Cloud Drops, Rain Drops and Mushroom Caps

If Clouds are NOT Lighter than Air,
How Do They Stay Up?

by Steve Horstmeyer, Meteorologist, Cincinnati, OH

A cloud is a mass of air, liquid drops of many sizes, solids like dust, minute sea-salt particles, pollutants and gasseous water, aka water vapor. Some clouds have a mixture of ice and liquid water, some are all water, some are all ice, but most clouds in the middle latitudes (that's around here) are a mixture of ice and liquid.

Even a small cloud, that looks like a floating cotton ball with a flat bottom ( a cumulus cloud) weighs many, many tons.
So what keeps a cloud up in the sky?

When you wonder if clouds are lighter than air, you are asking if ice, liquid and gasseous water can be lighter than air. There is no magic in science, so in no way the things you find in a cloud can be made lighter than air. So clouds are HEAVIER THAN AIR.

There is a logical explanation for everything ... so let's try a thought experiment.

I. Thought Experiment

Have you ever observed (observing is different from just looking at something) a camp fire? As a log burns some of the carbon in a log is oxidized (that is what burning is) and the combination of oxygen and carbon becomes carbon dioxide. Ash is what is left, it is stuff that was not oxidized.

As the fire burns, the air around it is heated and because the warm air is less dense than the cool air surrounding it, the warm air is forced to rise. The upward moving air (in meteorology we call it an updraft) carries some of the ash with it. Now think about it, ash from burning logs is not lighter than air, but it floats upward, pushed by the updraft.

Apply that same reasoning to a cloud and you have the answer. The liquid water drops and ice crystals are held aloft by rising air.

Every cloud, has an updraft, in stratus clouds, the updraft is gdntle but in violent thunderstorms the updraft can be over 100 miles per hour, and the faster the updraft, the larger the drops, ice crystals or hail stones that can stay aloft.

II. Drop Size, Air Resistance and A Balancing Act

A typical cloud drop is very small, only 20 micrometers (.002 millimeters, .00008 in. or 8 one hundred thousandths of an inch). Gravity pulls every cloud drop, no matter how small towards the ground, but as a drop falls through the air the air resists the fall of the drop. SOOO...let's call it air resistance.

Try This Experiment (first get permission from the driver)

You can experience air resistance. Next time you are riding in a car with the window rolled down, stick your arm out the window with the palm of your hand facing down, then turn your had so the palm faces forward.

What happens?

What happens if you repeat the experiment going faster?


Because the palm of your hand is very much wider than your hand's thickness there is more air resistance when your palm is facing forward. As you go faster, the force of the air on your hand increases and so does wind resistance.

Apply the Results of Your Experiment to a Cloud

  1. Gravity pulls a drop down, and a bigger drop is pulled more.

  2. Air resists gravity, a bigger drop experiences more air resistance and rising air creates more air resistance.
In the battle between gravity and air resistance if gravity wins the drop falls, if air resistance is the clear winner the drop rises.

What if a typical cloud drop is neither rising or falling, just floating in place? In this situation there is a balance between the force applied to the drop by the updraft and the force applied by gravity.

A typical cloud drop, remember only 2 one-thousandths of a millimeter, requires an updraft of 0.02 miles per hour, or 1.8 feet per minute to stay aloft. On the other hand if there is no updraft the typical cloud drop will fall at .02 miles per hour, this is called the terminal velocity (sometimes referred to as the "fall speed").

If the drop is one mile up how long will it take to hit the ground?

Answer: 50 hours, a long, long time.

Click Here for Cloud Drop Table CLICK FOR MORE ON CLOUD DROP SIZES

III. Bigger Drops Fall Faster and Get Smushed Too!

In the most simple of cases, cloud drops collide and coalesce (merge) to form a larger drop. The physics of raindrop formation is much more complicated than this but to illustrate what happens to a falling drop we will go no further into raindrop formation than this.

As drops get bigger they fall faster, but something else happens too. They begin to deform, that is change shape because forces inside the drop are becoming overwhelmed by gravity, the pressure of air pushing on the drop as it falls and turbulence in the air surrounding the drop.

Forces in the drop? To what is he referring? Is this some sort of Mickey Mouse Just Ask Steve?

Funny Mickey Mouse should come up, to see what he has to do with all this click icon below.

Click here for more on forces in water molecules. CLICK FOR MORE ON FORCES IN WATER MOLECULES

Those electrical forces are very strong when the drop is a very small size but the large the drop becomes the more important external forces become. This is very common in science, forces are frequently scale specific. For example at astronomical distances gravity is the most powerful force, but inside the nucleus of an atom gravity is unimportant.

Like cloud drops, rain drops can be larger if there is a greater updraft, so in a severe thunderstorm with updrafts at times apporaching 100 mph large drops can be kept aloft. Rain drop size also varies with the amount of moisture in the cloud and the number of pollutants (dust and gasses can change the drop size distribution).

A stratus cloud (stratus means layer, so these clouds are layered clouds where the horizontal dimension is much greater than the vertical) typically forms with gently rising air and will have smaller drops than a thunderstorm.

As we observe larger and larger drops we find that the larger a drop is the more it becomes deformed, but not into the typical "teardrop" shape shown in the background. The teardrop shape is a common public mis-conception. More on rain drop deformation in the "Complications" section below.

Click Here for a summary of rain drop sizes CLICK FOR MORE ON RAIN DROP SIZES

IV. Complications...and there are many!

The figures in this explanation are representative and future research may indicate changes need to be made. A few factors that may complicate the picture:

  1. The atmosphere is less dense aloft and the terminal velocity will decrease closer to sea level due to increasing air density.

  2. Rain drops deform (change shape) when falling <<<<(CLICK HERE FOR MORE ON DROP DEFORMATION>>>>, larger drops deform more and look like the caps of mushrooms. This increases the wind resistance.

  3. Larger drops may be broken into smaller drops as they fall due to turbulence. The largest drop that reaches the ground is thought to be about 5mm (.20'')<<<<(CLICK HERE FOR MORE ON DROP DEFORMATION AND DROP BREAKUP>>>>

  4. Rain drops often evaporate as they fall, the drop gets smaller slowing the fall.

  5. Hail stones may be smooth spheres which fall easily, but sometimes hail stones freeze together forming an ice mass with an irregular shape and a rough surface that often tumlbe which slows the rate of descent.

2008 Steven L. Horstmeyer, all rights reserved

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