With bright twilight now persisting well into the night, observers of the sky should watch out for a particularly beautiful type of cloud formation seen only at this time of year.  Known as Noctilucent cloud, these delicate formations appear above the northern horizon, long after sunset, often around midnight.  Shining quite brightly, Noctilucent cloud is filamentary in structure, having a characteristic silvery-blue colour. 

It forms almost exclusively between latitudes 50 and 60 degrees north, high in the upper atmosphere: 80 km (50 miles) up - five times higher than normal clouds.  The cloud forms when water vapour condenses at the low temperatures that prevail at such altitudes onto particles suspended in the air.

More frequent sightings of Noctilucent cloud over the last 40 years may indicate that these particles could be a result of industrial pollution, perhaps from increased air traffic.  As far as observers of the sky are concerned, the right type of cloud, for the wrong reasons?