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; shown in the photo above as the small black line in the column) farthest from the bulb indicates minimum air temperature since its last reset; resetting the index is accomplished by moving the thermometer to a vertical position with the bulb at the top, allowing gravitational force to pull the index back up the tube to the meniscus of the alcohol column (current temperature); surface tension of the alcohol prevents the index from breaking past the meniscus. The photo I took above shows this thermometer recorded a minimum temperature of about 43 degrees Fahrenheit.
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.)
~ Steve Woodruff and Devin Lussier