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Noctilucent clouds are thin ice clouds that appear around the summer polar mesopause. Recently, the Cloud Imaging and Particle Size instrument on the AIM satellite discovered nearly circular ice free regions within the clouds-denoted as "ice voids." The origin of these ice voids is not known. Their existence has so far only been reported by Cloud Imaging and Particle Size, which only can give very limited information of the time scales involved. On 4 July 2010, such an ice void was registered by our ground-based camera taking images with 30-s time interval. We thus here present the first full temporal development of an ice void. Surprisingly, the void did not drift with the prevailing wind as cloud features around it, but instead remained notably stationary for its entire existence of approximately 1 hr. This indicates that that the origin is of stationary character, rather than a rapid change of the local atmosphere.

Plain Language Summary Noctilucent clouds are ice clouds that appear high in the atmosphere, about 80 km above the summer pole. By observing them we have learned a lot about the remote and inaccessible region where they form. Recently, a satellite borne instrument discovered nearly circular ice-free regions within the clouds, denoted as "ice voids." The origin of these voids is a mystery-we do not know what causes the clouds to disappear in large circular areas. So far these voids have only been observed from satellites, which only can take pictures of the clouds when they pass above once every 1.5 hr-longer than most ice voids exist. This means that until now we completely lack observations of the development and disappearance of the voids. Here we therefore present the first full temporal development of a void, as observed by our ground-based camera taking images every 30 s. Surprisingly, the void did not drift with the wind as cloud features around it, but it remained notably stationary for approximately 1 hr. These observations give important clues to help us solve the mystery of the origin of these voids-they suggest a steady local heating of the atmosphere as the cause.