|
|
|
|
Thermometer
Mon Dec 08, 2003
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 4 and
6 feet above the ground (or, eye level), and be 2 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 and The National Science Foundation.
|
|
