Level: Middle School to H.S.
Time involvement: 15-25 minutes.
Materials: Plastic juice or soda bottle, water, smoke (matches). This lesson creates a visible cloud in a plastic jar. The cloud can be made to appear and disappear by squeezing and relaxing your grip. Rain simulation is by gently and slowly adding a fine mist to a vertical Teflon cookie sheet. Gradually small drops join other small drops. With enough pumps of the sprayer the droplets grow and begin to run down the Teflon sheet. The simulation shows how clouds that are fed by enough moisture cannot hold the microscopic droplets in suspension and larger and larger droplets form until they begin to fall out of the cloud as rain.
Discussion: How nature makes clouds. Water vapor from evaporation of the ocean, lakes, plants, etc. adds moisture to the air. In effect water vapor dissolves into air which is actually a mixture of approximately 79% Nitrogen and 21% Oxygen. Because the water molecules are very small they are invisible. Depending on several factors the air can become saturated to the point where the water molecules begin to clump together and become visible. The exact point depends on how much water vapor is present and the air temperature. The warmer the air the more moisture it can hold before the vapor becomes visible. Conversely if the air is very saturated and no more water vapor is added, merely cooling the air forces the molecules to clump together. These large “clumps” interfere with the passage of light waves. This scattering of the light is similar to the problem of seeing underwater if the water is contaminated with dirt. Sort of like underwater fog.
A well known principle to scientists is called adiabatic cooling or heating. Because the principle involves gasses, which are generally invisible, a temperature change in the gas is not visible to the eye. Amazingly, an interesting example of this principle is no further away than the closest diesel truck or car. Diesel engines require no spark plugs. The reason is that the fuel/air mixture in the cylinder is ignited by the mere action of the upward moving piston compressing the mixture many times. Typical compression ratios are 15 or 20 to 1! That would be like squeezing a cup of air into a Tablespoon. The important consequence is that the temperature is immediately raised to about 400°F. causing the fuel to burn and release energy for the engine.
Atmospheric adiabatic cooling is the reverse. As the pressure is reduced (caused by lifting to a higher altitude) so is the temperature. In the atmosphere this process is occurring frequently wherever, high and low pressure areas are found and air is either being lifted to higher altitudes or sinking from higher altitudes. In the diesel engine, rapid and extreme pressure changes bring extreme temperature changes. In the atmosphere a change in altitude of a thousand feet brings about 3.5°F temperature change. However, because dew point is a very specific temperature, merely making a small pressure change in the juice bottle will trigger enough of a temperature change to cause saturated air to move across the dew point. This short trip across the dew point is the secret behind making a cloud appear or disappear!
For our cloud in a bottle we have introduced a small amount of water. A half cup or so is plenty. In reality, one additional factor must be present. This factor is virtually always present in the air so we may take it for granted: dirt. Tiny, mostly invisible particles of actual dirt, or pollen, pollution, or smoke, are called condensation nuclei. They serve as a “seed” to begin the condensation. If you wish to frost a cake you need a cake. If you wish to create a visible cloud particle you have to “build” it on something. For this demonstration to work well we introduce a bit of smoke by lighting a match, letting it burn for a few moments, then extinguish it and suck a small amount of the smoke into the jar. Please note: the cloud in the jar is not the smoke. The amount introduced is barely visible.
The last step of cooling the air is easy. Start by squeezing the bottle to slightly raise the temperature. Give the bottle a few good shakes to insure that the air is fully saturated (100% humidity or at the dew point). Get ready for the magic! Release your grip and the cloud instantly appears. Do not expect a thick milky cloud. Holding the bottle near a light will enhance the viewing. Holding the bottle in front of you while wearing a white shirt will make it difficult to see. (Wear a dark shirt.) You may make the cloud appear or disappear as many times as you wish by merely squeezing or relaxing your grip. Take a bow.
Eventually the smoke particles will dissolve into the liquid water and the effect will diminish. Introducing more smoke will reactivate the demonstration. On a foggy day it may be difficult to see across the street. It does not take “pea soup” thick fog to create clouds. The cloud in the bottle is probably at least as “thick” as a cloud in the sky.
The reason this demonstration is scientifically accurate in that in the atmosphere clouds are constantly forming and dissipating. Low pressure areas cause lifting, cloud formation, and often rain. High pressure areas bring air down from higher altitudes warming the air and dissipating clouds, bringing clear skies and fair weather.
Learn more: Since clouds can be formed in different ways: by solar heating, by warm fronts and cold fronts, high altitude ice crystals etc. they will appear different. Meteorologists have dozens of cloud names. Three of the simplest are: Stratus, Cumulus, and Cirrus. Adiabatic cooling, (discussed above) can occur by four means: winds forcing air up a slope, solar heating (thermal lifting), warm or cold fronts riding up and over or pushing under and lifting another air mass above, and low pressure areas where air is being lifted to higher altitudes by being squeezed by neighboring high pressure areas.
This simple demonstration contains two important concepts: how water vapor is almost always present in the air, and the principle that changes in pressure always bring change in temperature. Many YouTube “cloud in a jar” demonstrations use pumps to create extreme and rapid pressure changes with dramatic effects. More dramatic yet is pouring hot water into a bowl of liquid Nitrogen. The eye-popping explosion ranks with “Action” movies (car chases, explosions, gun play, etc) unfortunately the demonstrator seldom delves into the important physics behind the excitement. This demonstration is less awesome but is faithful to the actual process in the atmosphere!