What are the FLAASH units of measurement: atm-cm?

FLAASH calculates the total amount of gas in the atmospheric column as part of the atmospheric correction process. The units used to describe the amount of gas, atm-cm, are often not found in physics textbooks. This Help Article discusses what atm-cm measures and how it is related to a more common measurement, gm/cm^2.

The atm-cm units is specific to the atmospheric science community, which might be why this measurement unit often does not turn up in physics books. The atmospheric science community customarily uses either of two types of units to measure the total amount of a gas in the atmospheric column from the ground to the top of the atmosphere (where 200-300 km is generally a good number for the location of the top). When "atm-cm" of water is used, for example, it could be thought of as bringing all the water molecules down to a thin layer of pure water vapor right at the Earth's surface, at one atmosphere of pressure and 0 degrees C. That layer would have a thickness of some number of centimeters, and it is said that the water column is that many atmosphere-centimeters. If the pressure were doubled, then the thickness would be halved. Thus the units atm-cm, not just cm, is used to describe the amount of gas in the atmospheric column to emphasizes that the height and pressure are interdependent.

Of course, it's not physically possible actually to bring these molecules into such a condensed layer. All the molecules in the layer would condense into a liquid under these conditions, even if they could be gathered in such a way. The layer is imaginary.

The second set of units, gm/cm^2, is more easily understood as the mass of water molecules in the atmospheric column over each cm^2 of ground surface. Since liquid water has a 1 gm/cm^2 density, this value is numerically equal to the number of centimeters of water we would have on the ground if all the atmospheric water rained out at once.

Review on 12/31/2013 MM