Chapter 3
Air Temperature
Summary
Daily, seasonal, and geographic variations in temperature have important practical and economic implications and are examined in this chapter. The chapter begins with a discussion of the daily heating and cooling cycle in a thin air layer near the ground. Daily temperature is controlled by incoming energy, primarily from the sun, and outgoing energy from the earth's surface. While energy from the sun is generally most intense at noon, daytime temperatures continue to rise into the afternoon as long as energy input exceeds output. Because most of the incident sunlight is first absorbed at the ground and then transported into the atmosphere, large temperature gradients can develop between the ground and the air just above, especially under calm wind conditions. At night, the ground cools more rapidly than the air above and a radiation inversion will often form. With an understanding of the factors which promote the formation of an inversion layer, it is often possible for farmers and growers to reduce the severity of a nighttime inversion and to protect cold-sensitive plants and trees.
Temperature varies considerably on a global scale and mean and record temperatures observed throughout the world are summarized. The main factors that affect the range of temperatures at different locations around the world are latitude, elevation, and proximity to land, water, or ocean currents. Variables, such as mean daily temperature and normal temperatures, mean annual temperature, and annual range of temperature, can be used to characterize the climate of different regions. Additional parameters such as the number of heating or cooling degree-days can be used to estimate a region's heating or cooling needs.
The chapter concludes with an examination of how the human body's perception of temperature is influenced by atmospheric conditions and discusses the different types of thermometers and instruments that are used to measure temperature.
Teaching Suggestions, Demonstrations and Visual Aids
1.Students may not understand why the ground cools more quickly than the air above at night. The soil radiates nearly as a black body. The atmosphere is a selective absorber and also a selective emitter of infrared radiation.
2.Make an inversion. Fill a container with crushed ice or dry ice. Attach 3 or 4 thermometers to a vertical ringstand such that the thermometers are several inches apart and the lowest thermometer is about 1 inch above (but not touching) the ice. Cover the entire setup with a large glass bell jar.
Make a graph of the change in temperature with time for each thermometer. At first, the readings will be isothermal, but eventually the coldest air will be observed just above the ice and a strong inversion will have formed.
This demonstration can be applied to important concepts presented in this chapter. Explain why the air cools faster near the ice and relate this to the formation of a radiation inversion. Ask what ingredients are necessary to maintain the inversion. Explain how the inversion would be destroyed See also the article "Temperature Inversions Have Cold Bottoms" (ref: C.F. Bohren and G.M. Brown, Weatherwise, 34, 273-276, 1981).
3.Pass around a piece of metal, a piece of wood, and a piece of foam (all three could be attached to a larger piece of plywood perhaps). Explain that all three objects have been sitting in the classroom and have the same temperature. Ask the students whether all three objects feel like they have the same temperature. This is a good demonstration that our perception of temperature is often a better indication of how quickly our body is loosing heat rather than absolute temperature and leads into a discussion of the wind-chill effect.
Student Projects
1.Have students measure the temperature of a variety of different surfaces on a sunny afternoon. Are there differences in the temperature values? Ask the students to explain what causes the differences.
2.Have several students, who live in different parts of a city or town, make simultaneous early morning temperature measurements. Are there appreciable differences? Can these differences be attributed to topography? Is there evidence of a "thermal belt" in their city?
3.Students could design a simple weather station for their school. Where would the instrument shelter be located? What types of temperature instruments would be needed? How would the data be collected? See "Basic Meteorological Observations for Schools: Temperature" by J.T. Snow and S.B. Harley (Bull. Am. Meteorol. Soc., 68, 486-496, 1987) for a detailed discussion of temperature measurement, instruments, and activities.
4. Have the students plot an early morning and late afternoon sounding. Is a radiation inversion visible on the morning sounding? If so, how deep is the inversion layer? Did the students observe any visual evidence of a strong radiation inversion that morning? How has the sounding changed by that afternoon? How much of a change was observed at the ground and at different levels above the ground? (Tabulated data may use pressure as the vertical coordinate; in this case it is probably sufficient to assume a 1 mb decrease per 10 meters).
5.Students can study radiative cooling at night in a variety of ways. Have the students place a piece of wood and a plate of aluminum outside on the ground on a cold, clear night. The wood will often be covered with frost the next morning while the aluminum is not (ref: C.F. Bohren, "An essayon Dew," What LightThrough Yonder Window Breaks, John Wiley and Sons Inc., New York, 1991). A surface that is insulated from the ground may radiatively cool to well below the air temperature (ref: "The Cold Night Sky," Phys. Teacher, 248, April, 1984).
6.Have students determine the record high and low temperatures for their state or region. Then, review the controls of temperature and have students try to understand why these record values occurred where and when they did.
7.Plans for simple air and water thermometers are given in Chapter 4 of Hands-On Meteorology. The student-made thermometers could be calibrated using a laboratory-grade thermometer.
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8.Using the Atmospheric Basics/Layers of the Atmosphere section of the ThomsonNow web site, find an upper-air site in the continental U.S. that has an inversion. What weather conditions do you think are causing this inversion?
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9.Use the Atmospheric Basics/Energy Balance section of the ThomsonNow web site to explore the model of energy exchange between the surface and the atmosphere. Use the time slider to step the model through three-hour increments. For each time step, note the values of energy flow to and from the earth’s surface, and plot the total gains and losses versus time. Based on this information, when would you expect the temperature to reach its peak for the day?
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10.Using the Weather Forecasting/Forecasting section of the ThomsonNow web site, click on World Weather to show current temperatures in different hemispheres (that are experiencing different seasons). Do the temperatures you find match your expectations? Why or why not?
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Answers to Questions for Review
1.Even though incoming solar radiation decreases in intensity after noon, it still exceeds outgoing heat energy from the surface for a time. This yields an energy surplus for two to four hours after noon and substantially contributes to a lag between the time of maximum solar heating and the time of maximum air temperature several meters above the surface.
2.Because the calm conditions prevent forced convection from mixing the heat upward.
3.Warming occurs when incoming energy exceeds outgoing energy. Cooling occurs when outgoing energy exceeds incoming energy.
5.The ground, being a much better radiator than air, is able to cool more quickly.
Consequently, shortly after sunset, the earth’s surface is slightly cooler than the air directly above it. The surface air transfers some energy to the ground by conduction, which the ground, in turn, quickly radiates away.
6.Clear skies and calm or very light winds.
7.Cold, heavy surface air slowly drains downhill during the night and eventually settles in low-lying basins and valleys. Valley bottoms are thus colder than surrounding hillsides. In middle latitudes, these warmer hillsides (thermal belts) are less likely to experience freezing temperatures than the valley below.
8.Orchard heaters: warms the air by setting up convection currents close to the ground. Also, heat energy radiated from the orchard heaters is intercepted by the buds of the trees, raising their temperature. Wind machines and helicopters: mix the cold air at the ground with the warmer air above, thus raising the temperature of the air next to the ground. Flooding: Because water has a high heat capacity, it cools more slowly than dry soil. Consequently, the surface does not become as cold as it would if it were dry. Furthermore, wet soil has a higher thermal conductivity than dry soil. Hence, in wet soil, heat is conducted upward from subsurface soil more rapidly, which helps to keep the surface warmer. Sprinklers: the water freezes around the branches and buds, coating them with a thin veneer of ice. Latent heat, given off as the water changes into ice, keeps the ice temperature at 0°C (32°F). The ice acts as a protective coating against the subfreezing air by keeping the buds (or fruit) at a temperature higher than their damaging point.
9.Because the lowest temperatures on a clear, still night occur near the surface.
10.Latitude: distance from the equator. Controls the sun elevation and length of day. Land and water distribution: controls the heat capacity of the surface. Ocean currents: controls the horizontal mixing of heat. Elevation: controls the distance from the earth's surface, which provides energy to the atmosphere.
11.Land is generally cooler than water in winter because it has a smaller heat capacity, and also because the water temperature cannot drop below 0oC.
12.Thermometers are generally placed a few meters above the ground. Temperatures at the ground may actually be quite colder than temperatures a few meters above.
13.Because these areas are closer to the earth's surface, which is losing energy (cooling) by infrared radiation.
14. (a) In humid regions, the diurnal temperature range is usually small. Here, haze and clouds lower the maximum temperature by preventing some of the sun’s energy from reaching the surface. At night, the moist air keeps the minimum temperature high by absorbing the earth’s infrared radiation and radiating a portion of it to the ground. (b) Again, clouds lower the maximum temperature by preventing some of the sun’s energy from reaching the surface, and at night keep the minimum temperature high by radiating infrared energy to the ground.
15. Continents are generally less humid as compared to oceans, thus their temperature ranges tend to be larger than over oceans. The larger heat capacity of water as compared to land further moderates the temperature ranges.
16.The monthly pattern and ranges of temperatures may differ greatly, even though the annual means are the same.
17.The heating degree-day is based on the assumption that people will begin to use their furnaces when the mean daily temperature drops below 65°F. Therefore, heating degree-days are determined by subtracting the mean temperature for the day from 65°F. Thus, if the mean temperature for a day is 64°F, there would be 1 heating degree-day on this day. The cooling degree-day, is used during warm weather to estimate the energy needed to cool indoor air to a comfortable level. The forecast of mean daily temperature is converted to cooling degreedays by subtracting 65°F from the mean. The remaining value is the number of cooling degree-days for that day. For example, a day with a mean temperature of 70°F would correspond to (70 65), or 5 cooling degree-days.
18. Because thermometers are designed to measure air temperature and are not placed in direct sunlight. Our bodies, on the other hand, are influenced by both air temperature and direct sunlight.
19.a. -19oF. b. An ordinary thermometer, unlike human skin, does not have an insulating layer ofwarm air that would be swept away by the wind, with a resulting removal of heat.
20.Cold, wet and windy weather.
21.If the thermometer was placed in the sun, the measurement represents the temperature of the thermometer heated by the sun, not of the air surrounding the thermometer.
22.The minimum thermometer is similar to other liquid-in-glass thermometers except that it contains a small barbell-shaped index marker in the bore. The small index marker is free to slide back and forth within the liquid. It cannot move out of the liquid because the surface tension at the end of the liquid column (the meniscus) holds it in.
23.a. Liquid-in-glass thermometers are often used for measuring surface air temperature because they are easy to read and inexpensive to construct. These thermometers have a glass bulb attached to a sealed, graduated tube about 25 cm (10 in.) long. A very small opening, or bore, extends from the bulb to the end of the tube. A liquid in the bulb (usually mercury or red-colored alcohol) is free to move from the bulb up through the bore and into the tube. When the air temperature increases, the liquid in the bulb expands, and rises up the tube. When the air temperature decreases, the liquid contracts, and moves down the tube.Hence, the length of the liquid in the tube represents the air temperature. Because the bore is very narrow, a small temperature change will show up as a relatively large change in the length of the liquid column. b. A bimetallic thermometer consists of two different pieces of metal (usually brass and iron) welded together to form a single strip. As the temperature changes, the brass expands more than the iron, causing the strip to bend. The small amount of bending is amplified through a system of levers to a pointer on a calibrated scale. The bimetallic thermometer is usually the temperature-sensing part of the thermograph, an instrument that measures and records temperature. c. Highly accurate temperature measurements may be made with electrical thermometers, such as the thermistor and the electrical resistance thermometer. Both of these instruments measure the electrical resistance of a particular material. Since the resistance of the material chosen for these thermometers changes as the temperature changes, the resistance can be calibrated to represent air temperature. d. Air temperature may also be obtained with instruments called infrared sensors, or radiometers. Radiometers do not measure temperature directly; rather, they measure emitted radiation (usually infrared). By measuring both the intensity of radiant energy and the wavelength of maximum emission of a particular gas (either water vapor or carbon dioxide), radiometers in orbiting satellites are now able to estimate the air temperature at selected levels in the atmosphere.
Answers to Questions for Thought and Exploration
2.The Nevada desert site. The dry conditions of the desert allow for warmer afternoons and colder nights.
3.The jacket absorbs the infrared radiation leaving your body, and radiates it back to your body.
4.The temperature sensor is often located in direct sunlight.
5.You would have to place reflective material between the instruments and the asphalt to avoid heating the instruments by radiation from the asphalt. A fan would also be helpful.
6.San Francisco's average summer temperature is much lower than Richmond's average summertemperature. Consequently, more heating degree-days are computed at San Francisco during the summerthan are computed at Richmond for the same time period.
7.Because water heats and cools more slowly than land, the lag in daily temperature over water isusually much larger than that experienced over land.
8.Valleys get colder at night than hillsides, so frost and freezing is less of a problem on the hillside.
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