If you’ve ever gone outdoors on a foggy day, you’ve been a lot closer to a cloud than you thought. In fact, you’ve probably been right in it, without even touching the sky. That’s because both fog and clouds are microscopic water droplets similar to those seen in a hot, steamy shower. The processes of evaporation and condensation form clouds. When the sun warms water in lakes, rivers, oceans, or puddles, it evaporates into water vapor. By heating water, you may disappear it and see it dissipate like vapor.
Invisible water vapor spontaneously rises from the Earth’s surface into the atmosphere in the form of heated bubbles, similar to those seen rising in a lava lamp. The temperature decreases until the water vapor condenses back into liquid water as it ascends.
Clouds are composed of millions of these tiny droplets of liquid water. The droplets evenly spread the hues of the sun, giving clouds their white appearance. While clouds may seem to be comfortable puffballs, they are incapable of supporting your weight or supporting anything other than themselves. The evaporation and condensation processes in the atmosphere are analogous to those that occur in your bathroom when you take a hot shower: warm water evaporates and then condenses back into the water on the cool mirror.
Water vapor does not naturally condense. It requires either small particles or a surface – such as your bathroom mirror – to make a drop. Atmospheric scientists refer to these microscopic particles as cloud condensation nuclei or CCN. These CCN are just dirt or dust particles carried into the sky by the wind.
Does this suggest that areas with high levels of dust and pollution, such as cities, have more drops than areas with low levels of pollution? Researchers discovered more small droplets and clouds in locations with a high concentration of these cloud condensation nuclei. In contrast, fewer clouds are recorded in areas devoid of them, such as over the ocean or the Arctic.
The air temperature lowers as cloud droplets climb in the atmosphere. When the temperature falls below 32 degrees Fahrenheit, the tiny cloud droplets freeze (0 degrees Celsius). It’s precisely the same as freezing ice cubes. Ice crystals have formed from the frozen drops. They expand when water vapor condenses to create ice and adheres to them.
Scientists refer to this gas transformation into a solid as “deposition.” It is responsible for the lovely branching ice crystals seen in snowstorms. Constant air currents keep these very light water droplets or ice crystals floating in the cloud. Thus, how can they become rain and snow and eventually fall to the ground? Simple, they band together. On their journey to the Earth as raindrops, larger droplets attract smaller ones.