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Blowin' in the Wind, Issue #012 - A list of thermodynamic temperature conversions. November 1, 2004
November 02, 2004

Fourteen Temperature conversions

This strange list of temperature conversions gives you an idea of what thermodynamics forecasters deal with. We now explore some of the deeper, fundamental concepts used in meteorology.

The most important two:

1) Dry bulb temperature - T, the usual degree of sensible heat, shown by the isotherms on the tephigram.

2) Web bulb temperature - Tw, the temperature you get when you go up the dry adiabat until saturation, reaching the correct mixing ratio line, then down the pseudo-adiabat to the original pressure.

3) Isobaric wet bulb temperature - Tiw, we get this one by evaporating moisture into it until saturated, for instance air moving over the open sea with no pressure change. Forget about using the tephigram for this one.

4) Virtual temperature - Tv, with pressure being equal, the temperature dry air would have to be in order to have the same density. It's a little higher than the actual temperature because water molecules, which weigh less than air molecules, occupy space in moist air.

5) Adiabatic equivalent temperature - Tae, the temperature of the air after going up the dry adiabat until saturated, up the pseudo-adiabat from this point up to the top and all moisture is removed, with its latent heat left behind, then down the new dry adiabat to the original pressure and new higher temperature. Chinook (and other foehn) winds go partially through this process.

6) Dew point temperature - Td, if you cool the air without changing the pressure or vapor content, dew will start to form once you reach the dew point temperature. If you move in the tephigram, go left along the isobar to symbolize no pressure change, where the correct mixing ratio line crosses that isobar, the isotherm at that intersection gives the dew point temperature. In real life, the air loses heat, like on a foggy night, when this happens - therefore we do not use the adiabats.

7) Frost point temperature - Tf, same idea as dew point, only relative to ice and frost. Usually a degree or two higher than dew point.

8) Saturation temperature - Ts, if you suddenly depressurize air, dew may form at the saturation temperature. On the tephigram, go straight up the dry adiabat until reaching the mixing ratio line and record the temperature isotherm there.

9) Potential temperature - Theta, the temperature if you quickly compress or decompress air to exactly 1000 mbar. Go up or down the correct adiabat on a tephigram until reaching the 1000 isobar and read the temperature.

10) Saturated potential temperature - Theta-w, we get this by going up the dry adiabat until saturation, then down the pseudo-adiabat until 1000 mbar. It gives us the best idea of how warm that air mass actually is.

11) Virtual potential temperature - Theta-v,

12) Equivalent potential temperature - Theta-ae,

13) Isobaric equivalent potential temperature - Theta-ie

14) Isobaric wet bulb potential temperature - Theta-iw

Scientists can define these last few, but normally have no use for them.

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