The low angle of the sun below the horizon also allow the high altitudes at which the NLC reside to be in direct sunlight. Therefore NLC are visible due to reflected sunlight and do not generate any light of their own (Figure 1).

Figure 1 The visibility of NLC
Figure 1. The visibility of NLC.

The latitudes of visibility are also thought to be a compromise regarding NLC formation and evaporation of NLC. At higher latitudes, above 65, NLC particles may not have had time to form to an observable size and so will be invisible by observers there. At latitudes below 50 NLC may have evaporated as they move towards the equator.

NLC are also visible from space. NLC observed from space are called Polar Mesospheric Clouds, with some scientists e.g. Thomas (1996) calling both phenomena "mesospheric clouds" or PMC/NLC.

Altitude

The visibility of NLC is due to their great altitude. Their altitudes have been accurately determined by ground based parallactic photography and in-situ rocket measurements. The clouds reside at an altitude of 80-85Km with an average altitude of 83Km. This places the clouds just below the temperature minima of the mesopause which is approximately 150K (-123C). It is theorised that the clouds nucleate at the mesopause over several hours and during nucleation descend by a few Km to the altitude at which they are observed (Gadsden and Parviainen, 1995). Compare this with weather clouds which form and reside in the troposphere at altitudes of between 0-3Km.

Formation

Their formation is still highly contested. Originally they were thought to form due to the nucleation of ice around meteoric particles. Rocket flights penetrating NLC have collected particles of nickel and iron, the major constituents of iron meteorites. However, as of late this hypothesis has lost favour to one in which the ice crystals nucleate around terrestrial dust particles and not particles of extraterrestrial origin. There is even some hypotheses that suggest deposition of dust particles from libration clouds provide the nuclei upon which NLC grow.

The mesosphere is extremely dry and cold so it is unusual that NLC form at all. Nucleation occurs due to super-saturation in the mesosphere. This is possible due to the very low temperatures at and near the mesopause, temperatures as low as 111K (-162 degrees C) have been measured just a few Km above a NLC, with temperatures at the mesosphere itself being slightly higher at ~150K (-123C). These temperatures are set by mixing in the atmosphere due to hemisphere to hemisphere circulation from the movement of upward cooling air in the summer polar regions and the downward, warming movement of air over the winter polar regions (Gadsden and Schroder, 1989). These temperatures are further maintained or adjusted by the attenuation of upwardly propagating gravity waves (buoyancy currents).

Gravity waves also supply the water molecules from which NLC form by upward diffusion from lower down in the atmosphere. Water vapour is also supplied by the photodissociation of methane molecules in the mesosphere by ultraviolet light. Even with these sources of water the levels of water in the mesosphere available for NLC formation are extremely low. Water molecules are present in only a few of the million or so molecules composing the surrounding air. Mixing ratios for water at 80Km show that 3ppm is a good estimation of the amount of water present (Gadsden and Schroder, 1989). Further calculations show that this amounts to 7x10E15 m2 water molecules above 80Km. For such a large area this is extremely small, consequently NLC are very thin, measurements have suggested the optical thicknesses are in the region of 10-4 with cloud particle diameters in the range of 50nm.

The physics of NLC formation are quite advanced and hence great detail has not been entered to here until an article of the necessary technical level has been written, hopefully this should be in the near future.

Strucutre and colour

Each NLC is composed of different forms, of which there are four major groups, veil (Type I), bands (Type II), billows (Type III) and whirls (Type IV). There are further sub divisions of these four major classes. For NLC which possess forms which are not described by the four major categories a four point "complex structure" scale exists to classify these NLC. These scales are further explained in the section on observing NLC.

NLC are a very colourful phenomena, something which is part of their attraction. Generally NLC are an electric blue colour, but red, gold and white are not uncommon and are due in part to the angle of the sun below the horizon. The general blue colouring is caused by absorption of incident sunlight by ozone in the Chappuis bands which reside in the yellow portion of the spectra (Gadsden and Parviainen, 1995).

NLC are very similar to cirrostratus clouds, which are also composed of ice but form and reside in the troposphere. On rare occasions, for example when an NLC is faint and there is tropospheric cloud present, it is hard for a beginner to ascertain which cloud formation is the NLC. There two reliable methods to be able to recognise an NLC. An NLC will shine against the twilight sky whereas tropospheric cloud will be a darker silhouette. An NLC can bear magnification and fainter structures can be seen in a higher resolution. Tropospheric clouds resemble smudges under magnification. The use of a polarising filter can detect whether a cloud is an NLC or not. NLC will be enhanced whereas tropospheric cloud will not be. Also calculation of the sun elevation angle can reveal whether the sun is at the correct position for NLC to be illuminated and observed. Once an NLC is witnessed for the first time it will become much easier to recognise an NLC.

Polar Summer Mesospheric Echoes and their relation to NLC

Polar Summer Mesospheric Echoes (PSME) are very strong radar echoes which appear during the months of the NLC season, May to August, with their greatest frequency occurring in June and July, at the time when NLC are also most frequent. The echoes are caused by plasma densities in the atmosphere which are thought to be controlled by the break up of upward propagating gravity waves. It has been hypothesised that PSME are the embryonic stages of NLC formation. This hypothesis is still contested and no general consensus has been reached on its validity.

Amateur observations

The vast majority of NLC observers are amateurs due to the fact that justification of an extended program of scientific study is very difficult because of the visibility and altitude of NLC. Gadsden and Schroder (1989) present an interesting and thought-provoking view of why this is so, which is briefly summarised here. NLC occur on approximately 1 in 5 nights during the three months which make up the NLC season. In most locations on average the sky is likely to be cloudy for 4 nights a week. This provides the NLC observer with 10-11 nights per 365 days on which an NLC observation can be made. Half of these nights NLC will be very difficult to see and unmeasurable. Accounting for the sky brightness further reduces observation time which, on average, is only dark enough for 4 hours a night during the NLC season. Taking into account the above estimations this means that there is only aboout 25 hours worth of data collection time a year (Gadsden and Schroder, 1989). Hence NLC observing programs are not good proposals to place before a scientific funding body!

Direct observations are also made extremely costly and difficult due to the great altitude of NLC. The mesosphere is a sort of "no-mans land" in the atmosphere. Getting to the mesopause where NLC are present is very difficult. It is too high for balloons to reach which have ceilings of 30-40Km and too low for satellites to survive a single orbit before re-entry. Rocket borne instruments provide a precise but highly spatially limited method of sampling. One measurement is taken on the ascent, one on the descent. The only problem is that these point measurements are separated by a several minutes at distances of 2-3Km which limits their use.

Because NLC are the only means of observing the region near the mesopause over longer time scales, as well as being a possible indicator of global climate change, it is of paramount importance that they are observed and only amateurs have the time and means to carry out substantial long term observations. It is therefore one of the few branches of astronomy and atmospheric science where the amateur can make a scientifically valuable contribution.