Noctilucent Clouds Over Scotland

July 08, 2013


Photographer: Alan Tough 
Summary Author: Alan Tough

The photo above showing a spectacular display of noctilucent clouds (NLC) was captured from Hopeman, Moray, Scotland on May 30, 2013. NLC form in the mesosphere at an altitude of about 50 mi (80 km). These extremely tenuous clouds are composed of water ice. The process of NLC formation begins when water vapor condenses around small particles, possibly of meteoric origin, in the upper atmosphere. It's uncertain just where the water comes from -- one possibility is the dissociation of methane by solar ultraviolet radiation.

NLC can only be seen under specific conditions: The temperature in the mesosphere needs to be low enough for the clouds to form, around minus 120 degrees C (150 Kelvin). At this altitude the temperature is, counter-intuitively, coldest in the summer months. In addition, the clouds need to be illuminated by the Sun at the proper angle. This happens when the Sun is between 6 and 16 degrees below the horizon. Residents in northern temperate latitudes can look for NLC from the end of May through mid-August. The best displays usually occur in June and July.

Photo details: Camera Model: Canon EOS 6D; Lens: Sigma EX DG 20mm; Focal Length: 20mm; Aperture: f/3.2; Exposure Time: 1.000 s; ISO equiv: 320; Software: Adobe Photoshop 7.0.