Noctilucent Cloud is a phenomenon which generally occurs in the summer months between latitudes 50 - 70 degrees.  They are the very highest clouds which occur in the Earth's mesosphere at an altitude of around 80kms. Formed from Tiny crystals of water ice it is beleived by some that the increasing propensity of these to appear in recent years may be linked with climate change.

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10 Second Exposure, Canon 300D

This phenomenon can only be observed in deep twilight when the Sun is just at the right angle below the horizon to illuminate these tenuous clouds.

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15 Second Exposure, Canon 300D