20. Periods are
and represent the number of
in the electron configuration

In meteorology, a  cloud  is an aerosol consisting of a visible mass of minute liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body. nephology is the science of  clouds, which is undertaken in the  cloud  physics branch of meteorology.cirrus clouds  usually describe clouds high in the atmosphere that can include wispy clouds, typically with ice crystals. variations include cirrus, cirrocumulus and cirrostratus as defined by the cloud's position in the atmosphere.layered stratus clouds  have both flat tops and bases, and can appear as if they take up the whole sky, stretching from horizon to horizon. other combinations and variations include stratus, stratocumulus, nimbostratus and altostratus.cumulus clouds  often pile up in multiple layers of the atmosphere, representing clouds that develop vertically. cumulus clouds often look like pillars with anvil-type tops or columns of clouds stacked vertically. variations include cumulus, cumulus-congestus, cumulonimbus and altocumulus.