Record the reading, also the time, the temperature, and the humidity.
(2) Take the barometer to the higher elevation. Record the pres-
sure, the time, the temperature and the humidity.
(3) Return the barometer to the original position and take the
pressure reading, the time, the temperature and the humidity.
(4) Interpolate between the two sets of readings made at the lower
elevation to find the pressure, temperature and humidity values at the
lower elevation which corresponds in time to the pressure, temperature
and humidity values at the higher elevation.
(5) The following is an example of such interpolation:
(a) Assume that the pressure reading is taken at the station of
lower elevation at 1 0: 30 . The pressure is 101 5 .0 . mb; temperature is
1 0 0 F.; relative humidity, 62 per cent.
(b) Take the barometer to the station of higher elevation. At 1 1:15
the pressure, corrected for temperature, is 89 1.3 mb; temperature is
9 7 F.; relative humidity, 57 per cent.
(c) Return the barometer to the original station at 1 1:45. Pressure,
corrected for temperature is 1015.3 mb; temperature is 101 F.; rela-
tive humidity, 60 per cent.
(d) Tabulating the pressure values:
mb. Since the reading is taken only to the nearest tenth millibar, this
is 1 0 15 .2 m b .
(e) The influence of temperature and humidity on the values of the
correction is sufficiently small that these values need not be known
closer than the nearest 2 F. and 10 per cent relative humidity. Thus
the average temperature of the two stations is 980 F. and the relative
humidity 60 per cent.
(f) Using the conversion table, the altitude corresponding to 1015 .2
mb is -70 feet. The altitude corresponding to 9 8 1.3 mb is 855 feet.
Subtracting the values, the approximate difference in elevation is 855
- ( - 70 ) = 92 5 feet.
(6) It is now necessary to correct the approximate difference in
elevation for temperature and humidity. (As noted in (e) above, the
average temperature is 980 F., the relative humidity 60 per cent.)
(a) Locate 98 F. on the horizontal scale along the bottom of the
curves for Air Temp. and Relative Humidity Correction Factor for
Altitude (fig. 6). Follow the 98 F. line diagonally u ward until it
intersects the 60 per cent relative humidity curve.
The correction
factors are given in the horizontal scale at the top of the curves.
(b) Follow the intersection of the 9 8 F. diagonal and the 60 p e r
cent relative humidity curve vertically upward to this correction factor
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