The sky is blue with white clouds, and the green meadows create one of the most beautiful scenes in our imagination. Sometimes, you lie on the grass and watch white clouds drifting by, and a question arises: How much does this cotton candy-like cloud weigh? Will it suddenly fall from the sky?
Don’t underestimate those blooming “cotton candy” clouds; each one can weigh hundreds of tons! Clouds are a crucial component of the Earth’s water cycle. When the sun shines down on the Earth’s surface, water evaporates into water vapor, gradually rising into the atmosphere along with the wind. As altitude increases, temperature decreases; for every 100 meters of elevation, the temperature drops by 0.65 degrees Celsius.
When the temperature drops to a certain value, water vapor condenses upon encountering cold air and gathers around particles in the atmosphere, forming tiny water droplets. More and more tiny droplets cluster together to create clouds. Even an average-sized cloud can weigh over 500 tons.
An average-sized cloud can weigh over 500 tons.
How can hundreds of tons of clouds float in the sky without falling due to Earth’s gravity?
A water molecule consists of one oxygen atom and two hydrogen atoms. The main components of the air on Earth are nitrogen and oxygen, with nitrogen being heavier than hydrogen, making water molecules lighter than air. Because the number of molecules in the same volume is equal at the same temperature and pressure, areas with a higher concentration of water molecules are lighter than surrounding air, causing them to rise due to their lower mass. However, even the lightest water molecules will eventually fall due to Earth’s gravitational force. Thus, clouds high up also experience the effects of gravity and will eventually descend. The only difference is that these tiny water droplets fall very slowly.
In physics, there is a term used to refer to the falling speed of small objects like water droplets, known as “terminal velocity.” Essentially, this means that terminal velocity relates to the mass of the object and its cross-sectional area. The smaller the mass of the object, the slower its terminal velocity.
The smaller the mass of the object, the slower its terminal velocity.
The mass of tiny water droplets is so minimal that they fall extremely slowly, often appearing to float in the air. These droplets will eventually fall, but they may evaporate during their descent and re-form into smaller droplets. Therefore, clouds represent a dynamic equilibrium.
So why do clouds sometimes produce rain, snow, or even hail?
This is actually quite understandable in terms of temperature. Initially, in clouds at high altitudes, water condenses into ice crystals or snowflakes, forming clouds. A solitary cloud or two won’t cause winds and waves that lead to isolated rain. A significant number of clouds must accumulate through strong air convection. Through compression and mutual collisions, ice crystals bump and rub against each other, increasing in mass and weight, causing them to fall to the ground. If the temperature near the ground is above 0 °C during this descent, it will rain.
If the temperature near the ground is above 0 °C, it will rain.
Some may argue that in summer, it can only rain; how can there be snow or even hail? The reality is that extreme weather changes in summer can indeed lead to hail. The occurrence of hail is associated with strong wind currents and vigorous convective activity that tends to happen in summer. When convective clouds develop strongly, their tops can reach heights of over 10,000 meters, where temperatures can drop to -30 to -40 °C. The water inside the cloud continuously pushes larger droplets from the lower part of the cloud to its midsection, forming supercooled droplets.
The downward airflow in the cloud can carry ice crystals and snowflakes upward, where supercooled droplets collide with ice crystals to form the core of hail. The hail’s core rises and falls, moving upward and downward with the air currents in the cloud’s 0 °C layer. It gradually grows larger, resembling a snowball. When the upward winds can no longer support the increasing hail, it will fall to the ground, resulting in the hailstorm we see.
Hail.
Clouds throughout the seasons carry the essence of each season. In summer, the sky is a bright blue; in autumn, the sky is adorned with beautiful white clouds; in winter, the sky is overwhelming, hiding the sunset; and in spring, gentle winds carry the clouds. When you have some free time, look up at the sky more often and find your favorite cloud.
