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Clouds are formed from evaporated water from bodies of water (lakes, oceans, rivers, etc.) and from moist soil and plants (transpiration). The water vapour (evaporated water) expands and cools as it rises into the air. Air can only hold a certain amount of water vapour at any given temperature. Warm air has a larger capacity for water vapour than cool air. When the air cools, some of the water vapour condenses into tiny water droplets. In order for water vapour to be able to condense, microscopic particles must be present. The particles, condensation nuclei, become the centres of the water droplets. Most condensation nuclei are tiny salt or smoke particles. If the temperature is above -40°C, and ice crystals or freezing nuclei (resembling condensation nuclei) is present, sublimation (gaseous to solid state) can occur. If the temperature of a cloud is between 0°C to -40°C, the cloud may contain both water droplets and ice crystals.

Water vapour rises to form clouds in different ways. Three of the ways are convectional, relief (lifting or orographic) and frontal.

Evolution of A Cumulus Cloud

Convectional

Intense heating of the earth’s surface causes the air above it to be heated and eventually rise. The warm, accumulating air rises quickly. As it ascends, the water vapour in it condenses into water droplets or maybe even ice crystals. These particles gather to form clouds. As the air near the earth’s surface continues to be heated and to rise, more water particles are added to the clouds.[Click here for animation]

Relief

Prevailing winds blows from a large body of water to a mountain or upland area. The wind carries with it much moisture as it passes a large body of water. The wind hits the elevated area and is forced to rise up, by the prevailing wind behind it that is constantly blowing in the same direction, which causes the mass of air to pile up. The relative humidity of the air is high as the winds are moisture-laden. Thus, the air will only need to rise a little before condensation occurs and clouds are formed quickly. Relief rain often accompanies this kind of uplifting of air. The rain usually falls on the side, facing the prevailing wind, called the windward side. After that, the clouds will be very dry and the area in which the dry clouds pass will be dry. This side is called the leeward side and the process is called the rain shadow effect. [Click here for animation]

Frontal

The boundary where two air masses of different temperature meet is called a front. A cold front forms when a cold air mass moves into a warm air mass. The cold air mass will stay at the ground, forcing the warm, moist air mass to move above it. The warm air mass rises rapidly and cools as it rises, condensing the water vapour to form clouds. The clouds of this kind are normally very great in height, but very narrow. A warm front is formed when a warm air mass moves into a cold air mass. The warm air mass rises over the cold air mass, which is denser. The steepness of the warm front is less dense than the cold front and the warm air travels over a greater distance. As the warm air rises, condensation occurs and clouds are formed. Clouds of this kind are normally widely spread and not very great in height.