Category: Earth Science
The Sun's heat evaporates water from the oceans. Evaporated water, or water vapor, is dry, invisible substance, just like a gas. Water vapor, is carried upwards into the atmosphere by rising air currents.
Warm air can hold more water vapor than cold air. The hot deserts contain more water vapor than cold air over temperate regions. When air contains all the water vapor it can hold at a given temperature, called the dew point, it is saturated. Saturated air has a relative humidity of 100%.
Relative humidity is a measure of the amount of water vapor in the air, as a percentage of the total it can contain when saturated, at that temperature.
When air with a relative humidity of 100% is cooled, the process of condensation must lose some water vapor. This means that some water vapor is then turned into a liquid. When condensation occurs, water vapor liquefies around specks of dust or salt in the air, forming tiny visible water droplets. A mass of these droplets forms a cloud. Some clouds consist of masses of tiny ice crystals and very cold-water droplets. Both are condensed at temperatures below the freezing point. The water droplets may have a temperature of -15 degrees C, but they remain liquid. However, when these super-cooled droplets collide with ice crystals, they freeze around them.
Clouds occur in various shapes. There are two main kinds; Cumulus clouds are high heaps of cloud, while Stratus clouds form in thin layers.
Cumulonimbus clouds are formed when warm air rises rapidly. The air is cooled and the invisible water vapor condenses into visible water droplets.
The main low clouds (under 1.5 miles into the air) include layered stratus, heaped cumulus, stratocumulus, nimbostratus, and cumulonimbus-thunder clouds. Medium clouds are altocumulus and altostratus. High clouds (over 3.7 miles in the air) are cirrocumulus, cirrostratus, and cirrus.
Precipitation is the general name of the moisture lost from clouds, and the most familiar example is rain. Rain is formed in several ways within clouds. In warm regions, tiny water droplets are blown around. As they move, the hit against and merge with other droplets. Finally, the droplets become so heavy that they fall to the ground as large raindrops.
In temperate regions, where the clouds are below the freezing point, the ice crystals grow as super-cooled water droplets freeze around them. When the ice crystals become large they fall downwards as snowflakes. Near the ground, they melt and become raindrops, but if the air near the ground is cold enough, the ice crystals fall as snow.