Specific gas constants for dry air and for water vapour

R_d 287 J / (kg.K)

R_v 462 J / (kg.K)
Miscellaneous properties of air which we shall treat as roughly constant over the environmental temperature range.

specific heat c_p 1010 J / (kg . K)

Ltnt heat vap L_v 2.5e6 J / kg
Saturation vapour pressure e_S(T) in [millibars] versus temperature T in [^oC] can be calculated from the following formula (range of validity unknown, but certainly ok over range 0 - 30^oC; T is to be inserted in degrees Celcius; see also Table below):
e_S(T) = 6.112 exp [ 17.67 T / (243.5+T) ]
It is useful to have the inverse of this formula, which for example will give us the depoint temperature for a given value of the actual vapour pressure. Rearrangement of the above formula gives
T(e) = 243.5 / [ 17.67 / ln(e / 6.112) - 1 ]
where e is to be inserted in millibars, and T will be in degrees Celcius. When using this formula, you might want to check at least once against the table given below, just to be sure you are doing the operations in the correct order. Note that ln( ) denotes the natural logarithm.

R_d	287	J / (kg.K)
R_v	462	J / (kg.K)

Back to the EAS372 home page.