This blog post provides an easy-to-understand explanation of the process of water vapor condensation, cloud formation, and atmospheric changes that lead to precipitation.
When the atmosphere contains the maximum amount of water vapor it can hold at a given temperature, it is said to be saturated. The pressure exerted by the water vapor is called saturated vapor pressure, and this value increases as the temperature rises. The vapor pressure of the atmosphere when it reaches saturation is an important point at which condensation begins. When condensation begins, water vapor turns into water droplets and clouds form.
The process of cloud formation is complex and multi-step, but in general, clouds can form when the vapor pressure in the atmosphere exceeds the saturation vapor pressure due to cooling of the air temperature. In other words, when the air temperature cools and the saturated vapor pressure decreases, excess water vapor condenses and forms small water droplets. These water droplets continue to collide and merge, growing into clouds. Various factors influence the formation and growth of water droplets.
Water droplets in the early stages of formation are extremely small and spherical, so they have a large curvature. The greater the curvature, the more difficult it is for water droplets to grow. In water droplets with high curvature, surface tension makes it difficult for water vapor molecules in the surrounding area to enter the water droplets, while making it easier for water molecules on the surface of the water droplets to evaporate into the surrounding area. When water vapor molecules enter water droplets, the surface area increases, so the smaller the water droplets, the greater the water vapor pressure required. In addition, when water vapor molecules leave the surface of a water droplet, the surface area decreases, making it easier for the droplet to evaporate. For this reason, the greater the curvature of a water droplet, the higher the water vapor pressure must be for condensation to occur easily. The curvature of water droplets affects the amount of water vapor pressure required for their growth, and this is called the curvature effect.
However, in nature, water droplets are formed even when the atmosphere is not saturated with water vapor. This is because various particles suspended in the atmosphere, called aerosols, help form water droplets. Many aerosols are hygroscopic and can act as nuclei for condensing water vapor even at relative humidity below 100%. Most condensation nuclei in the atmosphere occur naturally, but they can also be caused by air pollution. Condensation nuclei provide a surface on which water vapor molecules can easily condense, so large aerosols are more effective at forming water droplets than small ones. The formation of water droplets using hygroscopic aerosols as condensation nuclei is called heterogeneous nucleation.
Let’s look at an example of how water molecules are adsorbed and water droplets are formed even when the atmosphere is not saturated due to heterogeneous nucleation. When sea salt (sea salt) particles in the atmosphere act as condensation nuclei, the sea salt melts during the droplet formation process. In other words, the droplets formed by the condensation of water vapor act as a solvent and dissolve the sea salt, which is the solute. When a solute is dissolved in a droplet, the surface of the droplet is composed of water molecules and solute molecules, and the number of water molecules is smaller than when it is pure water. Therefore, the evaporation rate of water molecules on the surface is lower than that of pure water. As a result, the water vapor pressure required for growth is lower for water droplets containing dissolved sea salt than for pure water droplets. The degree of water vapor pressure required for water droplet growth varies depending on the concentration of the solution, which is called the solute effect.
This condensation process affects not only cloud formation in the atmosphere but also precipitation. When water droplets in clouds become large enough, they begin to fall under the influence of gravity, and this process is the principle of rain. The precipitation process involves various complex physical phenomena, and the amount and form of precipitation vary depending on various atmospheric conditions. For example, when there is a lot of dust or other fine particles in the atmosphere, the number of condensation nuclei increases, which can lead to the formation of more water droplets and an increase in precipitation. On the other hand, when there are few fine particles in the atmosphere, it is difficult for water droplets in clouds to grow large enough, which can lead to a decrease in precipitation.
As such, cloud formation and precipitation are closely related to complex atmospheric dynamics, and understanding these processes plays an important role in weather forecasting and climate change prediction. Due to recent climate change, extreme weather events are becoming more frequent around the world, making research on atmospheric water vapor and condensation processes even more important. In order to accurately predict and respond to the effects of climate change, in-depth research and analysis of atmospheric water vapor dynamics, cloud formation mechanisms, and precipitation processes are necessary.
