How Does a Thermometer Measure Air Temperature?
By
Tiffany Means
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."
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.
Tiffany Means
Meteorology Expert
Education
B.S., Atmospheric Sciences and Meteorology, University of North Carolina
Introduction
Studied
atmospheric sciences and meteorology at the University of North Carolina
Former
administrative assistant for the National Oceanic and Atmospheric
Administration
Member
of the American Meteorological Society
Experience
Tiffany
Means is a former writer for ThoughtCo who contributed articles about weather
for five years. She has interned with the domestic and international weather
departments at CNN, written monthly climate reports for NOAA’s National Centers
for Environmental Prediction, and participated in a number of science outreach
events, including the Science Olympiad Competition. Means has personally
experienced such weather greats as the Blizzard of 1993 and the floods of
Hurricane Francis (2004) and Ivan (2004).
Education
Bachelor's
degree in atmospheric sciences and meteorology from the University of North
Carolina at Asheville
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https://www.thoughtco.com/how-does-a-thermometer-work-3444248
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