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Thermometer
Fri Jan 28, 2005
Listen in RealAudio 
A thermometer is one of the most basic, most used, and most misunderstood of all
weather instruments. Hi, I'm Bryan Yeaton for The Weather Notebook.
A thermometer, as you probably already know, measures the temperature of the air. Air
is a gas, made up of molecules. Basically, the faster those molecules are moving, the
hotter they are, and the more room they need.
The most common device used to measure temperature is called the "expansion
thermometer" -- that's the kind you probably have out the kitchen window, or use when
someone has a fever. A liquid, usually alcohol or mercury, expands as it gets warmer,
and contracts when it gets cooler. Most meteorological stations use a mercury
thermometer for high temperatures since it is more accurate, and alcohol for the lows
since it has a lower freezing point than mercury.
Where you put the thermometers is important, too. They need exposure to the airflow,
but also have to be sheltered from solar heating. The solution: the Cotton Region
Shelter. This instrument shelter, sometimes called a Stevenson Shelter, is a slatted,
white wooden box 30 inches by 20 inches, with a door which opens to the
north.
There are reasons for these curious requirements. The slats allow air to move freely
through the shelter, and the white paint reflects sunlight, keeping the thermometers
from heating above the air temperature. The shelter also is required to house the
instruments between four and six feet above the ground (or, eye level), and be two feet
above the maximum snow depth. All that just to take the temperature.
To see pictures of the shelter and standard thermometer setup, go to our website at
www.weathernotebook.org. The Weather Notebook is a production of the Mount
Washington Observatory, and is supported by Subaru.
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