Learning Lesson: Smoking Clouds

Overview

In a clean-air chamber (one where all solid particles are filtered out) a cloud will not form even with a relative humidity greater than 200%. While clouds are made up of droplets that are essentially distilled water, each droplet is not 100% pure. At the cloud droplet's core is a tiny water attracting particle upon which moisture collects and grows. This demonstration will show that just a small amount of nuclei (smoke) is all that is needed in the production of a cloud.

TOTAL TIME	3 minutes
SUPPLIES	1 glass one-gallon milk jug; Rubber stopper; A long-stemmed tire valve; Bicycle pump; Box of matches; Rubber Cement
PRINTED/AV MATERIAL	None
TEACHER PREPARATION	Obtain a one-gallon glass milk jug (can be found on the Internet or antiques store). From a local hardware store obtain a rubber stopper that fits snuggly in the neck milk jug. A tire valve stem can be obtained at a local auto parts store. Drill a hole in the center of the stopper just large enough for the tire valve stem; should be a tight fit. Insert tire valve stem through opening in stopper in a way where the valve will point up when placed on the milk jug. Use rubber cement to secure and seal the valve stem to the stopper.
SAFETY FOCUS	Flash Flood Safety

Procedure

1. Add about an inch of water to the jug. (Warm water works better.)
2. Placing you hand over the opening, shake the jug for a few second to allow the water to add moisture to the air inside the jug.
3. Insert the stopper/valve stem onto the jug's opening.
4. Ask the students what they think is inside the jar. (Just air and water.)
5. Attach a bicycle pump to the valve stem and have a student pump about 5-10 times to pressurize the jug. (Hold the stopper on the jar as air may begin to escape.)
6. Again, ask the students what they think is inside the jar. (Just more air and water.)
7. Quickly remove the stopper and ask the students for their observation of the result. (There may a small amount of haze form but for the most part nothing will happen.)
8. Repeating the demonstration first strike a match and hold it inside the jug for 2-3 seconds then drop the match into the jug.
9. Ask the students what they think is inside the jar. (Air, water, smoke and a burnt match. The smoke will not be visible.)
10. Pressurize the jug again then quickly remove the stopper to release the pressure.
11. Ask the student what they observe. (It will be obvious a cloud has formed inside the jug.)

Discussion

When air is compressed its temperature increases. As air expands its temperature decreases. This why air cools as it rises into the atmosphere due to decreasing pressure. Pressurizing the jar raises the air temperature inside the jug.

Quickly removing the stopper will produces a rapid decrease in pressure cooling of the air. This is the temperature component needed for cloud formation.

In the first demonstration no major discernible change in the opacity of the air inside of the jug will be observed. (There may be a little fogging inside the jug due to pollutants in the jug's 'atmosphere'.)

However, in the second demonstration, with the addition of a little smoke, the air's opacity will noticeably change as a cloud forms. The addition of a small amount of smoke particles act as cloud condensation nuclei.

Some students will think that smoke has formed and not a cloud. By looking at the jug's opening where the students will see the cloud dissipate as it moves into the drier surrounding atmosphere.

The demonstrated can be repeated but to purge the air inside the jug fill it with water to overflowing. Then empty the jug of all water and repeat the procedure.