English: A series of images captured by the Visual Monitoring Camera on ESA's Mars Express of the recurring Arsia Mons Elongated Cloud. Image sequences were captured between 20 October and 1 November 2018 and represent a daily cycle of the cloud. The cloud begins growing before sunrise on the western slope of Arsia Mons volcano before expanding westwards for two and a half hours, growing remarkably fast — at over 600 km/h — at an altitude of 45 km. It then stops expanding, detaches from its initial location, and is pulled further westwards still by high-altitude winds, before evaporating in the late morning as air temperatures increase with the rising Sun.

In the gif the black region to the left is the nightside of Mars, while the tan-coloured mask on the right hand side is covering part of the planet that was not imaged. The blue line marks the ‘terminator’ – the boundary between the day and night sides of the planet. Because the cloud starts growing before sunrise, it can sometimes be seen even on the nightside. This is possible because it is high over the surface, so the Sun’s rays can still illuminate it close to the terminator.
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