Atmospheric Density

Atmospheric Density

Atmospheric density is the ratio of the mass of air to the volume occupied by it.

The density of an air parcel is affected by several variables including volume, pressure, temperature, and water vapor which are all interrelated. In discussing each as it relates to density we are describing it with all other variables being equal, but in the real world they are not. For example, hotter air is less dense than colder air because it occupies a larger volume.

• Density varies inversely with volume. More air in a smaller space results in higher density.
• Density is directly related to pressure. Higher pressure air is more dense.
• Density varies inversely with temperature. Hotter air is less dense.
• Density varies inversely with water vapor quantity. This is because air molecules have a larger mass than water vapor molecules and density is directly related to mass.

High and Low Pressure

An area of high pressure is an air parcel that is more dense than the surrounding air. Being more dense, this air will tend to descend. Conversely, an area of low pressure is an air parcel that is less dense than the surrounding air. Low pressure air tends to rise. This is a key concept that we’ll refer back to later in this section.

Remember that volume, pressure, temperature, and water vapor are all interrelated factors with regards to air density. One illustration of this is that, excepting significant weather systems, atmospheric pressure tends to decrease throughout the day as a result of higher temperatures.