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The air above us is constantly moving. Convection currents, the rising and falling of currents in the air, and wind keep air circulating around the Earth. Wind is caused by atmospheric pressure differences resulting from differential heating of the Earth's surface by the sun. Convection is the rise and fall of air depending on temperature. Cool air sinks because it is more dense, warm air rises because it is less dense. Air movement is an important factor in keeping the air free of pollutants. A temperature inversion occurs when a warm air mass overlies cold air and inhibits convection, there by trapping any pollutants in place. An inversion prevents the pollutants form rising and traps them at lower levels.
Topography can affect the amount of pollutants concentrated in an area. In valleys or basins surrounded by mountains and hills cold air and pollutants tends to settle. Cold air can strengthen an existing surface inversion or carry pollutants downhill from the surrounding hillsides. The surrounding mountains tend to block the prevailing wind. The valley becomes poorly ventilated, little mixing occurs, and pollutants are trapped.
Purpose: This demonstration will show how a temperature inversion is formed and how pollutants get trapped close to the surface.
Materials: Large glass bowl or aquarium, large glass measuring cup or pouring container, funnel, plastic or rubber tubing that fits the funnel, food coloring: blue, red, and green, salt water, large spoon.
Procedures
A. Fill the large glass bowl or aquarium a little less than half full with water. Put a few drops of red food coloring. Explain that the food coloring represents warm air and warm air is lighter than cold air.
B. Fill the measuring cup with water and saturate it with salt (keep adding salt until no more dissolves). Put several drops of blue food coloring in the salt water. Explain that this represents cold air and cold air is more dense than warm air.
C. Pour the blue saturated water slowly and carefully through the funnel making sure that the other end of the tubing is on the bottom of the bowl. You should have two layers, blue on the bottom and red on the top. Ask the students why this happened. Explain that this is what happens in an inversion where you have cold heavy air under warmer, lighter air. This is a stable condition that inhibits vertical mixing and thus dilution of pollution.
D. Demonstrate what can happen to pollutants that are released under these conditions. Take green food coloring and put it into the bowl drop by drop. It will layer out between the blue and the red. Ask the students to describe what they see. Explain that the pollutant does not mix but becomes confined in the area it is emitted into.
E. Show what happens when the wind begins to blow or a storm comes in and vertical mixing occurs. Mix up the contents of the bowl and the pollution will be diluted.
Student Experiment
Materials: Same as above.
Before handing out fresh and salt water, put food coloring in each.
A. Hand out the materials to the students.
B. Explain to them that the blue liquid represents cold air, the red liquid represents warm to hot air, and the food coloring represents the pollutants.
C. Let them explore with the liquids and try to figure out how the pollutant becomes trapped between the two layers.
D. Have them show how the pollutant becomes dispersed in the atmosphere. Have them explain what kind of weather conditions would keep pollutants from becoming trapped.
Assessment