Explain how different types of thermometers work and why the placement of thermometers is an important factor in obtaining accurate readings. Distinguish among Fahrenheit, Celsius, and Kelvin temperature scales.

During a typical day, many of us routinely check the current air temperature several times. Radio stations frequently report the current temperature, and the time and temperature are often placed on digital business signs. Many cars show the air temperature as part of the dashboard display. Of course, many people rely on a cellphone app to retrieve temperature information. Which of these sources are accurate and reliable?

Thermometers

A thermometer is an instrument that measures temperature—either mechanically or electrically (Figure 3.21). To accurately measure air temperature, a thermometer must be placed in the shade and mounted at 1.5 meters (5 feet) above the ground.

Mechanical Thermometers

Most substances expand when heated and contract when cooled, and many common thermometers operate using this property. More precisely, they rely on the fact that different substances react to temperature changes differently.

The liquid-in-glass thermometer shown in Figure 3.22 is a simple instrument that provides relatively accurate readings over a wide temperature range. Its design has remained essentially unchanged since it was developed in the late 1600s—consisting of a bulb containing a fluid and a stem that has been bored to form a thin capillary tube. When temperature rises, the molecules of fluid grow more active and spread out, causing the fluid to expand. Expansion of the fluid in the bulb is much greater than the expansion of the enclosing glass, forcing a thin “thread” of fluid up the capillary tube. Conversely, when temperature falls, the liquid contracts, and the thread of fluid moves back down the tube toward the bulb. The movement of the end of this thread, known as the meniscus, is calibrated against an established scale to indicate the temperature.

The highest and lowest daily temperatures can be measured using specially designed liquid-in-glass thermometers. Mercury is the liquid used in the maximum thermometer, which has a narrowed passage called a constriction in the bore of the glass tube, just above the bulb (Figure 3.23A). As the temperature rises, the mercury expands and is forced through the constriction. When the temperature falls, the constriction prevents the mercury from returning to the bulb. As a result, the top of the mercury column remains at the highest point (maximum temperature attained during the measurement period). The instrument is reset by shaking or whirling it to force the mercury through the constriction back into the bulb, a process that must be completed daily.

In contrast to a maximum thermometer that contains mercury, a minimum thermometer contains a liquid of low density, such as alcohol. Within the alcohol is a small dumbbell-shaped index that rests at the top of the column (Figure 3.23B). As the air temperature drops, the column shortens, and the index is pulled toward the bulb by the surface tension of the alcohol meniscus. When the temperature subsequently rises, the alcohol flows past the index, leaving it at the lowest temperature reached. To return the index to the top of the alcohol column, the thermometer is simply tilted. A minimum thermometer must be mounted horizontally; otherwise, the index will fall to the bottom.

Another commonly used mechanical thermometer is a bimetal strip. As the name indicates, this thermometer consists of two thin strips of metal that are bonded together and have widely different expansion properties. When the temperature changes, both metals expand or contract, but they do so unequally, causing the strips to curl. This change corresponds to the change in temperature. Bimetallic strips are often used in home thermostats and electrical breaker boxes.

Electrical Thermometers

Modern observing stations are automated and use an electrical thermometer, called a thermistor, to measure temperature. A thermistor works on the concept that the flow of electricity through a metallic oxide disk or bead, called a resistor, is temperature dependent. As the temperature of the resistor changes, it alters the flow of electricity in a predictable way. Thus, an electric thermometer measures the flow of electricity and calibrates and displays that data as degrees of temperature. One advantage of an electric thermometer is that it provides an instant reading in any temperature scale. Thermistors, also called digital thermometers, are used in the medical field to measure temperature, as well as to measure the oil temperature in your vehicle.

Thermistors are used in radiosondes, where rapid temperature changes are often encountered. The National Weather Service also uses thermistors for ground-level readings. A sensor is mounted inside a shield made of louvered plastic rings, and a digital readout is placed indoors (Figure 3.24).

Instrument Shelters

How accurate are thermometer readings? Accuracy depends not only on the design and quality of the instruments but also on where they are placed. Placing a thermometer in direct sunlight will give an excessively high reading because the instrument itself absorbs solar energy much more efficiently than does air. Placing a thermometer near a heat-radiating surface, such as a building or the ground, also yields inaccurate readings. False readings will also be recorded if air is prevented from moving freely around the thermometer.

The ideal location is an instrument shelter (Figure 3.25). An instrument shelter is a white box with louvered sides to permit the free movement of air through it, while shielding the instruments from direct sunlight, heat from nearby objects, and precipitation.

Furthermore, the shelter is placed over grass whenever possible and as far away from buildings as circumstances permit. Finally, the shelter must conform to a standardized height so that the thermometers are mounted at 1.5 meters (5 feet) above the ground.

Temperature Scales

In the United States, standard temperature information is provided in degrees Fahrenheit, which we have shown in figures throughout this chapter. However, meteorologists and geoscientists, as well as most other countries, rely on the Celsius scale. In some circumstances, scientists also use the Kelvin, or absolute, scale. Figure 3.26 compares the three commonly used temperature scales.

Fahrenheit Scale

In 1714, Daniel Fahrenheit, a German physicist, devised the Fahrenheit scale. Like all temperature scales, the Fahrenheit scale is based on reference points, although the original reference points chosen by Fahrenheit were later modified. Today, the Fahrenheit scale is defined by two fixed points, the temperature at which ice melts (32°F) and the boiling point of water (212°F). The difference between the two fixed points on the Fahrenheit scale is 180 degrees.

Celsius Scale

In 1742, 28 years after Fahrenheit invented his scale, Anders Celsius, a Swedish astronomer, devised a decimal scale on which the melting point of ice was set at 0° and the boiling point of water at 100°. For many years it was called the centigrade scale, but it is now known as the Celsius scale after its inventor.

Because the interval between the melting point of ice and the boiling point of water is 100 degrees on the Celsius scale and 180 degrees on the Fahrenheit scale, a Celsius degree (°C) is larger than a Fahrenheit degree (°F) by a factor of 180/100, or 1.8. Therefore allowance must be made for this difference in degree size when converting from one system to the other. Also, conversions must be adjusted because the melting point of ice on the Celsius scale is at 0° rather than at 32°. This relationship is shown graphically in Figure 3.26.

The Celsius–Fahrenheit relationship is shown by the following formulas:

You can see that the formulas adjust for degree size with the 1.8 factor and adjust for the different 0° points by adding or subtracting 32.

Kelvin Scale

For scientific purposes, a third temperature scale is used: the Kelvin, or absolute, scale. On this scale, degrees Kelvin are called Kelvins (abbreviated K). This scale is similar to the Celsius scale because its divisions are exactly the same; 100 degrees separate the melting point of ice and the boiling point of water. However, on the Kelvin scale, the melting point is set at 273 K, and the boiling point is at 373 K (Figure 3.26). The zero point represents the temperature at which all molecular motion is presumed to cease (called absolute zero). Thus, unlike the Celsius and Fahrenheit scales, there is no negative value on the Kelvin scale, for there is no temperature lower than absolute zero.

• absolute zero: The zero point on the Kelvin temperature scale, representing the temperature at which all molecular motion is presumed to cease.

• bimetal strip: A thermometer consisting of two thin strips of metal welded together, which have widely different coefficients of thermal expansion. When temperature changes, the two metals expand or contract unequally and cause changes in the curvature of the element. Commonly used in thermographs.

• Celsius scale: A temperature scale (at one time called the centigrade scale) devised by Anders Celsius in 1742 and used where the metric system is in use. For water at sea level, 0° is designated the ice point and 100° the steam point.

• Fahrenheit scale: A temperature scale devised by Gabriel Daniel Fahrenheit in 1714 and used in the English system. For water at sea level, 32° is designated the ice point and 212° the steam point.

• instrument shelter: A white box having louvered sides to permit the free movement of air through it while shielding the instruments from direct sunshine, heat from the ground, and precipitation.

• Kelvin scale (a.k.a., Absolute scale): A temperature scale (also called the absolute scale) used primarily for scientific purposes and having intervals equivalent to those on the Celsius scale but beginning at absolute zero.

• liquid-in-glass thermometer: A device for measuring temperature that consists of a tube with a liquid-filled bulb at one end. The expansion or contraction of the fluid indicates temperature.

• maximum thermometer: A thermometer that measures the maximum temperature for a given period in time, usually 24 hours. A constriction in the base of the glass tube allows mercury to rise but prevents it from returning to the bulb until the thermometer is shaken or whirled.

• minimum thermometer: A thermometer that measures the minimum temperature for a given period of time, usually 24 hours. By checking the small dumbbell-shaped index, the observer can read the minimum temperature.

• thermistor: An electric thermometer consisting of a conductor whose resistance to the flow of current is temperature dependent; commonly used in radiosondes.

• thermometer: An instrument for measuring temperature; in meteorology, a thermometer is generally used to measure the temperature of the air.

• Thermometers measure temperature either mechanically or electrically. Most mechanical thermometers are based on the property that a material expands when heated and contracts when cooled.

• Electrical thermometers, called thermistors, use resistors that measure the rate of flow of an electrical current to measure temperature.

• To obtain an accurate air temperature, the best place to locate a thermometer is in a properly situated instrument shelter.

• Three common scales are the Fahrenheit scale, Celsius scale, and Kelvin, or absolute, scale.