Traditionally maximum and minimum temperatures were measured using the set-up shown in the photo above. Now many weather stations are equipped with electronic devices to measure temperature. However, there are still many cooperative weather stations that still use “mercury-in-glass” thermometers. These thermometers are mounted on a Townsend Support which places each thermometer in the proper alignment to measure the high (maximum) and low (minimum) temperatures.
The minimum thermometer uses alcohol for the fluid to measure the temperature. The red fluid is alcohol, which has a freezing point (-173 degrees F), much lower than mercury (-37.9 degrees F). The Townsend Support holds the thermometer tilted down slightly to the left. Inside the tube is a black index which always marks the lowest reading since it was last reset. The index allows alcohol to move past when the temperature warms rises. When the temperature cools the surface tension of the alcohol drags the black index down. Once the temperature reaches its lowest point and begins to warm the alcohol moves up the scale again allowing the marker to remain in place, marking the lowest reading. To reset the thermometer the observer tilts it down to the right and the black index moves down the tube stopping at the current temperature.
The maximum thermometer works like a fluid in glass thermometer used to take your temperature. There is a constriction just above the bulb which allows expanding mercury to move through when temperatures warms but stays in place when readings cool. When the mercury expands (warming) it is forced out of the bulb but when it contracts (cooling) it cannot go back into the bulb. As a result, the mercury stays at the highest point until it is reset by the observer. To reset the maximum thermometer the observer spins it to force the mercury down through the constriction.
The photo above was taken inside a medium size Cotton Region Shelter. The maximum-minimum thermometers are mounted on a cross bar (visible in the photo). The shelter keeps the thermometers in the shade to measure the air temperature, not the temperature of the sun shining on the thermometers, which is what would happened if they were exposed in the open. Sun shining on the thermometers would read too warm. The shelter also keeps the thermometers dry. Wet thermometers would tend to read too cool as water evaporates off them. In the background on the left is a mercury-in-glass thermometer that reads the current temperature. The minimum thermometer also reads the current temperature. The maximum thermometer does not.