These weird blue clouds form at around 50 miles (80km) altitude, on the very edge of space. This is around 10 times higher than normal cirrus clouds, so are they are still lit by sunlight at this altitude, even though it is dark at ground level. Normal clouds, if there were any, would be silhouetted as dark outlines against them, or more likely orange outlines, lit by the lights of London, whose centre lies 20 miles (32 km) away to the north, the direction in which this picture was taken.
I have seen noctilucent clouds only once before from this particular location in southern England - the latitude (52 degrees N) is rather too far south for a view high in the sky. They normally form in polar regions - hence their official name Polar Mesospheric Clouds - so usually appear quite low down in the northern sky from this latitude. However, this was by far my best view to date, when they reached nearly 30 degrees into the sky.
This good display was perhaps the result of water put into the upper atmosphere during the Space Shuttle launch 10 days earlier - the hydrogen and oxygen propellant burns to make water. The atmospheric pressure at this altitude is only a millionth of that at ground level, and at a temperature of around -150 degrees Celsius the water readily forms minute ice crystals roughly the size of cigarette smoke particles. They therefore scatter light at shorter wavelengths, thus creating the blueish colour of cigarette smoke and as seen here.
Timing and location to see these clouds is critical. The Sun must be between 6 and 16 degrees below the horizon - less than this and they are masked by the bright sky, and greater than this they are no longer lit, so are invisible. This display peaked shortly before I started shooting (22:25 local time, about 1.5 hrs after sunset), and were diminished, though still visible, by 23:00, 2 hours after sunset. Luck also comes into the equation - the previous and following nights were all clear but no trace of noctilucent clouds was seen!
Picture added on 19 July 2006