CHAPTER 3 (Moran and Morgan, 1997) In everyday conversation, the terms heat and temperature are often used interchangeably, and as a result, not always precisely. To most of us, temperature, one of the weather elements, is a measure of comfort. Strictly speaking, temperature is a measure of the average energy of motion (kinetic energy) of the molecules of any substance. Heat represents the total amount of molecular energy for the given substance. However, when heat is transferred from one substance to another by one or more of the energy transport modes (radiation, conduction and convection), the temperature of the substance will change. Heat and temperature are related, since a gain or loss of heat by a substance usually will result in a temperature change in that substance. The magnitude of the temperature response of the particular substance depends upon its specific heat, a characteristic of that substance. The thermal response of the various materials that comprise our earth-atmosphere system to radiative heating and cooling influences our climate. In particular, the high specific heat of water provides the oceans with greater thermal inertia, resulting in maritime climates that have smaller annual temperature variations than that of more continental climes. A thermal sensor called a thermometer, and the reading given by one of three temperature scales usually measures the temperature of any material. Usually the heat is determined from the magnitude of the measured temperature change. In addition to air temperature, several temperature-related statistics might be of value, to include heating and cooling degree-day units and the wind-chill equivalent temperature. STUDY NOTES Look at Figure 3.1 to compare the fixed points that help define the three temperature scales currently used. From this diagram you should realize that 100 units separate the ice point and steam point on both the Celsius and Kelvin scales, while 180 units separate these two points on the Fahrenheit scale. In Figure 3.3, notice the mechanisms used in those maximum and minimum recording thermometers. The text provides the details employed. Make a quick comparison of the heat conductivity values for various substances listed in Table 3.1, arranged from the most to least conductive. You will not need to memorize numerical values nor the particular units used in this table, but you should realize that air is a poor conductor of heat, while liquid water is more conductive and metals have the highest thermal conductivity. Think about this comparison when reading the text. Take a moment to consider Figure 3.6. Note that for a fresh snow cover with a low density (the mass to volume ratio is low because of the large air spaces between individual snow crystals), the heat conductivity is small (near the lower left portion of the figure). When the snow settles and the density increases due to compaction to a smaller volume with less trapped air, the thermal conductivity of the snow cover increases. In Figure 3.7, notice that the simple model of convection involves air that is warmed near surface becomes buoyant since it is less dense than its surroundings and rises. Cooler and denser air sinks to replace the air that has risen. Make a quick comparison of the specific heat for various substances in Table 3.2, arranged from the largest to the smallest specific heat values. You will not need to memorize numerical values nor the particular units used in this table, but you should realize that water has the highest specific heat of all the materials listed here. You should also notice that air has a specific heat that is roughly an order of magnitude smaller than for water. Also note that the value of the specific heat of liquid water is 1.00, which is a consequence of the way in which the calorie is defined. Think about the comparisons between the specific heat values for the various substances when reading the text. Study Figure 3.9, comparing the seasonal cycles of monthly temperatures for San Francisco, a maritime climate, and St. Louis, a continental climate. While both have essentially the same annual mean temperature (depicted by the dashed line), the green curve representing the variations in monthly average temperatures for St. Louis has larger amplitude than that of the red curve for San Francisco. In other words, St. Louis experiences a colder January and a warmer July than does San Francisco. Furthermore, the oceanic influence causes the maximum temperature to occur in September, lagging the July maximum in St. Louis. Figure 3.10 -- This figure is included to show how some climatologists have devised a scheme to assess the moderating influence of the oceans upon the climate of the continental margins. Use Figure 3.11 to estimate the average annual number of heating degree-day units that are accumulated in the vicinity of your home. You should then compare how this annual total compares with other locations in the continental United States and Canada. In particular, note that few heating degree-day units are accumulated annually in warm Florida where little fuel is needed for space heating. On the other hand the largest number of heating degree day units in the United States is found in cold Minnesota, where much more fuel is needed for heating purposes. Follow the example in the text to correctly determine the wind-chill equivalent temperature from Table 3.3A or 3.3B, using the ambient air temperature and the wind speed. Read the Weather Fact (The Maritime Influence on Western Europe) page 77. One factor that was not mentioned is the influence of warm Gulf Stream in the North Atlantic Ocean. This warm water current forms near the Florida Straits off South Florida, flows northward along the Eastern Seaboard before turning eastward near Cape Cod, MA to flow across the North Atlantic to the British Isles and western Europe. As will be discussed in the following chapter, these ocean currents from the tropics to subpolar latitudes transport a sizable amount of heat energy. Skim Special Topic (Temperature and Human Comfort) on pg. 84- 86 -- The focus is on the biological and physiological aspects of extreme temperature upon the human body. For your personal safety, you should become aware of the consequences associated with those times when your body temperature would become either too low (hypothermia) or too high (hyperthermia) for maintaining normal body processes. Special Topic (Temperature and Crop Yields) on p 87-88 -- Skim this topic that describes an example of another type of degree-day unit, specifically, the Growing degree-day Unit. CHAPTER 3 (Moran and Morgan, 1997) HEAT AND TEMPERATURE This chapter is concerned with the distinction between heat and temperature, how the two quantities are related and measured, and basic heat transfer processes. Temperature is a measure of the average kinetic-molecular energy whereas heat is the total kinetic molecular energy in some substances. Heat is transferred within and between substances through conduction, convection, and radiation. The temperature of a substance changes in response to a gain or loss of heat, with the magnitude of temperature change depending on the specific heat. In response to the same heat gain, substances with relatively low specific heats (such as land surfaces) warm up more than substances with relatively high specific heats (such as the ocean surface). The greater thermal inertia of the oceans means that downwind localities have maritime climates, that is, there is less temperature contrast between summer and winter. CHAPTER OBJECTIVES After reading this chapter, the student should be able to: - Distinguish between heat and temperature. compare and contrast the various temperature scales. - Describe how temperature is measured. explain how heat is transported via conduction and convection. demonstrate how heat and temperature are related through specific heat. contrast a continental climate with a maritime climate. Compute heating degree-days and cooling degree-days. Explain the significance of the wind-chill equivalent temperature. 3 Heat and Temperature 68 Distinguishing Heat and Temperature 69 Temperature Scales 70 Temperature Measurement 70 Heat Units 73 Transport of Heat 74 Thermal Response 76 Heating and Cooling Degree-days 78 Wind-chill 81 Conclusions 83 Weather Fact: The Maritime Influence on Western Europe 77 Special Topic: Temperature and Human Comfort 84 Special Topic: Temperature and Crop Yields 87 Key Terms 85 Summary Statements 85 Review Questions 88 Quantitative Questions 89 Questions for Critical Thinking 89 Selected Readings 89 3