Thermometer
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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 heat. More specifically, the heat of the atmosphere. The atmosphere is a gas, made up of molecules. Basically, the faster those molecules are moving, the hotter they are, and the more room they need.

Cotton Region Shelter, also known as a Stephenson Screen. View Movie

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, or 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 weathernotebook.org. Special thanks today to Nicole Plette. The Weather Notebook is a production of the Mount Washington Observatory.