A cloud is a visible mass of condensed water vapor, either in the form of tiny drops of water or ice crystals, suspended in the atmosphere. Generally clouds are classified according to their height: high (16,500 to 45,000 feet), middle (6,500 to 23,000 feet), and low (0 to 6,500 feet). The four chief forms of clouds are: cirrus, which are high only white clouds; stratus, which means spread out and most often are long, low, gray layered with a uniform base; cumulus, a Latin word meaning heap or puffy, are white clouds resembling cotton balls or globs of whipped cream; and nimbus, a kind of shapeless dark gray rain cloud, heavy with moisture.
The cirrus clouds generally are predictors of fair weather if they do not thicken. They appear to be light and wispy. Once they lower and merge with other types of cloud formations they thicken, which usually indicates rain or snow will occur in twelve to twenty-four hours. The much lower level stratus clouds are grayish in color and extend in a long layer with a uniform base. They tend to signify fair weather. The dome- shaped cumulus clouds most often foretell fair weather when they stand alone in the sky in a billowy manner. During the summer months, however, when the warm moist air rises rapidly into the atmosphere, they have a tendency to become heavy and flatten by late afternoon. When that takes place they become altocumulus (middle level) or patches of dark stratocumulus type clouds, and they can bring on a thunderstorm or rain before the end of the day.
The most obvious and ominous cloud type of all is the nimbostratus, an extensive, dark low-level mass of gloomy murkiness, easily leading the sky observer to conclude that the "rain clouds" are an imminent threat. The nimbostratus clouds arrive slightly ahead of a warm front and can bring long steady rain or snow in winter.