Physicist explains why the clouds do not fall to Earth

In childhood, many of us have often wondered why the clouds do not fall. Read on to discover how these soft balls are formed above our heads and how they stay high in the sky.

We remember from school that a cloud is an accumulation of water vapor or ice crystals. But the principle of "holding" these "marshmallows" in the air can be compared to the flight of a balloon filled with helium, writes IflScience.

How a cloud forms

According to scientists, when the air near the Earth's surface heats up, it begins to rise, "taking" water vapor with it, which is less dense than gases such as nitrogen and oxygen. Balloons filled with hot air can rise in the same way.

The higher the air rises, the colder the temperature becomes, resulting in its cooling. At this time, small drops of water and ice are formed from water vapor, which "unite" into a cloud, physicists say.

Interestingly, people can see clouds because light is reflected from a large number of water droplets and ice floes.

How much clouds weigh and why they do not "fall"

According to scientists, the actual weight of a cloud depends on its size (volume) and the amount of condensed water vapor in it. On average, a cirrus cloud can contain about half a gram of water for every cubic meter. That is, a cloud with a volume of 1 cubic kilometer can weigh about 500 tons.

Scientists note that the principle of keeping clouds in the air is similar to the flight of a balloon filled with helium. The secret is in the ratio of pressure, temperature, weight, and air density.

Another example is oil, which stays on the surface of the water and does not sink because it is less dense and lighter, physicists say.

The important thing is that the moist air in clouds is less dense than the dry air around them. Therefore, at the same pressure and temperature, water vapor will have a lower specific gravity than air (i.e., moist air is lighter than dry air).

So, despite the huge total weight of numerous drops inside the cloud, each of them remains in the air, scientists explain. And when the drops become larger in the process of merging, their weight increases in comparison to the surface area.

According to experts, then the radius of the drop exceeds the resistance of the air moving upward, and gravity makes such drops fall to the ground, i.e., rain begins.

However, they point out that water droplets in clouds make up only a small fraction of the total volume. For example, a typical cumulonimbus cloud contains about 0.5 grams of water per cubic meter. This adds up quite quickly, given the huge volume of clouds. That is, a cloud measuring 1 cubic kilometer would weigh 500,000 kilograms, scientists estimate.

They emphasize that water droplets inside the clouds do not depend on how heavy the air around them is in general. Numerous drops in the cloud remain in the air due to the large surface area.

Physicists point out that the upward force of air drag on small water droplets is sufficient to keep their terminal velocity -where the drag force of the air through which an object is moving is equal to the downward force of gravity - low for small droplets. As a result, smaller droplets are constantly falling, but the air beneath them is easily pushed upward.

"A droplet of 10-micron radius falls at a speed of 1 cm/sec [0.02 mph], while droplets of 50-micron radius fall at a speed of 26 cm/sec [0.6 mph]," explained scientist Louis J. Battan in his book Cloud Physics.

Interestingly, as soon as the radius of a raindrop exceeds 0.1 millimeters, the resistance of the air flowing upward is not enough to resist gravity. At this time, it is better to take an umbrella, researchers advise.

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