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At high latitudes in the summer months, night-shining or noctilucent clouds form between 76 to 85 km (47 to 53 mi) above the surface of the Earth. Their high altitude allows them to reflect sunlight after the sun has set. In this image, centered on the North Pole and acquired on 11 June 2007, white indicates clouds with the greatest density of ice particles and dark blue indicates clouds with the lowest. Because ice particles reflect sunlight, a greater concentration of such particles creates a higher albedo – the ratio of reflected light to total incoming light. Areas of no data appear in black, and landmass outlines appear in blue-green. Image courtesy of NASA.