Air Pressure and Density

Air Pressure and Density

Present-day barometers measure the atmospheric air pressure in a number of ways. These barometers are used by meteorologists to predict the weather.

Many times, mercury was used in barometers where changes in air pressure would change its level in a vacuum-sealed tube. Mercury is highly toxic however, and other liquids might better serve as a substitute. The most plentiful and harmless liquid would be pure water and using it in a vacuum tube would seem to be a logical choice.


Mercury is a very dense liquid and the average air pressure at sea level is 29.9 inches. This means our air can apply a force strong enough to lift a column of mercury just under 30 inches high in a sealed tube on average. (Note: the height is measured in the English system.)

An early model barometer using mercury in the sealed tube

Water has a density of 1.0 grams per cubic centimeter. (Note: density is usually measured in the metric system.)

Your challenge: 1. Look up the density of mercury in the metric system. Using its value, compare it to the density of water. 2. How high, on average, would our air pressure force up a column of water (in the English system)? 3. Would water be a logical choice in the make-up of a barometer? 4. Why, or why not?