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A Cloud Weighs Nearly 500,000 Kilograms, so Why Doesn't It Fall?

Updated on August 5, 2019

I'll explain this from the macroscopic atmospheric movement.

How do clouds form? In other words, it is because the rising air on the ground cools at high altitude, and after reaching the dew point below, the steam begins to condense into small droplets, thus forming clouds.

But why is the cloud relatively stable?

In fact, this problem is not correct, not all clouds are relatively stable. There are many unstable clouds, such as cumulonimbus.

There are aerodynamic reasons. The ground air is heated by the ground temperature and begins to rise. We can imagine a stream of air (like wrapped in a hot-air balloon). When it rises, it expands due to the decrease of pressure, which causes work to be done to the outside world, and consequently leads to a decrease in temperature. When there is no steam condensation, the temperature drops at a rate of about 10 C/km (this is called dry adiabatic direct reduction rate, DALR, Dry Adiabatic Lapse Rate). The natural temperature drop in the atmosphere is about 6.5 C/km (this is called ELR, Environment Lapse Rate). Therefore, at this time, the updraft cooling speed is faster than the environmental cooling speed.

When the temperature drops to dew point, the water vapor in the air begins to condense. During the condensation process, the heat is released, so the temperature drop rate of the air flow will be slowed down, generally at about 6 C/km (MALR, Moist Adiabatic Lapse Rate). At this time, the cooling speed of updraft is often slower than that of ambient cooling.

In any case, as the updraft continues to rise, its temperature drops and its density increases. When its cooling rate is faster than that of the environment (DALR > ELR), due to buoyancy effect, it will gradually have a downward trend and gradually offset its upward trend.

If the downward trend of droplets is still insufficient to fully resist the upward trend when they begin to form, they will continue to rise. There is nothing left to stop it, because at this time, its cooling rate slows down, and its cooling rate is not as fast as that of the environment (MALR < ELR). So it is always higher than the ambient temperature, the direct consequence is that it is rising, the temperature is getting lower and lower, more and more condensation, and eventually the cloud collapses, forming rain. This is called an unstable cloud.


However, when it begins to condense, it is already cooler than the ambient temperature, so there is a downward trend, then it will not continue to rise, but scattered, and then it will sink in a convective way. Therefore, droplets will not continue to condense, there will be no rain. That's why the clouds we see on sunny days are roughly flat at the same height.


So the question arises, since the airflow begins to sink at this point, why don't the clouds follow suit?

Simply, when the air sinks, the temperature starts to rise. So the droplets evaporate again. That is to say, the cloud is falling, but after falling, it is not a cloud, so you can not see it. Clouds at this altitude are supplemented by moisture from rising air currents. So what you see is a stable cloud.

Of course, atmospheric convection and cloud change are very complex phenomena. There are different mechanisms of cloud formation. This is just a simplification. Please refer to the meteorological principle.

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