How Does a Thermometer Measure Air Temperature?

How warm is it outside? How cold will it be tonight? A thermometer -- an instrument used to measure air temperature—easily tells us this, but how it tells us is another question entirely.

To understand how a thermometer works, we need to keep one thing in mind from physics: that a liquid expands in volume (the amount of space it takes up) when its temperature warms and decreases in volume when its temperature cools.

When a thermometer is exposed to the atmosphere, the surrounding air's temperature will permeate it, eventually balancing the thermometer's temperature with its own—a process whose fancy scientific name is "thermodynamic equilibrium." If the thermometer and it's inside liquid must warm to reach this equilibrium, the liquid (which will take up more space when warmed) will rise because it is trapped inside of a narrow tube and has nowhere to go but up. Likewise, if the thermometer's liquid must cool to reach the air's temperature, the liquid will shrink in volume and lower down the tube. Once the thermometer's temperature balances that of the surrounding air, its liquid will stop moving.

The physical rise and fall of the liquid inside of a thermometer is only part of what makes it work. Yes, this action tells you that a temperature change is occurring, but without a numerical scale to quantify it, you'd be unable to measure just what the temperature change is. In this way, the temperatures attached to a thermometer's glass play a key (albeit passive) role.

Who invented it: Fahrenheit or Galileo?

When it comes to the question of who invented the thermometer, the list of names is endless. That's because the thermometer developed from a compilation of ideas through the 16th to 18th centuries, starting in the late 1500s when Galileo Galilei developed a device using a water-filled glass tube with weighted glass buoys that would float high in the tube or sink depending on the hotness or coldness of air outside of it (sort of like a lava lamp). His invention was the world's first "thermoscope."

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In the early 1600s, Venetian scientist and friend to Galileo, Santorio, added a scale to Galileo's thermoscope so that the value of temperature change could be interpreted. In doing so, he invented the world's first primitive thermometer. The thermometer didn't take on the shape we use today until Ferdinando I de' Medici redesigned it as a sealed tube having a bulb and stem (and filled with alcohol) in the mid-1600s. Finally, in the 1720s, Fahrenheit took this design and "bettered it" when he began using mercury (instead of alcohol or water) and fastened his own temperature scale to it. By using mercury (which has a lower freezing point, and whose expansion and contraction is more visible than water's or alcohol's), Fahrenheit's thermometer allowed temperatures below freezing to be observed and more precise measurements to be observed. And so, Fahrenheit's model was accepted as the best.

What kind of weather thermometer do you use?

Including Fahrenheit's glass thermometer, there are 4 main types of thermometers used to take air temperatures:

Liquid-in-glass. Also called bulb thermometers, these basic thermometers are still used in Stevenson Screen weather stations nationwide by National Weather Service Cooperative Weather Observers when taking the daily maximum and minimum temperature observations. They're made of a glass tube (the "stem") with a round chamber (the "bulb") at one end that houses the liquid used to measure the temperature. As the temperature changes, the volume of liquid either expands, causing it to climb up into the stem; or contracts, forcing it to shrink back down out of the stem toward the bulb.

Hate how fragile these old-fashioned thermometers are? Their glass is actually made very thin on purpose. The thinner the glass, the less material there is for the heat or cold to pass through, and the quicker the liquid responds to that heat or cold—that is, there's less lag.

Bi-metallic or spring. The dial thermometer mounted on your house, barn, or in your backyard is a type of bi-metal thermometer. (Your oven and refrigerator thermometers and furnace thermostat are other examples, too.) It uses a strip of two different metals (usually steel and copper) which expand at different rates to sense temperatures. The metals' two different expansion rates force the strip to bend one way if heated above its initial temperature, and in the opposite direction if cooled below it. The temperature can be determined by how much the strip/coil has bent.

Thermoelectric. Thermoelectric thermometers are digital devices that use an electronic sensor (called a "thermistor") to generate an electric voltage. As the electric current travels along a wire, its electrical resistance will change as temperature changes. By measuring this change in resistance the temperature can be calculated.  

Unlike their glass and bi-metallic cousins, thermoelectric thermometers are rugged, respond fast, and don't need to be read by human eyes, which makes them perfect for automated use. That's why they're the thermometer of choice for automated airport weather stations. (The National Weather Service uses data from these AWOS and ASOS stations to bring you your current local temperatures.) Wireless personal weather stations also use the thermoelectric technique.

Infrared. Infrared thermometers are able to measure the temperature at a distance by detecting how much heat energy (in the invisible infrared wavelength of the light spectrum) an object gives off and calculating a temperature from it. Infrared (IR) satellite imagery—which shows the highest and coldest clouds as a bright white, and low, warm clouds as gray—can be thought of as a kind of cloud thermometer.

Now that you know how a thermometer works, watch it closely at these times each day to see what your highest and lowest air temperatures will be.

Sources: 

• Srivastava, Gyan P. Surface Meteorological Instruments and Measurement Practices. New Delhi: Atlantic, 2008.