So we know how to measure the temperature, what the temperature means, and how to do some basic statistics with the data, but what is the thermometer?

As the name implies, a thermometer allows us to measure (-meter) the temperature (thermo-).

There are several different types of thermometer:

• Mechanical thermometers
• Liquid-in-glass thermometer (alcohol or mercury)
• Maximum thermometer
• Can be mounted in any orientation
• Usually mercury filled
• Constriction in tube allows the mercury to only rise
• Mercury is reset by vigorously shaking it
• Minimum thermometer
• Usually horizontally mounted
• Usually alcohol filled
• Floating index lowers with decreasing temperature
• As temperature increase, the alcohol flows around the index
• To reset the index, tilt the thermometer
• Bi-metallic strip thermometer (mostly used in thermographs)
• Two different metals are mounted next to each other
• Differing coefficients of thermal expansion cause the strip to curl

• Electrical thermometers
• Thermistors (thermal resistors) make use of the fact that electrical resistance varies with differing temperature
• As T increases; resistance increases

Temperature measurements are made in a standardized environment, or shelter, i.e., not in the open sun or next to a hot rock.

Shelters that shade the instruments, but allow free air circulation are located 1.5 meters above a grassy spot and away from buildings.
 

Heating and cooling degree days

Data are calculated based on difference in the mean temperature from 65°F.

E.g., a day has 10 heating degree-days if the mean temperature for the day is 55°F or a day has 5 cooling degree-days if the mean temperature for the day is 70°F.

An easy way to remember heating and cooling degree days is to remember that 65°F is the temperature that is desired.  Therefore if the actual temperature is greater than 65°F we must cool; if the actual temperature is cooler than 65°F we must heat.

A similar calculation can be made, and called growing-days, based on the minimum required temperature for plants to germinate.