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Teacher General Air Quality Resources: In a Polluted Cloud - Temperature Inversion Activity
"In a Polluted Cloud" would be a good activity/demonstration to use if students wonder what impact cold weather might have. Winter weather often brings about temperature inversions, where wood smoke and other pollutants are trapped by the cold air near the ground. As the temperatures warm during the day, the air warms up, allowing the pollutants to rise and disperse.


  1. One large, clear plastic bowl OR six smaller, clear plastic bowls
  2. Heavy cloth gardening gloves
  3. Water
  4. Solid carbon dioxide (dry ice)

Dry ice exists at approximately minus 110 degrees Fahrenheit. Frostbite can quickly occur if it is placed in contact with bare skin. Handle only with gloves and watch students carefully to insure their safety as well.

Pre-demonstration Discussion:
Many cities, especially in the West, are situated in a valley or basin surrounded by mountains. Any pollution generated in that bowl tends to linger within the valley for a period of time.

Hot air rises and cold air sinks. During the night, the temperature of the ground and the air just above it drops or cools. The air above this ground layer is generally warmer. Therefore, there is a body of warmer air lying above a layer of colder air. This is a temperature inversion layer.

This occurs frequently during the winter months. In desert environments, this is common throughout the year.

Any pollution that is generated during the night and in the morning hours will be trapped in this colder ground layer of air. This lingers in the valley or basin, resulting in the characteristic brown cloud or urban haze. Slight breezes will have little or no effect in moving this layer past the mountains. Only when the heat of the sunlight warms this cooler ground layer, causing it to rise, will the skies be cleared of the haze.

Describe the bowl (or bowls - 1 per lab group) as a simulation of the valley or basin, with the city in the bottom.

Add enough water to the bowl(s) to make a layer at least one inch deep.

To simulate the pollution generated by vehicles, fireplaces, stoves, etc. add a small piece (a cube about 2 inches on a side) of dry ice to the water. NOTE: This is not steam (which is invisible); it is a cloud of condensed water vapor, like a cloud or fog.

Gently fan with your hands or blow across the top of the bowl to simulate morning winds.

Demonstrate warming effects of the sun by blowing down into center of bowl, forcing fog up and over the sides of the bowl.

Click here for a printable version of "In a Polluted Cloud."




Northern Arizona University Institute for Tribal Environmental Professionals

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Environmental Education Outreach Program (EEOP)
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E-mail: eeop@nau.edu

Last updated: May 26, 2005