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
- Add about an inch of water to the jug. (Warm water works better.)
- 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.
- Insert the stopper/valve stem onto the jug's opening.
- Ask the students what they think is inside the jar. (Just air and water.)
- 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.)
- Again, ask the students what they think is inside the jar. (Just more air and water.)
- 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.)
- Repeat the demonstration, but first strike a match and hold it inside the jug for 2-3 seconds, then drop the match into the jug.
- Ask the students what they think is inside the jar (Air, water, smoke and a burnt match. The smoke will not be visible.)
- Pressurize the jug again then quickly remove the stopper to release the pressure.
- 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 is why air cools as it rises into the atmosphere and decreases in pressure. Pressurizing the jar raises the air temperature inside the jug.
Quickly removing the stopper results in 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, 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.
Building a Weather-Ready Nation
Rain comes from clouds, and when we get too much rain, we can have flooding. Most flood-related deaths and injuries could be avoided if people who come upon areas covered with water followed this simple advice: Turn Around Don't Drown.
The reason that so many people drown during flooding is because few of them realize the incredible power of water. A mere six inches of fast-moving flood water can knock over an adult. It takes only two feet of rushing water to carry away most vehicles. This includes pickups and SUVs.
If you come to an area that is covered with water, you will not know the depth of the water or the condition of the ground under the water. This is especially true at night, when your vision is more limited. Play it smart, play it safe. Whether driving or walking, any time you come to a flooded road, TURN AROUND DON'T DROWN!
