Literally, ‘night-shining’ blue–silver clouds seen in late or early twilight during the summer from high temperate latitudes. Noctilucent clouds result from water vapour condensing on nuclei (possibly meteoric or volcanic debris) at altitudes of around 82–85 km, close to the mesopause. The clouds are extremely tenuous, and are seen only when the Sun lies between 6° and 16° below the horizon, appearing bright by contrast with the twilight sky. Noctilucent clouds lie five times higher than the loftiest clouds in the troposphere, and are distinct from cirrus clouds to which they bear a superficial resemblance. In particular, noctilucent clouds may be identified by their delicate, feathery, interwoven herringbone structure. They are more common in years of low solar activity, and there is evidence that they are becoming more frequent.