The clouds in the sky can be high or low. The higher clouds can reach about 10 kilometers, while the lower clouds may only be a few dozen meters above the ground.
There are many reasons for cloud formation, but the primary cause is the rise of moist air. As this air moves upward, the external air pressure decreases with altitude, but its volume increases, causing it to consume heat in the process. Thus, the air both rises and loses heat.
We all know that the air’s capacity to hold water vapor is limited. At a certain temperature, the maximum limit of water vapor pressure in a unit volume of air is referred to as the saturation vapor pressure. This saturation vapor pressure decreases as the temperature drops. Therefore, when the temperature in the sky decreases, the saturation vapor pressure continuously declines as well.
The process of cloud formation.
When the saturation pressure of the air at high altitude drops below the actual vapor pressure, some of the water vapor will combine with dust particles in the atmosphere and condense into tiny water droplets (when the temperature is below 0°C, small ice crystals may form).
The volume of these droplets is very small, and they are the components that form clouds, with an average radius of only a few micrometers. However, their density is very high, and they fall at an extremely slow rate in the air, allowing them to remain suspended in the atmosphere, thus becoming clouds.
How can we observe the moist air rising to create clouds? Here are a few methods:
- First is the effect of thermal dynamics. On clear summer days, the sunlight and high temperatures heat the air near the ground, causing the warm, lighter air to rise. The towering and mountainous clouds we see on such days are formed in this way.
- Second is the effect of the “frontal boundary”. In meteorology, a “frontal boundary” refers to the interface between warm air and cold air. When the lighter warm air rises and encounters the heavier cold air, it will actively move over the inclined surface of the cold air mass, creating what is known as a warm front. The warm air rising over this inclined surface can form thick clouds over a wide area. Conversely, when cold air rises into warm air, it will infiltrate beneath the warm air, pushing it upward; this interface is referred to as a “cold front.” The warm air being pushed up over a “cold front” can also create thick layers of clouds.
- Third is the effect of topography. When the moist air from the stratosphere encounters geographical obstacles such as plateaus and mountains, it is forced upward, forming clouds or mist at the windward mountain peaks.
Additionally, disturbances in the airflow at right angles, along with the nocturnal cooling radiation from the cold air layer, can cause water vapor in the air to condense into clouds.
Regardless of how clouds are formed, due to their tiny size and slow fall rate, even weak upward air movement can keep them suspended, allowing clouds to float in the sky without falling.
