304
XIX
ADIABATIC CHANGES IN MOIST AIR
kilogramme-metre
R
=
29.27
degree C. x kilogr
One of
These adiabatics are straight lines in our diagram.
them is marked by the letter (a), and we may denote the
system by this letter. The individual lines are so drawn that
the value of the constant, the entropy, increases from one line
to the next by
0.0025-
calorie
degree C. x kilogr
Thus they are equidistant. One of them is drawn through
the point 0° C. and 760 mm. pressure.
Now the curves of the second stage satisfy the equation¹
0.6219.
const = c, log T- AR log p+
R ጥ e
R₁₂ T Ρ
·
R/R is the density of aqueous vapour referred to air, i.e.
r, according to Clausius, is equal to 607 -0.708
calorie
kilogr
(T-273)
The values of e for the various tempera-
tures I have taken from the table calculated by Broch.2 The
curves run from the right-hand top corner to the left-hand
bottom corner with a slight curvature. One of them is
marked B. They also are drawn so that the entropy per
kilogramme increases from one curve to the next by 0.0025
calorie/degree C., and so that one of them passes through the
point 0° C., 760 mm.
The portions of the curves which correspond to the third
stage coincide with the isothermal 0° C.
Lastly, the curves of the fourth stage are very similar to
those of the second, but yet are not quite the same; for their
equation is got from that of the former curves by putting
r+q for r, where q = 80 calorie/kilogr. They are marked y
1 Even though u is neglected in comparison with A, yet it is doubtful
whether cu should be neglected compared with cp; for c is four times as great
as Cp.
Though in the diagram u is no more than of A, still the ratio cu/cpλ
amounts to For meteorological applications we must, however, remember
that in these extreme cases the liquid water will not all be carried about with
the air. Frequently a large proportion will be deposited as rain, so that we
may be nearer the truth in neglecting the specific heat of the liquid water
altogether than in taking the whole quantity into account.
2 Traveaux du Bureau international des poids et mesures, tome i.