Cloud Demonstration
Steve Carson

Condensation and the Water Cycle
Air Pressure
Cloud in a Bottle
Temperature Changes in the Bottle
How Nature Causes the Pressure Change
Rising and Sinking of Hot and Cold Water
Convection in Air

Introduction

This is a series of demonstrations intended to lead to an understanding of what clouds are made of, how they form and why they are found high in the sky. Clouds can be made of tiny droplets of water or tiny particles of ice. The focus of these demonstrations will be on clouds made of droplets of water. The basic steps that lead to cloud formation that will be demonstrated (in a different order) are as follows:

  1. Something causes air to rise. The cause could be warming of air at the surface, wind encountering a mountain or activity at a front or low pressure system.
  2. Since pressure decreases with height in the atmosphere, the rising air is exposed to lower pressure and expands.
  3. Expanding air (or any gas) gets cooler.
  4. As air gets cooler the amount of water vapor that can exist in the air decreases (relative humidity increases).
  5. Eventually the air is cool enough that it reaches its capacity to hold water vapor (becomes saturated) and further rising and consequent cooling leads to condensation into droplets. Condensation occurs on the surfaces of particles.

This can be summarized with the diagram below that shows the basic "steps to cloud formation". This diagram can be used with older students. The basic cloud-in-a-bottle demo involves all of the steps that are not italicized. The italicized steps are dealt with in additional demos. The numbers below each step will be used to relate the steps to the different demos.

            Cloud - 7
          Water Condenses on Particles - 6  
        Air Cools
- 5 		   
      Air Expands - 4      
    Lower Pressure - 3        
  Air Rises - 2          
Particles in Air - 1            

To lead into the cloud demonstration, some understanding of evaporation and condensation is important. The cloud demonstration is also dependent on the concept of pressure. Evaporation,

Condensation and the Water Cycle
Students should understand that evaporation means that water is changed from liquid to vapor and that condensation is the reverse. They should be reminded that water vapor is invisible and is in the air around us all the time, but the amount varies. It is also worth pointing out that water can also exist as a solid (ice).

The basics of the water cycle can be reviewed: Water evaporates from oceans, lakes, rivers, etc. to become water vapor. Under conditions that will be demonstrated here, water vapor can condense into droplets (or ice particles) to form clouds. Some clouds produce precipitation (rain, snow, etc.) that reaches the surface to complete the cycle.

Air Pressure
There are many demonstrations of air pressure, but a simple one that works for these demos is a juice box. When you drink a juice box to the bottom and keep sucking, the juice box will collapse due to air pressure. Sucking air out of the box reduces the air pressure inside. The pressure outside is greater than inside and the box collapses.

Another way to show this in front of a class is to use a plastic soda bottle with one of the caps used for the cloud-in-a-bottle demo to suck air out of the bottle causing the bottle to collapse due to the greater pressure outside than inside. Air can be blown or pumped back into the bottle to get it back to its original shape.

Cloud in a Bottle
Materials:

  • Plastic soda bottles with a little water inside and with tubing fitted into caps (construction of the caps is described in a separate handout)
  • Inflating pumps
  • Flashlights
  • Water
  • Matches

Procedures:
Have students work in groups of 3 or 4. Each group gets a bottle. Have students describe what is in the bottle: liquid water (how do we know it is water?), air, water vapor, dust (which they can see floating in air sometimes). Drop a lighted match into each bottle to add smoke (like dust) and cap tightly. Each group gets a pump and flashlight. With room lights dimmed have each group shine the flashlight on the bottle, pump the bottle with the hand pump through the tubing in the cap (about 10 strokes) and then remove the pump. They should observe the bottle getting harder as they pump and softer when they remove the pump, note the feel and sound of the air leaving the bottle when they remove the pump, and see the development of a "cloud" in the bottle when the pump is removed. They should see that the cloud is made of tiny particles which are drops of water. The cloud and its droplets are visible because the droplets reflect light. Remember that water vapor is not visible. Remind students that some clouds are made of particles of ice. Have them pump again and they should see the cloud disappear. Remove the pump again and the cloud forms again. This can be done a number of times with students taking turns in different roles.

The cloud formed by the condensation of water vapor to form the cloud droplets when the air pressure was released. Then the bottle was pumped again the cloud droplets disappeared by evaporation.

This demonstration helps show steps 1, 3, 4, 6 and 7 from the first page. Step 5 is also involved, but will be demonstrated in the demo using a thermometer.

A simpler approach to the cloud demonstration is to have students squeeze a sealed soda bottle with a little water and smoke in it for a few seconds and then release. Squeezing can be done either with hands alone or by pushing the bottle against the edge of a desk. Squeezing the bottle increases the air pressure inside and releasing reduces the pressure and a cloud then forms. This avoids the need for the adapted caps and the pumps, but I developed and use the approach using pumps for a number of reasons.
  1. For younger children it is easier to get a cloud with the pumps than by squeezing. For any age a more prominent and long-lasting cloud can be produced with the pump than by squeezing. This helps for observing the cloud characteristics.
  2. Again for younger students in particular I feel that it is easier to conceptualize the changes in pressure by using pumps, by feeling the pressurized bottle, and by hearing and feeling the sound of the air rushing out of the bottle than by simply squeezing and releasing the bottle.
  3. As a visitor to classrooms I have always been wary of introducing an activity that young students could easily do at home that involves matches. I felt that young students would be less likely to try something that uses matches on their own if it is more involved.
Temperature Changes in the Bottle
Materials:
  • Plastic soda bottle with tubing fitted into the cap
  • Liquid crystal thermometer strip taped inside neck of bottle These can be found in the aquarium section of a pet store or can be purchased in packs of 10 for $9.95 plus shipping and handling of $5 for any size order prepaid from Project Atmosphere-AMS 1200 New York Ave., NW, Suite 410 Washington, DC 20005.
Procedures:
Have students read the temperature on the thermometer strip. Pump the bottle as before and read the temperature again. The temperature should have risen and may have even risen beyond the scale of the thermometer. Release the pressure and observe what happens to the temperature. It should decrease. If there is still enough smoke in the bottle, the cloud may be seen again, though it is not necessary.

This demo links step 5 ("Alr Cools") with steps 3 and 4 from page 1. If a cloud forms it also links to steps 1, 6 and 7. When air or any gas expands due to a decrease in pressure the temperature in the gas will decrease. When a gas is compressed due to an increase in pressure, the gas will get warmer.

This is important to cloud formation due to the importance of temperature to evaporation and condensation. It is common experience that warmer temperatures cause water to evaporate more quickly than cooler temperatures. That is why heat helps clothes or hair dry more quickly. Cold temperatures can be seen to make water vapor condense. When you breathe out on a cold day and see a fog from your breath, that results from water vapor in your breath condensing when it mixes with and is cooled by the outside air. You will not see a fog from your breath on a warm day even though there is still water vapor in your breath. As another example, on a humid summer day you can see a cold glass "sweat" not because it is leaking, but because water vapor from the air condenses on the cold surface.

What is happening in the bottle experiments? The presence of liquid water saturates the air in the bottle with water vapor. When pumping the bottle, the air inside gets warmed due to the compression and is kept saturated with water vapor by the evaporation of liquid water in the bottle. When the pump is removed the air in the bottle cools as it expands out of the bottle and since the air was already saturated with water vapor some of that vapor condenses due to the cooling forming tiny "cloud" droplets that can be seen floating in the air in the bottle. When the bottle is pumped again the air is warmed slightly and the cloud drops evaporate.

Some more detail:
It is often said that warm air can hold more water vapor than cold air or that warm air has a greater capacity hold water vapor than cold air. Although some meteorologists object to these statements, they are conceptually useful if correctly interpreted. At a given temperature there is a maximum amount of water vapor that can exist in contact with a flat surface of liquid water. When that amount is in the air, the air is said to be saturated. There is often less water vapor in the air than the saturation amount.

Relative humidity is a way of expressing the amount of water vapor in the air as a percentage of the saturation level. 100% relative humidity means that the air is saturated. 50% relative humidity means that 1/2 of the saturation level of water vapor is in the air. If air at 80% relative humidity, for example, is cooled without changing the actual amount of water vapor in that air, the relative humidity will increase and can reach 100% since the cooler air can "hold" less water vapor. Once air is saturated, further cooling can result in condensation if there are available surfaces (next section)

Do not think that air has a certain amount of space for water vapor that increases with temperature. Air or any gas at pressures we experience have plenty of space between molecules.

Air is also not necessary for water vapor to exist. Water vapor would exist in a container that had liquid water and some space from which all air had been removed. Evaporation of water would "fill" that space with water vapor. What determines the amount of water vapor in that space or in normal air are the relative rates of evaporation and condensation Although we think of those two processes as either/or, both happen simultaneously. Often, however, one is faster than the other leading to net evaporation or condensation.

In the bottles before any pumping, the rates of evaporation and condensation were equal resulting in saturation or the air "holding" as much water vapor as it could at that temperature in contact with a flat surface of water. With the increase in pressure, and hence, temperature, more evaporation could occur leading to more water vapor. Higher evaporation results because higher temperatures mean more energetic molecules that can escape the liquid water surface. With more water vapor, condensation on the water surface also increased as more water molecules in the air hit the surface. The new balance between evaporation and condensation led to a higher saturation level at the new higher temperature. The air could "hold" more water vapor.

When the air cooled due to the pressure decrease and expansion, the rate of evaporation decreased because cooler temperatures mean fewer energetic molecules that can escape the liquid water surface. Even though some water vapor escaped with the air that left the bottle, there was still enough water vapor left for the condensation rate to be greater than the evaporation rate at the cooler temperature, so some of the water vapor condensed into cloud droplets. This reduced the amount of water vapor left in the air in the bottle which reduced the condensation rate until it equaled the evaporation rate at the lower temperature. When the bottle was pumped again the temperature increased and so did the evaporation rate, so the cloud droplets disappeared by evaporation.

The Effect of Particles:
The importance of the particles can be demonstrated by comparing the results in a bottle without added smoke to one with smoke added. A thicker cloud forms in the one with the added smoke. In the workshop and in classes I demonstrate this with the large glass bottle, but it can also be done with the bottles used in the first demo.

A satellite photo of "ship tracks" (clouds enhanced by particles from ship smokestacks) can be used to show the effect of particles in the atmosphere. This can be used with older students.

What difference do particles make? In the simplest terms water condenses more easily on surfaces. Particles in air provide such surfaces. Every cloud droplet in the bottle condensed onto a particle in the air. The more particles in the air the more sites for condensation and the more cloud droplets form. With more cloud droplets the cloud looks thicker since there are more droplets in the same space blocking and reflecting light.

This relates steps 1, 6 and 7 from page 1.

Some more detail:
The tiny particles in the atmosphere have a number of sources. The most important both in quantity and for cloud formation are soil dust, sea salt (left from evaporation of drops of water that get into the air from waves and bursting bubbles) and smoke particles (e.g., from forest fires and combustion of fuels by humans). Other sources of particles include plants, volcanoes, other human activities, and chemical reactions in the atmosphere of gases also released by some of the above sources. The scientific term for tiny liquid and solid particles in the air is aerosols. Some types of particles attract water more readily than others and are better sites for formation of cloud droplets. Such particles are called cloud condensation nuclei (CCN).

Why are particles so important? This relates to the rates of evaporation and condensation. Evaporation occurs very rapidly from the very tiny invisible droplets that would need to form first in order to grow into cloud droplets in the absence of particles. In such tiny droplets there are fewer water molecules to attract each other so they can evaporate more easily. One role of particles is to provide a place that will attract water and can start out the drops at a larger size from which the water will evaporate more slowly. Water also evaporates more slowly when something is dissolved in it since there are proportionately fewer water molecules near the surface since some of the molecules near the surface are now the dissolved material. Some particles or portions of particles can dissolve in the condensing water. The effect of larger drops and dissolved material reduce the evaporation rate of drops and hence allow net condensation to occur with a lower amount of water vapor in the air at a certain temperature (or a higher temperature with a certain amount of water vapor in the air) than would happen without the particles.

How Nature Causes the Pressure Change
Materials:
  • Plastic soda bottle with small holes near bottom and in middle and filled with water
  • Sink, bucket, or aquarium to catch water
Procedures:
Are there bottles in the sky? No, but the same basic principles illustrated so far apply to cloud formation. In the atmosphere the changes in pressure are caused by air moving up or down. To demonstrate changes in pressure with altitude use the bottle with two holes filled with water to show that water shoots farther out of the bottom hole than the upper hole because there is more pressure at the bottom. Similarly in the atmosphere pressure is higher near the ground than at greater altitude in the atmosphere. To make a cloud, air must rise from the high pressure near the ground (where the air also acquires water vapor and dust) to the lower pressure above (like releasing the pump from the bottle). If air sinks it moves from low pressure above to higher pressure below (like pumping up the bottle) so clouds tend not to form.

This finally relates step 2 ("Air Rises') on page 1 to the rest of the steps.

How does the air rise in the atmosphere? There are three basic ways that can be easily described:
  1. Heating of the air causes the air to become less dense and to rise. The example of how a hot air balloon works can help students understand this. When the sun heats the ground the air above gets warmed in certain spots. This can make the individual puffy cumulus clouds that can dot the sky. (Two demos below show how heating can cause water or air to rise.) Note that as the air rises it cools due to expansion, but it remains warmer than surrounding air which also cools with altitude.
  2. Another way that nature can make air rise and sink is at mountains. Wind hitting a mountain range is forced to rise producing clouds and often precipitation. The air then sinks on the other side of the mountain which inhibits cloud formation and little rain occurs (rain shadow). Some deserts in the western US are found in the rain shadow of mountains.
  3. Fronts are narrow regions where warm and cold air meet. The warm air rises above the cold air at the front. Fronts are typically associated with cloudy skies. I do not discuss this with younger grades.
Remember that the names of fronts describe which air mass is pushing the other out of the way. A cold front is one in which cold air pushes the warm air so the front itself moves in the direction of the warm air side. A warm front is one in which warm air pushes the cold air so the front itself moves in the direction of the cold air side. A stationary front is one that is not moving. An occluded front is one in which a cold front has caught up to a warm front, forcing all the warm air aloft away from the ground.

Low pressure systems also have rising air for reasons explained in the Coriolis handout. In U.S. Iatitudes low pressure systems are often associated with fronts. In tropical latitudes there are no fronts.

Rising air can also result from turbulence or mixing in the atmosphere, from waves in the atmosphere, or from cooling of portions of the atmosphere above the ground causing sinking in some spots and forcing rising in others.

Rising and Sinking of Hot and Cold Water
Materials:
  • Aquarium 2/3 filled with water close to room temperature (mix hot and cold water) and with a white background
  • Baby food jar of hot water colored red loosely capped
  • Baby food jar of cold water colored blue loosely capped
Procedures:
For younger students the rising of warm air can be illustrated by an analogy using water in an aquarium and jars of hot water dyed red and cold water dyed blue. Point out that the colors are added just to allow us to see and distinguish the water from the two jars. Allow the students to feel the hot and cold jars of water so that they know which is which. Place the cold jar in the aquarium and gently remove the cap. A little blue water will come out due to the disturbance, but most will stay in. Place the hot jar in the aquarium and gently remove the cap. The red water will quickly rise to the surface. Turn over the cold jar and let the blue water pour out over the bottom. Tell the students that in the atmosphere warm air rises and cold air sinks just like in the water. Rising warm air can produce clouds.

Warm air is less dense than cold air so warmer air rises or floats upward in cooler air. The same is generally true in water. The exception is in pure water between 0°C and 4°C in which range the cooler water is less dense than the warmer water (remember that ice also floats on water), but that is not important to the demo above.

Convection in Air
Materials:
  • Two aquariums with aluminum pans for a lids and with black backgrounds and one of them with black paper or cardboard on the bottom at one end
  • Two "candles" of wax paper rolled tightly and mounted to stand upright (e.g., with a stand fashioned out of aluminum foil) placed on a stand (e.g., inverted bottle) to situate the top of the "candle" just below the lid inside the aquarium.
  • Bright light
  • Ice pack
  • Matches

Procedures:
For older students the convection demo can be used: Shine the light into the end of the aquarium with the black paper on the bottom to heat the paper. (This should have been started somewhat earlier). When the paper is heated, place the other aquarium next to the lighted one on the opposite end from the light. Light one of the wax paper "candles" and place it in the center of the unheated aquarium. Blow out the flame and quickly cover. Have students observe. They should see that the smoke stays near the top and moves somewhat randomly with some areas sinking. Light the second "candle", place it in the center of the heated aquarium, blow out the flame, cover the aquarium and place the ice pack on the lid on the end opposite to the light. Have students observe. They should see that the smoke now moves in the circular pattern of convection with it rising above the heated black paper and sinking beneath the cooled part of the lid. The air above the paper becomes heated and less dense so it rises. The air beneath the ice pack becomes cooler and more dense so it sinks. The smoke is just a marker to make the air motions visible.

The convection demo can be related to other processes in the atmosphere. Wind is often driven by differences in temperature in different locations which result in rising air in one location and sinking air in the other. Along the ground air moves from the region of sinking to the region of rising creating wind. This is the origin of local land and sea breezes as well as monsoons.

A similar effect occurs at a much larger scale to drive global winds. At the equator the sunlight is most direct on average, so the air at the equator is relatively hot and rises like the lighted side of the aquarium. In the upper atmosphere (actually the upper troposphere) the air that has risen spreads away from the equator toward the poles. Air sinks near 30°N and S latitudes like the side of the aquarium with the ice pack. Although the air does cool as heat is lost to space as it moves away from the equator, the reason for the sinking near 30°N and S is much more complicated and involves the rotation of the earth which is presented in the Coriolis handout.

In the rising air near the equator many clouds form and hence precipitation is high and tropical rain forests can be supported in many regions of the equator. (They are not found everywhere along the equator due to the complicating effects of the distribution of oceans, continents and mountains on the circulation of the atmosphere.) In the sinking air near 30° N and S clouds tend not to form and hence precipitation is low and deserts are common. (Again the complications noted above apply.) This can be seen in the distribution of clouds in a satellite picture. (The cloudy area in the mid--latitudes basically results from the meeting of cold air from high latitudes with warm air from low latitudes causing the warm air to rise above the cold air. The rising air forms clouds and precipitation. Of course, what really happens is more complicated.) A map of precipitation and a satellite photo of biomes also show the effects of rising air at the equator and sinking air near 30° latitude as described above.