Noctilucent Clouds

What are Noctilucent Clouds?

The literal meaning of 'noctilucent' is "lit at night" or "night shining". This refers to the fact that Noctilucent Clouds (NLC's) are only seen at twilight, when the clouds occupy a portion of the Earth’s atmosphere which is still sunlit: they are never seen in daylight skies. Therefore, the best time to look for NLC's is during the deep summer twilight when the Sun lies between 6 - 16 degrees below the horizon. The diagram on the right shows the basic geometry. Look west 60 to 90 mins after sunset, and if you see luminous blue-white tendrils spreading across the sky you've probably spotted a noctilucent cloud. Alternatively, book a trip to the ISS - they've been photographed from there as well!

NLC's are tenuous cloud-like formations in the high atmosphere, thought to be composed of small ice-coated particles; their precise nature remains a mystery however. They form at very high altitudes - around 50mls above sea level - and are thus a quite separate phenomenon from normal weather or tropospheric cloud. Each year, NLC incidence tends to peak just prior to and for a few weeks after the summer solstice - from mid-May to mid-August for northern hemisphere observers. The twilight conditions which render NLC's visible impose a latitude restriction on their visibility. They are, therefore, most often seen from locations which lie between latitude 50 and 60 degrees: through most of June and July in the northern hemisphere these locations never attain true astronomical darkness and twilit skies persist all night. There is some evidence that NLC's are being seen more & more frequently and at lower latitudes though - some theories link this to climate change.

Some pictures

I had never seen NLC's (or perhaps just had not recognised them for what they were!) so specially looked out for them during summer 2009 as this had been a particularly good "season" elsewhere. I was rewarded on 14th July when at around 22:25BST the sky lit up with the characteristic blue-white glow. The sun was 9deg below the horizon at this time - just right for NLC's. A patch of "normal" cloud is visible to bottom left, showing the great difference in appearance between it and the NLC: note that this cloud is dark, as it is in shadow (as illustrated by the diagram above).

The NLC's pictured show the typical characteristics of this type of phenomenon, which I show expanded here to give a better view. On the left we have the uniform white illumination of "stratus"-type cloud while on the right the rippled effect of "cirrus"-type. Note that the rippled sections are only lit on their lower edge i.e. from the direction of the setting sun.