To understand how clouds form, let's follow the movement of water from the surface of the earth (ground surface) to the sky ^above2).

First, when the earth's surface is heated by sunlight (solar radiation), water evaporates into vapor in oceans and rivers. At this time, sunlight also heats the air immediately above the earth's surface. As the temperature of the air rises, the amount of water vapor that can be contained per ^cubic meter (saturated water vapor content) increases, creating a mass of air with a high water vapor content near the ground surface.

Air has the property of becoming lighter as its temperature rises. Therefore, a mass of air heated near the ground surface becomes lighter and moves upward. This air flow is called "updraft. Because air pressure is lower in the upper atmosphere, the air that rides the updraft expands. The air then uses energy to lower its temperature. The higher the altitude, the lower the temperature of the air.

As the air temperature decreases, the saturated water vapor content decreases. The water vapor in excess of the saturated water vapor content then attaches to the dust (aerosol) in the air and condenses, forming small droplets with a radius of about 0.01 mm. These droplets are called "cloudbursts.

As the cloud particles rise further and the temperature drops to about -20°C, they solidify (freeze) into ice crystals. Ice crystals are another material used to make clouds. Clouds are made of cloud particles and ice crystals.

However, sometimes the temperature does not drop low enough to form ice crystals, and clouds are formed only from cloud particles. Clouds made entirely of cloud particles are classified as "water clouds," clouds made of cloud particles and ice crystals are classified as "multiphase clouds," and clouds made entirely of ice crystals are classified as "ice clouds. Water clouds are sometimes referred to as "warm clouds," while clouds composed of cloud particles and ice crystals are referred to as "cold clouds.