308
XIX
ADIABATIC CHANGES IN MOIST AIR
=
one. If the number of lines crossing each other appear con-
fusing, we may lay a strip of paper parallel to the system
of lines considered, when all confusion will be avoided. In
order to find out the state in the neighbourhood of the height
700 m., we seek out the point 700+100 800 in the scale
of heights, and go vertically upwards to meet our line a. The
point of intersection gives the pressure 687 mm., and the
temperature 19°3. But we must only use the line a down
to the point at which it cuts the dotted line 11. For reach-
ing this line means that we reach a state in which the air can
only just retain 11 grammes of water per kilogramme in the
form of vapour. And as we have 11 grammes per kilogramme,
any further cooling produces condensation. The pressure for
the point of incipient condensation is 640 mm., the tempera-
ture 13°.3. This is not the temperature of the initial dew-
point, but is lower. The dotted line 11 cuts the isobar 750
at 15°-8, which is the initial dew-point. But since our air
has increased in volume in addition to having cooled, the
water has been able to keep itself in the state of vapour down
to 13°.3. The height at which we find ourselves corresponds
to the lower limit of cloud formation; it is about 1270 m.
To trace still further the changes of state we draw through
the point of intersection a curve of the system B and follow
its course.
This curve is much less inclined to the axis of
abscissæ than the line a previously used; so that now the
change of temperature with height is much less than before,
owing to the evolution of the latent heat of the steam. When
we have risen 1000 m. above the point at which condensation
commenced, the temperature has only fallen to 8°2, i.e. only
0°.51 for every 100 m. We find ourselves on the dotted line
8.9, and thus see that 8.9 grammes of water still exist as
vapour, so that 2.1 grammes of water per kilogramme of air
have been condensed in this first thousand metres of the cloud-
layer. We reach the temperature 0° at a pressure 472 mm.
and at a height 3750 m., while we should have reached it at
a height of 2600 m. if the air had been dry and we had not
had to forsake the line a. 4.9 grammes of water, or 45 per
cent of the whole contents, are now found to have condensed;
and this portion on further expansion begins to freeze to form
hail. But until the last trace of water has frozen, the tem-