Visually demonstrate how pollutants can be trapped near the ground because of atmospheric conditions.
Air temperature can play an important role in the buildup or dispersion of surface air pollution. In general air temperature decreases as you move upward in the atmosphere. Under most circumstances the air close to the earth warms as it absorbs surface heat, and begins to rise. Winds occur when cool air rushes in to take the place of the rising warm air. The wind causes "mixing" in the atmosphere and can carry away or dilute pollution.
Air temperature can play a different role if air movement is influenced by topography or by air pressure (density). Cold air is generally "heavy" because low temperature causes slower molecular motion, contraction, and increased density. Gravity causes denser, heavier air to sink below lighter, less dense air. Warm air is "light"; higher temperature leads to faster molecular motion, expansion, and a decrease in density.
The cold dense air close to the ground does not readily circulate and mix. Pollutants such as carbon monoxide and particulate matter are "trapped" below the "lid" of warm air. The quantity of pollution tends to increase until the lid is lifted or a wind occurs. Using hot and cold water, you can simulate normal atmospheric conditions and a cold weather temperature inversion.