Formation of Clouds • A cloud is composed of millions of little droplets of water (or ice crystals when temperature is very low) suspended in the air. • There are several major processes for cloud formation. • Convection • Forceful Lifting • Meeting air masses • Horizontal motion Convection : • When the sun shines, the air at the ground level that contain water vapor is heated and it begins to rise, and as the air rises, it begins to cool. • The water then condenses on tiny suspended particles and forms droplets in the atmosphere. Convection: Forceful Lifting (mountains) • When air rises over the mountains, it cools and becomes saturated with water vapor. Condensation occurs and the water vapor becomes liquid. • When the air mass descends on the other side of the mountain, it is more dry and warm. Meeting of air masses • Not only mountains can force air to rise : when warm air meets a mass of heavier cold air, it is also forced to rise. The boundary between warm and cool air is called a « front ». As the warm air ascends it cools, and then clouds may be formed. Horizontal motion • Sometimes winds bring warm and moist air into a region. If the warm moist air flows over a much colder surface, it is cooled and moisture will condense and form fog. This mechanism is frequent on the coast of continents. Types of Clouds • Clouds are classified according to their height above and their appearance from the ground. The following cloud roots and translations summarize the components of this classification: • • • • • Cirro- curl of hair, high altitude Alto- middle altitude Strato- form in layers Nimbo – rain, precipitation Cumulo - heap High Level Clouds • High level clouds occur above 20,000 feet and are given the prefix “cirro.” Cold tropospheric temperatures the clouds are composed of ice crystals. • Three main types: cirrus, cirrostratus, and cirrocumulus Mid-level clouds • Appear between 6,500 and 20,000 feet. • Depending on the time of year, temp and altitude these clouds may be composed of liquid water droplets, ice crystals, or a combination of the two. – Two main types are: altostratus and altocumulus Low-level clouds • Not given a prefix. • Low clouds occur below 6500 feet and normally consist of liquid water droplets or even supercooled droplets, except during cold winter storms when ice crystals comprise much of the clouds.