Appearance

(Section 2.6.3.3.1)

Noctilucent clouds are generally observed during the twilight periods when the Sun is between 6° and 16° below the observer's horizon, with the best visibility when the Sun is about 10° below the horizon. When the Sun is less than 6° below the horizon, the sky background is too bright for a moderate display to be observed, and when the Sun is greater than 16° below the horizon, the level where the clouds are formed is no longer illuminated by the Sun's rays. Bright noctilucent clouds may just be detectable when the Sun is between 2° and 6° below the horizon, but it is difficult to distinguish them from high Cirrus at such times.

When bright, well-developed noctilucent clouds displays are observed under clear-sky conditions, identification is easy, even by an inexperienced observer. Difficulty may arise, however, when identifying noctilucent clouds that are faint and lack structural detail. The identification is made more difficult when obscured by tropospheric clouds. In cases of doubt, the observer should make sure that the object is not an aurora or a sunlit or moonlit tropospheric cloud or contrail.

What sets noctilucent clouds visually apart from tropospheric clouds is their visibility in the night, their obvious blue−white colour and their disappearance into the dawn, close to onset of civil twilight. Bright displays can be awe-inspiring and very obvious to the casual observer. In twilight after sunset, noctilucent clouds are at first, greyish or pale blue and, as time advances, they become more and more brilliant; appearing bluish white like tarnished silver, pearly white or electric blue. Sometimes, there are golden, reddish or greenish tints to the colour when the clouds are near the horizon. On some occasions, there may be a red upper edge to the clouds.

At midnight, noctilucent clouds may be seen rather low on the northern horizon (in the northern hemisphere), or low on the southern horizon (in the southern hemisphere). As sunrise approaches, they can appear in more of the sky, before disappearing at dawn.

The following comments will serve as useful aids in identifying noctilucent clouds:

• Noctilucent clouds are always brighter than the twilight sky; therefore, clouds that stand out as dark silhouettes against the background sky cannot be noctilucent clouds.
• Tropospheric clouds, when illuminated by the Moon, city light or light scattered from the bright part of the sky, may appear brighter than the sky background if the sky is fairly dark. However, these clouds can usually be distinguished from noctilucent clouds by their colour and form. They are milky white, whereas noctilucent clouds shine bluish white, and unlike noctilucent clouds, these clouds will continue to be visible during civil twilight and after sunrise.
• Cirrus clouds illuminated by the Sun when it is below the horizon are usually coloured yellow, orange and pink. Clouds having these colours usually are not noctilucent clouds.
• Binoculars can assist in the identification of noctilucent clouds. Under magnification, finer detail than can be seen with the naked eye can be sharply resolved. This is not the case with Cirrus cloud, which tends to be nebulous when viewed through binoculars.