Noctilucent clouds are rare in that they form high in the atmosphere near the edge of space where normally there is not enough moisture or molecular debris needed to form the water vapours that make up clouds. Such clouds generally appear only during the summer months and in the northern hemisphere. So high are the clouds that the sun, having already set below the horizon, can still illuminate them. These photographs were taken more than an hour after sunset when the sky would normally be dark.
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