The definitions of clouds given in the present Atlas apply, unless otherwise specified, to observations carried out under the following conditions:
(a) The observer is at the Earth’s surface, on land in areas without mountainous relief or at sea
(b) The air is clear, and no obscuring phenomena such as fog, haze, dust, smoke, etc. are present
(c) The Sun is sufficiently high to provide the usual luminance and coloration
(d) The clouds are so high above the horizon that the effects of perspective are negligible
It will be necessary to adapt the definitions to other conditions. In many cases, this can easily be done; for example, by night when the Moon is in its brighter phases, it may play, with regard to the illumination of clouds, a role analogous to that of the Sun.
The following steps should be performed during a cloud observation: