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:

lower elevation at 1 0: 30 . The pressure is 101 5 .0 . mb; temperature is

1 0 0 F.; relative humidity, 62 per cent.

the pressure, corrected for temperature, is 89 1.3 mb; temperature is

9 7 F.; relative humidity, 57 per cent.

corrected for temperature is 1015.3 mb; temperature is 101 F.; rela-

tive humidity, 60 per cent.

mb. Since the reading is taken only to the nearest tenth millibar, this

is 1 0 15 .2 m b .

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.

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.)

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.

cent relative humidity curve vertically upward to this correction factor

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