Normally, air must become saturated for condensation or sublimation to occur. Saturation may result from cooling temperature, increasing dew point, or both. Cooling is far more predominant. In aviation community the clouds are classified as 14 sorts. They are cumulus, cumulus congestus, cumulonimbus, fractocumulus, stratus, stratocumulus, nimbostratus, fractostratus, fractonimbus, altocumulus, altostratus, cirrus, cirrocumulus and cirrostratus. Clouds are the weather signposts in the sky to the pilots. They give them an indication of air motion, stability, and moisture. Clouds help to visualize weather conditions and potential weather hazards pilots might encounter in flight. Let’s examine these signposts and how to identify them. For identification purposes, you need be concerned only with the more basic cloud types, which are divided into four families. The families are: high clouds, middle clouds, low clouds. The first three families are further classified according to the way they are formed. Clouds formed by vertical currents in unstable air are cumulus meaning accumulation or heap; they are characterized by their lumpy, billowy appearance. Clouds formed by the cooling of a stable layer are stratus meaning stratified or layered; they are characterized by their uniform, sheet-like appearance. In addition to the above, the prefix nimbo or the suffix nimbus means raincloud. Thus, stratified clouds from which rain is falling are nimbostratus. A heavy, swelling cumulus type cloud which produces precipitation is cumulonimbus. Clouds broken into fragments are often identified by adding the prefix fractus; for example, fragmentary cumulus is fractocumulus. The high cloud family is cirriform and includes cirrus, cirrocumulus, and cirrostratus. They are composed almost entirely of ice crystals. The height of the bases of these clouds ranges from above 20,000 feet in middle latitudes. In the middle cloud family are altostratus and altocumulus. These clouds are primarily water, much of which may be supercooled. The height of the bases of these clouds ranges from about 6,500 to 20,000 feet in middle latitudes. In the low cloud family are the cumulus, cumulus congestus, cumulonimbus, fractocumulus, stratus, stratocumulus, nimbostratus, fractostratus，fractonimbus. Low clouds are almost entirely water, but at times the water may be supercooled. Low clouds at subfreezing temperatures can also contain snow and ice particles. The bases of these clouds range from near the surface to about 6,500 feet in middle latitudes. The vertically developed cloud includes cumulus congestus and cumulonimbus. These clouds usually contain supercooled water above the freezing level. But when a cumulus grows to great heights, water in the upper part of the cloud freezes into ice crystals forming a cumulonimbus. The heights of cumuliform cloud bases range from 1,000 feet or less to above 10,000 feet.4. What state is the water present in low clouds? （ ）