SM0486
(c) Hygrographs are recorders of relative humidity. A pen arm, which is
substituted for the pointer, traces an ink record on a clock-driven drum and graph-
paper mechanism.
(3) Dew point hygrometer. The dew point is that point where the humidity
in the air just starts to condense and form water droplets on exposed surfaces.
This condition exists when the relative humidity reaches 100 percent.
Basically,
the procedure for detecting dew point is to cool a mirror until dew or frost just
condenses room surrounding air.
The temperature at the surface of the mirror at
the instant dew or frost appears is defined as dew point. By using tables such as
tables 8 and 9, relative and absolute humidity may be calculated when the dew point
is known. One difficulty with this method is in measuring the exact temperature of
the mirror when dew or frost first occurs.
Another difficulty is that any two
observers would probably not detect dew or frost at the same instant.
It is
therefore, common practice for the dew point to be taken as the average temperature
at which dew or frost is first detected, on cooling of the mirror, and the
temperature at which the dew or frost vanishes when the mirror is warmed.
This
procedure does not assure a correct answer, but it is close.
TABLE 8
Water Vapor Capacity of Air
35
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