Noctilucent clouds

Early this morning (2.45 to be exact!) I dragged myself out of bed to look for noctilucent clouds. I've only seen these from the Heath on two previous occasions, possibly because this beautiful aerial phenomenon tends to occur low in the northern sky: coastal observers see them more regularly. The excellent bottom image (after my somewhat average attempts!) was taken by astronomer friend Martin Stirland: he lives on the east coast and enjoys beautiful clear horizons.

No-one knows for certain what causes noctilucent clouds: the best bet is that they are produced when sunlight from below the horizon (ie: pre-dawn or post-sunset) illuminates tiny dust or ice particles at the edge of the Earth's atmosphere. Until the final flight of the Shuttle, it was considered possible that metal oxides from the SRB exhausts were a contributory factor.

My photos were taken with a rather high ISO and shortish exposure, and more or less reflect what the clouds look like with the naked eye. Martin uses a low ISO (200) and a longer exposure of 10 -15  seconds. This set-up captures the electric blue colour and structure much more clearly. I didn't want to wake Linda up by clattering about in the garden with a tripod, so had to 'hand-hold'!