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The sky should be watched towards dusk, to obtain some guidance on the clouds, which are likely to be present after the daylight has gone. When the sun is just below the horizon, the lower clouds look grey, those at medium levels look rose-coloured, whilst those at high levels appear whitish. As nightfall approaches, medium-level clouds turn grey whilst cirrus and aircraft condensation trails turn yellow, then pink and finally grey. This colour sequence is reversed at dawn. After nightfall, the sky should be observed from a dark location, well away from lights. The observation should not be made until the observer's eyes have become adapted to the darkness. On moonlit nights, clouds are visible when the moon is more than a quarter full. All perceptible clouds appear black to grey, except those illuminated by the moon, which present a whitish appearance. Halo phenomena produced by the moon are always white. The colours of rainbows produced by the moon are much weaker than those produced by the sun and are sometimes absent. When the moon is less than a quarter full, there may be difficulty identifying clouds at large angular distances from the moon. Their existence and approximate amount may be deduced from the number of stars that are obscured in the sky. The difficulties are of course substantially increased, if there is no moon at all. Observation of cirrus is then a real problem, but if it is thick and extensive, it may be noted by its dimming effect on stars. Cirrostratus causes slight diffusion of light around each star, whose brilliance is at the same time dimmed. However in the absence of moonlight, it is almost impossible to differentiate between cirrus and cirrostratus. The brighter stars and planets are visible through thin veils of cirrus, cirrocumulus and cirrostratus. Altostratus is generally so dense, that the stars are masked. The gradual lowering of a sheet of altostratus may be difficult to detect. However as the base is rarely uniform, as it descends, small contrasts can often be discerned on all but the darkest nights. Nimbostratus usually develops from thickening altostratus. If on dark nights doubt exists regarding the choice of designation between altostratus or nimbostratus, by convention the cloud is called nimbostratus if rain or snow is reaching the ground. Nimbostratus is normally associated with moderate or strong winds, whilst stratus often forms during calm or light winds. However this criterion alone must not be used as a basis for distinction. Fog formed over the sea and driven across the coast by an onshore wind may appear inland as stratus cloud. It can often spread across the sky very rapidly. The intensity of the darkness is of some assistance in deciding
whether the sky is wholly covered or not with dense low cloud. If
there is any light at all, variation of contrast may indicate patches
of low cloud with medium or high cloud above. Near built-up areas,
clouds may often be revealed by illumination from below, especially
when snow is lying. Sodium street lighting often casts an orange
glow on the base of the cloud. A layer of cloud illuminated in this
way, may provide a bright background against which lower fragments
stand out in dark relief. Very low cloud may obscure known lights
on hills and tall buildings/structures. The lights of low-flying
aircraft or the noise from their engines (when the aircraft is hidden
by low cloud), may give a clue to the cloud which is present. When
the height of the cloud base can be measured, this can be useful
in determining the cloud types that may be present.
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