BAA / RMetS Joint Meeting, 2004 November 27


Noctilucent Clouds

Dr Espy opened with a gallery of images of the clouds he would be talking about: strikingly beautiful silvery blue streaks which appeared across the night sky at high latitudes at certain times of year. These particular images were from Scandinavia, and he explained that despite the unusual colours of the clouds, they were in fact composed of the same water ice as many more familiar clouds, though in this case the ice crystals were very small – smaller than the wavelength of light – and at high altitude. The small size of the crystals gave them the same Rayleigh scattering properties as the particles which gave the atmosphere its characteristic blue hue, hence the clouds' similar apparent colouration, the speaker added. They appeared to glow in the night-time as a result of their high altitude: after sunset at ground level, the upper atmosphere might remain outside the Earth's shadow for some time, and the noctilucent clouds (NLC) thus remain illuminated by the Sun.

The exact altitude of NLCs had been known to be very high for some time: images from research aircraft flying at 37,000ft, for example, showed the clouds to be still very much higher above the plane. Their exact height had recently been measured using ground-based laser radar (LIDAR), which reflected laser beam pulses from them and timed the travel time before a return pulse was detected. The average derived altitude was 82.5km, high in the mesosphere, and much higher than more familiar clouds at altitudes of only a few kilometres.

The discovery of clouds at such altitudes was somewhat surprising: the speaker explained that the mesosphere was very dry, and so very cold temperatures would be required for any condensation to occur. Yet the mesosphere was originally thought to be as warm as the lower atmosphere, and that would have required much more water vapour than available. In addition, as it was above the ozone layer it would be bathed in ultraviolet solar radiation energetic enough to break apart water molecules, and so the existence of clouds implied some mechanism to continuously replenish this large supply of moisture. It was not until the 1960s, when the mesosphere was discovered to be extremely cold during the summer months, that a suitable formation mechanism was suggested.

Whilst the actual details of this mechanism were somewhat complex, the speaker proceeded to summarise our current understanding. The two essential ingredients were two air movements: firstly gravitationally-supported vertically-propagating waves through the atmosphere, generated by the movement of air masses over mountains and the jet streams, and secondly the circulation of the stratosphere around the poles, driven by the warming effect of the absorption of solar ultraviolet radiation by the ozone layer, and the rotation of the Earth. In the summer months, these interacted to produce substantial vertical motion into the mesosphere, both injecting moisture into it from the lower atmosphere, and, less intuitively, influencing the thermodynamics of the mesosphere to produce a cooling effect, yielding temperatures around 130-140K. At this temperature, the ~6ppm water concentration in the mesosphere would be well saturated. In winter, the interaction was reversed, and the mesosphere thus around 60K warmer.

The speaker explained that whilst NLCs might be present at any time of day, they were only visible when the Sun was between 6-12° below the horizon. When the Sun was higher, the clouds were lost in the sky's own blue glow, but when the Sun sank too far beneath the horizon, they themselves entered the Earth's shadow and faded from view. For this reason, there were long observing windows from Scotland and Scandinavia in the summer, when the Sun remained not far beneath the horizon long after setting, but much shorter windows for observers further from the poles. Satellites could also observe them close to the Earth's day-night limb, though in this field they were generally referred to by the alternative North American term, Polar Mesospheric Clouds (PMCs).

The speaker explained that there were many outstanding puzzles: for example, what atmospheric dynamic processes gave rise to their small-scale, filament-like structure. There was also evidence of solar-cycle dependence, as well as differences between the NLCs observed in the two hemispheres, possibly attributed to the near-alignment of the southern hemisphere's summer with the Earth's perihelion, and the corresponding aphelic northern summer. Curiously, the first records of NLCs dated back to 1884: they appeared to have been absent before this time – interesting in view of recent modelling, which suggested NLC activity to correlate closely with the CO2 content of the atmosphere. An increase in greenhouse gases was thought to retain heat in the lower atmosphere, but reduce the upward heat flow, resulting in a cooling of the upper atmosphere. A doubling of the CO2 concentration might reduce the mesosphere temperature by 10K, helping to satisfy the temperature requirement for NLC formation.

However, whether they truly were the miners' canary for climate change remained a matter of intense debate. Amateur reports of NLC sightings from Scandinavia were becoming more frequent, but the speaker was unsure how scientific this was: the region's population was also increasing, and the number of people looking for NLCs on any given night difficult to determine. For example, one apparent dip in the number of sightings correlated remarkably well with the period during which one of the more dedicated observers had been on military service. Further questions also remained over the distribution of sizes of the ice crystals, and the nature of their nucleation sites.

Following the applause for Dr Espy's talk, a member remarked that these images might not be so useful for raising environmental awareness, as without global warming, we might not see these beautiful structures. In response to a question concerning the nature of the mechanism behind the solar-cycle dependence of the phenomenon, the speaker replied that it was thought to be primarily that the enhanced flux of ultraviolet radiation broke up water molecules in the mesosphere more rapidly when close to solar maximum. Mr Boles proceeded to introduce the morning's final speaker, Mr Mike Pinnock, Head of the Physical Sciences Division of the British Antarctic Survey.






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