Mounted on Townsend Support on
wooden bar in Instrument Shelter; Official Minimum Air Temperature thermometer
in highest position on Support; alcohol liquid-in-glass type thermometer with
uniform bore, but with linear marker in bore; the end of the marker (index)
farthest from the bulb indicates minimum air temperature since last reset; reset
accomplished by moving thermometer to a vertical position with the bulb high ó
gravitational force causes index to move to the present end of the alcohol
column; this thermometer indicates the ambient or current air temperature at any
time by observation of the length of the alcohol column.
For psychometric readings I use 2 matching thermometers: dry bulb (t)
and wet bulb (tw); aspirating fan draws outside air through thermal shields,
past 2 thermometers, and exhausts through ports; wet and dry bulb thermometers
observed when stabilized; these observed values used with U.S.N.W.S.
Psychrometeric Tables for obtaining Relative Humidity, Dew Point, Vapor
Pressure, and Absolute Humidity. (By the way, relative humidity is the RATIO
of the actual water vapor content of the air to its total capacity at a given
temperature, and absolute humidity is the MASS of water vapor per unit
volume of the air.)
