XIX
309
ADIABATIC CHANGES IN MOIST AIR
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perature cannot fall any further, and thus we shall for a certain
distance keep at a constant temperature of 0°. To ascertain
how far, we use the small supplementary diagram between the
larger one and the scale of heights. We follow the isobar
472 down to the dotted line of this diagram. Through the
point of meeting we draw a line parallel to the sloping lines
of the diagram, and follow this line to its intersection with
the horizontal line characterised by the number 11, the total
weight of water. This latter line is easily interpolated between
the horizontal lines 10 and 15 drawn. As soon as we reach
this line we read off the pressure p
463 mm. and return to
the large diagram. At the pressure thus found the process of
freezing is completed; the layer within which it took place
has a thickness of about 150 m. It will appear strange that,
according to the dotted lines, the amount of water in form of
vapour has increased a little during the freezing. But this is
quite true, for the volume has increased, without any corre-
sponding fall in temperature. At the pressure 463 mm. we
leave the temperature 0°. The water which henceforth con-
denses passes directly into the solid state. As in a short time
little water is left in the form of vapour, the temperature
begins to fall more rapidly with increase of height. We find
the various states by following that one of the lines Y which
passes through the point 463 on the isothermal 0°. The
temperature -20°, up to which the diagram is available, is
reached at the height 7200 m., and at the pressure 305 mm.;
only 2 grammes of water per kilogramme remain as vapour,
the other 9 are condensed. If we wish to know how the
density in this state compares with the initial density, we
draw two lines through the corresponding points parallel to
the line 8. These meet the isothermal 0° at the pressures
330 and 680. The densities are in the ratio of these pressures,
i.e. as 33: 68, and they are in the ratios 33 and 68 to 76 to
the density of air in the normal state at a pressure 760 mm.
and a temperature 0°.
All these results have been read off direct from the
diagram. Errors which could cause inconvenience are prob-
ably only to be found in the heights given. For these in
strictness relate to a rise through an atmosphere everywhere
at the same temperature 0°. But in most cases it may be