Max Air Temp Thermometer


Mounted on a Townsend Support bolted to a 1 inch by 3 inch wood support bar in the instrument shelter is the Maximum Thermometer. The townsend support allows the thermometer to be unlocked and spun rapidly to reset the mercury column after each observation.

The thermometer works by having a mercury-filled glass tube (your typical old-fashioned thermometer) with one unique features added. This unique feature is a bore constriction about 2 milimeters above the bulb at the base of the thermometer.


The bore constriction allows mercury to pass through from the bulb into the glass tube when it is heated due to the force of expansion, however, when the temperature falls from its hottest point, the constriction of mercury causes a gap to occur in the bore constriction disallowing the mercury to fall back into the bulb. In the photo above, you can see the separation (gap) between the mercury in the bulb (on the left) and the column of mercury in the tube (on the right). This gap keeps the mercury column at the exact point of maximum temperature and allows observers to see what the maximum temperature was since the last time the thermometer was reset.

To reset the thermometer, either shake it sharply in a downward motion so that the bulb is furthest from your hand (being careful not to lose your grip or hit something causing the thermometer to break and releasing the toxic mercury inside), or use the townsend support which allows you to simply spin the thermometer and allow centrifugal “force” to push the mercury back down the tube and into the bulb as much as current temperatures allow.

I put scare quotes (“”) around the word force, because it isn’t really a unique force separate from momentum, but merely the tendency for an object to want to continue in a straight line. Ever turn a sharp corner with something on your dashboard and watched as it slid from one side of your dashboard to the other? You’re turning, but that something on your dashboard is simply trying to continue in the straight line you were on before you began your turn.

Our station records max and min temperatures on a 24-hour cycle from 5pm to 5pm during Daylight Savings Time, and from 4pm to 4pm during Standard Pacific Time. The max and min temperatures called in to the NWS for media and forecasting use is on a 24-hour cycle from midnight to midnight. Official numbers, however, have come from our automated station since January 2005.

~ Steve Woodruff and Devin Lussier