Buck 1011C, Operating Manual

Page: 43 / 48

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NOTE 1: f1(DP) and f2(e) are variations on vapor pressure formulations found in Buck, A: J
Appl Met 20, pp 1527-1532 (1981). They are given by:
e vs. DP or es vs. T:
f1(DP) = EF x aw x exp [(bw - DP/dw) x DP/(DP + cw)] (over water)
= EF x ai x exp [(bi - DP/di) x DP/(DP + ci)] (over ice)
DP vs. e or T vs. es:
f2(e) = dw/2 x [bw - s - ((bw - s)2 - 4 cw x s/dw)1/2] (over water)
= di/2 x [bi - s - ((bi - s)2 - 4 ci x s/di)1/2] (over ice)
where:
aw = 6.1121 ai = 6.1115
bw = 18.678 bi = 23.036
cw = 257.14 ci = 279.82
dw = 234.5 di = 333.7
s = ln (e/EF) - ln (aw or ai)
EF
w
= 1 + 10-4 [7.2 + P (0.0320 + 5.9 x 10-6 T
2
)],
EF
i
= 1 + 10-4 [2.2 + P (0.0383 + 6.4 x 10-6 T
2
)],
where P is in kPa and T is in oC.
NOTE 2: RH is defined here using es with respect to ice below freezing. However, RH is also
frequently defined using es with respect to water, even below freezing.
NOTE 3: These conversions are intended for use with moist air rather than pure water vapor.
They therefore include EF, the enhancement factor, which corrects for the slight departure of
the behavior of water in air from that of a pure gas.
NOTE 4: The definitions f1 and f2 for ice agree with an extrapolation of NBS values down to -
120 deg C, within 0.5%.