How do you test the temperature of gasses?

Question

More specifically how do you test the temperature of greenhouse gasses

Answer 1

There are contact and non-contact methods.

A thermometer or thermocouple is used for contact measurements.  The probe must come to the region of interest, which can be a problem if the region of interest is much different than ground level.  Measurements can be made from aircraft, but the aircraft itself can affect the measurement.  Balloons work best.  

Non-contact methods are based on spectroscopy.  All gasses with two or more atoms per molecule have vibrational and rotational spectra that can be used as a very sensitive temperature probe.  Here's how.  The internal energy of a molecule is described by two quantum numbers, n for the vibrational energy and j for the rotational energy.  Vibrational energies are in the mid infrared region whereas rotational energies are much less corresponding to the microwave region.  A molecule can absorb or emit a photon only at discrete frequencies corresponding to integral changes in the quantum numbers n and j.  The molecular symmetry determines which transitions are allowed as well as the preferred method (Raman or IR).  The interesting bit is that the energy distribution of the molecules is determined by a partition function:  that is the probability of a quantum state described by n and j is weighted by the  factor exp{-E(n,j)/kT} where the energy E is a function of the quantum numbers n and j, T is the temperature in degrees Kelvin and k is the Boltzmann constant.  The bottom line is that the spectra are temperature dependent and the temperature can be measured by comparing integrated emission line intensities.  Furthermore, the emission line profile is related to temperature by the Doppler effect, pressure through molecular collisions (usually the dominant effect) as well as the natural line width related to the state lifetime.  A high resolution spectrometer is needed to measure line profiles, but again the temperature can be inferred.  The downside to non-contact measurements is that gasses along a line of sight may vary in composition, temperature and pressure.  This problem can be solved by measuring a target region of the atmosphere from differing lines of sight so that the contributions from different layers vary in a known manner.  

The basics of molecular spectroscopy were worked out by Gerhard Herzberg.  

http://nobelprize.org/nobel_prizes/chemi...

I attended Dr. Herzberg's lectures as a PhD student many years ago.  Molecular Vibrations, Wilson Decius & Cross is an easier read for a novice.

Answer 2

Thermometers work and you can also use thermocouples.