Temperature is a measure of the energy of molecules. The more energy the molecules in air have, the hotter it feels. Thermometers are the instruments that measure temperature. Common thermometers work on the principle that a rise in temperature causes molecules to move farther apart. Therefore, most materials expand when heated.
Some thermometers contain a liquid such as mercury or alcohol as the expanding material. Mercury is silver in color. Alcohol is clear, so it is usually dyed red or blue to be more visible. The liquid fills the relatively broad bulb end of a long, narrow glass tube. When the temperature rises, the liquid expands into the tube’s narrow stem.
Mercury thermometers are more accurate than alcohol thermometers because mercury expands more evenly. However, mercury cannot be used at temperatures below about –40 degrees Celsius, where it freezes. Alcohol freezes at about –129 degrees Celsius.
Metal thermometers have two equally long strips of different metals. Brass and iron are a common pair. The two strips are bonded together, one on top of the other. This forms a device called a bimetal bar. Because the metals expand at different rates, a rise in temperature makes the bar curl. A drop in temperature curls it the other way. Usually the bar is shaped into a coil and fastened at one end. As the temperature changes, the coil winds or unwinds.
A thermograph is a self-recording thermometer. A maximum thermometer shows the highest temperature reached. A fever thermometer is a maximum thermometer that has a tiny constriction to keep the mercury from flowing back into the bulb. A minimum thermometer indicates the lowest temperature reached.
Temperatures are measured in degrees. A degree of temperature is a definite fraction of the difference between two fixed temperatures. Usually these points are the temperatures assigned to the melting of ice and the boiling of water at sea level pressure.
On the Celsius scale, the fixed points are labeled as 0 and 100. One degree Celsius is therefore 1/100 of their difference. On the Fahrenheit scale, the fixed points are labeled 32 and 212. One degree Fahrenheit is therefore 1/180 of their difference (212 – 32 = 180). Note that a Celsius degree is almost twice as large as a Fahrenheit degree. To be exact,
1 degree Celsius = 1.8 degrees Fahrenheit
or
1 degree Fahrenheit = 5/9 degrees Celsius
Isotherms are lines drawn on maps that connect places with the same temperature. Usually isotherms are spaced 5 or 10 degrees apart. Isotherms on a map for average July temperature show that the warmest temperatures are to the north of the equator. The sun’s rays are more vertical north of the equator in July. If Earth were all water, the isotherms would be east-west, following the parallels of latitude. However, land heats and cools more easily than water, so the continents are warmer in summer and colder in winter. This makes the isotherms more irregular in the Northern Hemisphere, where there is more land.
July is the warmest month in the Northern Hemisphere, and January is the coldest month. Thus, the isotherms shift their positions from January to July.
