Clouds consist of condensed atmospheric moisture or tiny ice crystals, which are light enough to float in the atmosphere by vertical currents. Cloud formation - a vital cog in the water cycle - is considered to be a transitive process, leading to rainfall. The presence and appearance of clouds are primarily influenced by climatic and atmospheric conditions. The interesting fact about cloud formation is that it could involve the three states of water. However, the liquid and gas states mainly influence how clouds form.
When water is heated and its atoms become sufficiently energised, it moves from the solid or liquid phase to the gas phase. In the gas phase, it is invisible and forms water vapour. Water vapour contributes to atmospheric moisture necessary for cloud formation. The visibility and appearance of clouds is due to the process of condensation.
Condensation is necessary for cloud formation, but it is inaccurate to call merely condensed air a cloud. Through condensation, water vapour is returned to liquid state by the cooling of the rising air or changes in atmospheric conditions. The tiny droplets, measured in microns, are formed around a nucleus of microscopic dust particles found in the atmosphere. The droplets are light enough to be supported by vertical air currents.
The condensation process occurs when the dew point- the temperature at which water vapour changes state - is reached. The expansion of air due to adiabatic processes also affects the point of condensation. Pressure (vapour partial pressure specifically) and changes in temperature and volume also affect this process. The condensation process also determines the type of cloud formed.
Clouds exist in an assortment of different shapes and sizes. This does not happen by chance, but is due to changes in the conditions that affect cloud formation, like temperature and pressure for example. Cirrus clouds, formed at high altitudes, are a result of self-nucleation. This process refers to the transformation of droplets into ice crystals at sub-zero temperatures. Thus, classification of clouds into different types is based on appearance and altitude.
In summary, water vapour is first formed through solar heating. The heated vapour rises due to vertical displacement by cooler air (cold fronts) near to the earth's surface (hot air rises over cooler air). The air expands as it rises, and cools adiabatically. If the air is saturated enough, microscopic droplets will form around a nucleus of microscopic dust particles. The type of cloud formed is based on a combination of atmospheric and climatic conditions.