Atmospheric State Variables

Atmospheric state variables refer to the different physical properties of the atmosphere that help describe its overall condition. These variables include temperature, pressure, humidity, and density. These state variables are interconnected and affect the behaviour and dynamics of the atmosphere.

The ideal gas law is a fundamental principle in thermodynamics that relates the pressure, volume, and temperature of a gas. It states that the product of pressure and volume is directly proportional to the absolute temperature of the gas. Mathematically, the ideal gas law is expressed as:

PV = nRT

Where P is the pressure, V is the volume, n is the number of moles of gas, R is the gas constant, and T is the absolute temperature.

The ideal gas law can be applied to the atmosphere to understand its behaviour and properties. For example, the ideal gas law helps explain why atmospheric pressure decreases as altitude increases. As the altitude increases, the atmospheric density decreases, resulting in fewer gas molecules per unit volume. This decreases the pressure, which can be explained by the ideal gas law. Similarly, the ideal gas law can help explain how temperature changes affect atmospheric pressure and density.

State variables are related to one another by the Ideal Gas Law (IDL)

IDL is often referred to as the “Equation of State”


Air is mostly made up of free molecules in constant motion (gases).State Variables

Air molecules have mass.

You can feel the mass of the air when the wind is blowing hard.

Surface Pressure

  • The pressure at the surface is caused by the weight of all the air molecules in the column above the surface.
  • Add more air molecules to the column and the pressure goes up. (High-Pressure areas)
  • Take away air molecules from the column and the pressure goes down. (Low-Pressure areas)


Air density is the mass of the air divided by the volume of measurement.

As one goes higher in the atmosphere the number of molecules in a given volume decreases, so like pressure, density also decreases monotonically with height.

Since don’t have as many molecules on top of you, the air pressure also decreases with height.

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