As seen from the edge of Six Acres at around 11.00 yesterday (23rd June)...
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| Silvery grey bands of noctilucent clouds over Cuxton... |
Noctilucent ("night-shining") clouds are extremely rare collections of ice crystals, occasionally appearing in late clear summer evenings after sunset, but before it gets completely dark. They become visible about the same time as the brightest stars appear and often stay visible after dark because they are still reflecting sunlight due to their great height (around 80,000m). They are higher up than any other clouds, occupying the layer of atmosphere known as the Mesosphere, and are only seen at latitudes between 45°N and 80°N in the Northern Hemisphere, and equivalent latitudes in the southern hemisphere.
Like other clouds, noctilucent clouds need water vapour, dust, and very low temperatures to form. Low temperatures are easily attainable in the Mesosphere, but water vapour and dust are in short supply. The high-altitude dust that seeds noctilucent clouds is thought to come from tiny meteors from space. Scientists believe that the moisture comes through gaps in the tropopause, or perhaps forms from the chemical reaction of methane and other chemicals.
This is only the third time I have ever seen them. I saw the display out of the window just when I was thinking about going to bed, so I grabbed a camera and tripod and raced up Church Hill to through to Six Acres where I could get a clear northern horizon. The picture is a 6 second exposure at F8.
I luckily managed to catch them pretty much at their peak - thirty minutes later they couldn't be seen.
