1.5: Hypsometric Equation

You have learned that air pressure decreases with height, but sometimes we want to know the distance or thickness between two different pressure levels. The atmospheric thickness varies depending on the average temperature in the layer. Warmer air is spaced out more, so a warmer layer of air will be thicker, while a cooler layer of air will be thinner. By knowing the average temperature of the layer, and the top and bottom pressure levels, you can calculate the thickness of the atmospheric layer. The hypsometric equation allows you to calculate how pressure varies with height in an atmosphere regardless of the temperature profile. It is the result of combining the ideal gas law with the hydrostatic equation. The hypsometric equation allows you to calculate the thickness (\(z_2-z_1\)) between two pressure levels, \(P_2\) and \(P_1\). The \(z_2\) and \(z_1\) values are the heights at pressure levels \(P_2\) and \(P_1\), respectively.

\[z_2-z_1 \approx \frac{R_d}{g} * \bar{T} \times \ln \left(\frac{P_1}{P_2}\right) \nonumber \]

\[P_2=P_1 \times \exp \left(\frac{z_1-z_2}{\frac{R_d}{g} \times \bar{T}}\right) \nonumber \]

In the above equation, the average temperature is shown. If the atmosphere is very moist, you may wish to use the average virtual temperature between the two heights \(z_2\) and \(z_1\) instead which includes the effects of water vapor.

\(T_v\) is known as the virtual temperature, defined as:

\[T_v=T \times [1+(0.61 \times r)] \nonumber \]

where the mixing ratio (r) is the mass of water vapor per mass of dry air and uses units of kilograms of water vapor per kilograms of dry air (kg·kg^-1). You will learn that many times virtual temperature can be used inside of equations in place of temperature if the effect of water vapor needs to be included.

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