Density may be defined as mass per unit volume and may be expressed as:
• Grams per cubic metre.
• A percentage of the standard surface density – relative density.
• The altitude in the standard atmosphere to which the observed density corresponds density altitude.
Effect of Changes of Pressure on Density
As pressure is increased, the air will be compressed which reduces the volume and increases the density. Likewise, if pressure is decreased, the air will expand which will increase the volume and decrease the density.
We can therefore say that:
DENSITY IS DIRECTLY PROPORTIONAL TO PRESSURE
In the atmosphere density can be decreased by raising the volume of air to a greater height since we know that pressure decreases with an increase in altitude. Similarly, density can be increased by lowering the volume of air to a lower height.