Weather: The Take on Temperature

The Take on Temperature

The temperature is measured with a thermometer. No kidding! Galileo Galilei (1564-1642) is often credited with being the first to come up with this instrument. But the discovery is not without controversy. More than 2,000 years ago, Hero of Alexander designed a temperature measuring device. Galileo's contemporaries Sanctorius Sanctorius and Cornelius Drebbel also did work on thermometers. The standard version consists of a liquid enclosed in a glass. The liquid usually consists of mercury or alcohol, which expands and rises in the tube when the temperature increases and contracts and drops when the temperature decreases.

But what about temperature scales—Fahrenheit, Celsius, and all that? It's all relative. In the early 1700s in his hometown of Leipzig, Germany, G. Daniel Fahrenheit defined the zero-point as the lowest point the column would fall when the thermometer was immersed in a mixture of ice, water, and salt. He then assigned the number 32 to the level that the column reaches when ice melts, which automatically made 212 degrees the point at which water boils. Now Fahrenheit could have defined the zero-point and assigned the freezing point an infinite number of ways. The intervals are completely arbitrary. You could define the zero point to be the lowest point on the column ever reached in your hometown, and go from there.

The Fahrenheit (F) scale suffers from bad press, and most countries have abandoned it—but it is a detailed scale and familiar to many. The Celsius scale is most commonly used around the world because it is a metric scale. Swedish astronomer Anders Celsius came up with this scale in the eighteenth century. On the Celsius scale, ice melts at 0 degrees, and water boils at 100 degrees. Because there are one hundred divisions between melting and boiling, this scale is also referred to as the centigrade scale.

In any case, because there are 180 degrees between melting and boiling on the Fahrenheit scale, each Celsius degree is 180 ÷ 100 or 1.8, times larger than a Fahrenheit degree. That's why the Fahrenheit scale is more detailed. An increase of 1 degree C is equivalent to an increase of 1.8 degrees F. The conversion from degrees F to degrees C is given by the following formula: degrees F = ⁹⁄₅ degrees C + 32. So 20 degrees C would come to ⁹⁄₅ (20) + 32, or 68 degrees F. A quick, although not precise, conversion would be to double the Celsius reading and add 32. It at least puts you in the ballpark. So if you happen to be an American in Paris and hear that the temperature is 25 degrees C, double it and add 32 to get the Fahrenheit equivalent (50 + 32, or 82 degrees). It's really 78 degrees, but at least you'll know how to dress without getting out a calculator.

The most scientific temperature scale is the Kelvin (K) scale, which also has 100 divisions between the freezing and boiling points of water, but the zero-point is defined as the point where all molecular motion comes to a grinding halt. That is the absolutely coldest temperature possible and called absolute zero. British scientist Lord Kelvin defined this scale in the nineteenth century. Here, ice melts at 273 degrees, and water boils at 373 degrees. The temperature could never be negative on this scale, and because of that, it's used in all mathematical calculations where the temperature is a variable. The following figure shows the three scales: Fahrenheit, Celsius, and Kelvin.

In addition to standard liquid-in-glass thermometers, air temperature can be measured in more hi-tech ways. Electrical thermometers are very accurate. They work on the principle of the electrical resistance of a wire often made of platinum or nickel. The resistance increases with increasing temperature. A meter will measure the resistance, and that reading will be set to correspond to the temperature. Along these lines, thermistors, which are made of ceramic materials, are used in remote sensing. These run on the same principle of electrical resistance.

On the subject of remote sensing, satellites use infrared instruments, which record the strength of long-wave radiation emitted through the atmosphere. That strength is also proportional to the temperature. We'll get to satellite measurements later in "Radar and Satellites."

Another popular instrument is a thermograph, which measures and records the temperature on a revolving drum (see the following figure). A metal strip consisting of two different pieces of metal provides the mechanism for this instrument. The two metals are usually iron and brass. One expands more than the other when the temperature increases, so the strip bends in one direction. Then when the temperature drops, the strip moves the other way. This bimetallic thermometer is used in thermostats. In thermographs, the motion of the strip moves an arm up or down, and a pen at the end of the arm records its position on graph paper. The paper is put on a clock-driven rotating drum so that the change of temperature with time can be seen.

Last but not least, some thermometers measure the maximum and minimum temperature of the day. The maximum thermometer looks like a fever thermometer—it goes up but doesn't come down, unless it is shaken. When the minimum thermometer column goes down, it drags a barbell-shaped marker along with it. But that marker goes one way—it can't go up when the temperature increases. Its position marks the lowest temperature. The minimum temperature is reset by turning the thermometer upside down until the barbell marker hits the top of the liquid column.