The Six’s Thermometers (on south side of pan) is on the lower left.
These thermometers measure the maximum and minimum water temperature during each day. Sometimes, they are referred to as Maximum minimum thermometers for this reason. Six’s thermometers may also be used to measure air temperatures.
In simplest terms, the six’s thermometer is merely a U-shaped thermometer, where one leg of the “U” records the minimum temperature, and the other leg of the “U” records the maximum temperature. An index rests on top of either end of the mercury columns. On the minimum side, the index will rest at the point of lowest temperature, whereas on the maximum side, the index will rest at the point of maximum temperature; this allows the observer to see the max and min temperatures recorded since the last time the thermometer was reset.
The six’s thermometers are reset by shaking them in a downward motion which resets the mercury columns.
It should be noted, that mercury is no longer used in newly manufactured thermometers due to the high toxicity levels of mercury. Nowadays, six’s thermometers are filled with a red fluid which is usually either ethanal alcohol with red dye added, or mineral spirits with red dye added. Always use caution when resetting any type of thermometer filled with gray liquid, as it is most likely mercury filled.
Notice in the picture that there are two separate “lines” above the two mercury columns. These are the indexes. The left side of this six’s shows the minimum temperature (about 15 degrees C), and the right side shows the maximum temperature (just about 25 degrees C). Fahrenheit is shown by the numbers between the columns forming the “U”.
Water temperature is an important consideration when determining evaporation rates since the energy required to evaporate warmer water as opposed to cooler water is less.
Six’s thermometers are always placed under water on the south side of the pan (opposite the still well) and adjacent the surface cup anemometer.
~ Steve Woodruff and Devin Lussier