For astronomical observations, clouds are nuisance as they block star and sunlight and distract the observations. But some (e.g. cirrus) clouds also convey information about a change in weather or the approaching weather storms that astronomers could use for planning of their observations. Some cirrus clouds may exhibit irisation, when a thin clouds is rainbow-colored as in soap bubbles. Here is one example of such clouds from Boulder, Colorado. Cloud iridescence is caused by the diffraction of the sunlight on small water droplets or ice crystals uniformly distributed inside a cloud.
Cloud iridescence may also be observed in much more rare noctilucent clouds (or polar mesospheric clouds). These clouds are formed at 75-85 km in Earth atmosphere and contain tiny ice crystals, which can scatter sunlight. These clouds are observed at twilight when the sun is several (5-15) degrees below the horizon. Second photo shows example of noctilucent clouds.
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