Temperature Stratification and Pollution of the Air
Most students understand stability and instability when it is referenced to their everyday activities. Atmospheric stability and instability are caused by different temperature changes with height. For the most part, the atmosphere is stable. Roughly, if the temperature decreases by more then 1/2° F per 100 ft. height increase, the air layer is unstable; if the temperature decreases by less than this amount, the air layer is stable as long as no clouds form. The most stable case exists, when the air temperature increases with increase in height, a condition called inversion.
The various conditions of temperature stratification can come about in various ways. For example, if the air is cooled near the ground by contact with a cold earth's surface, the air aloft will not be affected and an isothermal condition or even an inversion will be established. By contrast, when the earth's surface is warmer than the air, the air in contact with it will be warmed, while the air aloft will remain cool, so that the temperature will drop rapidly with increase in height.
Any transparent tall cylinder can be used for the experiment, but a transparent acrylic (Plexiglas TM or Lucite TM) cylinder 24" long by 3" diameter is best for the purpose. You can make one out of transparent plastic sheets with some ingenuity. Drill or punch a hole for some rubber or plastic tubing, about 9 to 10" above the bottom, push the tubing through the hole and tape the end of the tubing about 1" above the bottom of the cylinder. Seal the hole where the tubing goes into the cylinder. Tape some aluminum foil over the bottom of the cylinder and then place the bottom of the cylinder in a pan which will contain some hot or cold water. You will probably need some supports to hold the cylinder, ring stands or clamps, as to work best, the bottom of the cylinder should be wet. You may need to use a small wooden block will keep the cylinder off of the bottom of the pan.
Fasten two thermometers to a thin stick of wood or 3/8" dowel so when the rod rests on the bottom of the can, the lower thermometer bulb is about 2" from the bottom and the upper thermometer bulb is about 2" below the top rim of the cylinder. The stick should be taped so the thermometer bulbs are in the center (axis) of the cylinder. Measure the distance between the two thermometer bulbs.
Read the thermometers, which should read practically the same; this is the room temperature stratification. Pour ice water with small pieces of crushed ice into the pan at the bottom. Read the two thermometers again after 5 minutes. They should differ dramatically. This is the inversion with cool air underlying warm air. The temperature change can be calculated over the distance of the thermometers to give the lapse rate or change of temperature with height. It should be a stable inversion.
Empty the water and pour very hot water into the pan. Observe the temperatures after a few minutes. Again you can calculate the lapse rate. It should be a strong decrease of temperature with height.
The Effects of Atmospheric Stability on Smoke and Pollution
Repeat the ice water and the hot water while smoke is introduced. During an inversion (stable), the smoke does not move up the cylinder upward very far. The hot water will produce unstable atmospheric conditions within the cylinder and the smoke should move upward to the top.
Material: Transparent acrylic cylinder 24" long by 3" diameter or a sheet of transparent plastic 13 x 24"
18" of thin rubber or plastic hose
3/8" dowel rod, 30" long