Sunlight and Temperature

Transcription

1 Sunlight and Temperature Name Purpose: Study microclimate differences due to sunlight exposure, location, and surface; practice environmental measurements; study natural energy flows; compare measurements; and make inferences from findings. Background When sunlight enters the Earth system, most of it passes through the atmosphere and reaches the surface. Some is then reflected back to space and the remainder is absorbed, heating the ground. This heating is uneven, however, and can vary a great deal over a small area. These variations help produce distinct microclimates, that is places with different temperature, humidity, and sunlight within a small area. In this lab, we will study local variations in surface heating from sunlight. Below is a brief overview of some of the many factors that influence the temperature of a surface. Sunlight: Sunlight provides the energy that warms Earth's surface. Anything that reduces sun exposure will lower temperature. That might include shading, reflection, and the angle at which the sun's rays strike the ground. Albedo: The percentage of sunlight reflected from a surface is called its albedo. The energy in sunlight remaining after reflection is absorbed by the surface. All else being equal, the greater the albedo, the cooler the surface because less sunlight is absorbed. In general, darker surfaces, like asphalt, have low albedos and thus, under the same sun exposure, heat up more than reflective surfaces such as snow, grass, or concrete because they reflect less and absorb more of the available sunlight. Evaporation: When water is available, evaporation absorbs large amounts of energy and reduces heating. Cooling by evaporation is one of the fundamental ways that plants and animals, including humans, shed excess heat. When no water is available, all sunlight energy absorbed is used in heating the surface and it can become quite hot. Which is hotter: bare sand or nearby grass? Grass roots send soil water to the leaves where it evaporates and cools the plant. Surface Material Properties: One of the most important properties that affect temperature is a material's specific heat capacity, which is ratio of energy added to temperature increase. Water has the highest specific heat of any common substance in nature; meaning that it takes a lot of energy to heat it up compared other surfaces like concrete, soil, forest or anything else. This explains, in part, why lake and ocean surfaces heat up much more slowly than the nearby land areas during sunny days. 1

2 Part 1: Classroom Exercise - Energy Balance The temperature of a surface is influenced by the factors mentioned above albedo, sunlight, evaporation, material properties -- and the amount of energy available to heat the surface. For this lab, it will be helpful to introduce the concept of an energy balance to get an idea of how all these influences work together to produce differences in temperature between surfaces. A simplified energy balance is: Sunlight Reflected Sunlight = Longwave + Sensible Heat + Evaporation Sunlight is the amount of sunlight falling on the surface Reflected Sunlight depends on albedo, higher albedo means more reflection Longwave is net terrestrial radiation energy emitted by the surface Sensible Heat is measured as temperature Evaporation is the energy absorbed when water changes from liquid to gas state 1. Fill in the blanks in the Table below. Use the energy balance equation above to determine the missing values. Note that dry surfaces lack water and no water exists on the surface of the Moon. # Surface Sunlight Albedo Reflected Longwave Sensible Evaporation Sunlight Heat 1 forest % lava rock % grass % dry sand dry concrete % sugar cane moon % 800 2

3 Part 2: Outdoor Measurement 2. Today s date Sky conditions Use the infrared (IR) thermometer to record the following temperatures. When using the IR thermometer, put the instrument CLOSE to the surface being measured, maybe an inch or couple of centimeters away. You should turn the laser pointer off, it is misleading. DO NOT stand several feet away and just aim the instrument using the laser pointer, you will not get an accurate reading on the small surface area required for this lab. Sometimes the side of the IR thermometer shows the angle of measurement, this helps explain why you need to hold the instrument close to what is being measured. Same with the sky measurements below, try to find as large an area of clear sky or cloud as possible to aim at. Ambient Air Temperature Air temperature as reported by meteorological services around the world is recorded in a white, shaded box called a Stevenson screen, with open airflow and a thermometer mounted at two meters above the surface, hence sometimes called 2m-temperature. As we do not have a standard thermometer for measuring air temperature, we will use a proxy that should give a fairly reliable result. Measure the SHADED side of a tree trunk at around head height. The trunk should have been in the shade for an hour or more to ensure that it approximates the ambient air temperature. Be sure to put the instrument CLOSE to the tree trunk, within a couple of centimeters or an inch, when taking the measurement, not 2 or 3 meters away. 3. Temperature on shaded side of a tree at head height (about 2 meters). Blocked from the wind is best. We will assume this is a close approximation of the air temperature. Take the following temperature measurements: Temperature Profile to determine Heat Flow near the Surface In nature, heat flows from areas of higher temperature toward areas of lower temperature. We will explore the direction (up or down) of heat flow near the surface on a sunny day. In an open, sunlit area of bare dry soil, measure the temperature using the IR thermometer at the following places. For the below-ground measurements, use the digging tool to expose the soil at the listed depths and then use the IR thermometer to measure the temperature in the hole. Be sure to place the IR thermometer CLOSE, but not touching, the soil. 4. bare, dry soil in direct sunlight 5. about 1 cm (1/2 inch) below the soil surface (same or adjacent location as #4) 6. about 7 cm (3 inches) below the soil surface (same or adjacent location as #4) 3

4 Albedo, Surface Characteristics, and Surface Temperature Now measure the temperature of several different surfaces that have been exposed directly to sunlight for at least several minutes and compare to areas that have been shaded. 7. sunlit side of tree trunk 8. shaded green (not dead) grassy area 9. sunlit green (not dead) grassy area 10. shaded concrete 11. sunlit concrete 12. sunlit asphalt Effect of Evaporation on Temperature For this measurement, we will measure the temperature of an open surface exposed to sunlight, then lightly mist it with water, wait until the wetted area begins to disappear by evaporation and then measure the temperature again. 13. concrete in sunlight before wetting 14. concrete after wetted area begins to disappear 15. tree trunk in sunlight before wetting 16. tree trunk after wetted area begins to disappear Clouds and Greenhouse Warming Clouds are an important part element of greenhouse warming. For these measurements point your IR thermometer at blue sky and clouds, but DO NOT POINT DIRECTLY AT THE SUN!!!!! Do your best with these measurements, it will not be possible to get clean measurements of all items below, but you should be able to get reasonable enough measurements to show the general relationships. 17. large area of clear, blue sky overhead 18. base of large area of cumulus clouds as high overhead as possible 19. thin high clouds (if available, just put N/A if no thin clouds) Other Feel free to play around the instrument; there are lots of interesting variations in temperature in the environment. Try measuring your skin at different places. Where is it hotter and cooler? 4

5 Part 3: Back in the Classroom - Discussion Answer the following questions based on your measurements and the background information given at the beginning of this lab. Some of the answers will be speculative so do not be afraid to use your imagination, in other words - guess. Sun and Shade. 20. How many degrees warmer was the sunlight area than the shaded area of the: a) tree trunk, b) grass, and c) concrete? 21. Which surface had the greatest difference in temperature between sunny and shaded areas? Speculate: Why you think that surface was heated the most by direct sunlight? Temperature Profile 22. Daytime. In nature, heat flows from warmer areas toward colder areas. Using the soil and air temperature measurements (numbers 3-6 above), draw a profile on the Daytime graph. Plot each point and connect the points with straight lines in order (measurements 3 to 4, 4 to 5, 5 to 6). 23. Based on your measurements and graph profile, above the surface, which direction is heat flowing - from the surface upward into the air, or from the air downward to the surface? (Hint: heat flows from warm areas toward cooler areas) 24. Based on your answer above, during the daytime, does the ground heat the air or does the air heat the ground? 25. Based on your measurements and graph profile, where is the higher temperature - at the surface or in the soil below the surface? In the ground below the surface, which direction is heat flowing - from the surface downward into the soil, or from the soil upward toward the surface? 5

6 26. Night. Assume that, at night, the bare soil surface temperature (#4) falls to 20 o C and the other temperatures remain the same. Draw a profile on the Night graph above using your measurements in #3, #5, and #6 and substituting 20 o C for your measurement of the bare soil surface temperature (#4). 27. Look at your graph. At night, above the surface, does heat flow from the surface upward into the air, or from the air downward to the surface?? 28. Look at your graph. At night below the surface, does heat flow from the surface downward into the soil, or from the soil upward toward the surface? Albedo 29. Which surface was hotter, concrete or asphalt? Which was darker? 30. Do you think darker or lighter surfaces will be hotter in general? Why? Effect of Evaporation 31. How many degrees did wetting cool the concrete? How many degrees did wetting cool the tree trunk? What caused the temperature to drop? (Do not write "it got wet") 32. Did concrete or the tree trunk cool more when wet? Why do you think there was a difference? 33. Compare your measurements of the sunlit open grass (#9) and bare dry soil (#4). Which was hotter? Both are exposed to the same amount of sunlight, and grass has a lower albedo than bare soil and thus absorbs more sunlight energy, so why do you think there was a difference in temperature? Urban Heat Island 34. Which surface was hotter, grass or asphalt? Why do you think it was the hottest? 35. Ever hear of an "urban heat island?" The phrase refers to the observation that urban areas are almost always hotter than the surrounding rural areas. Based on your measurements and answers above, give two reasons that urban areas might be hotter than rural areas. First Reason: Second Reason: 6

7 Application 36. There was a lot of variation in the surface temperature measurements. Under what conditions would you expect all of your measurements to be approximately the same? 37. Based on your experience, would you rather walk on grass or concrete in your bare feet?? Why would it be cooler? Would you rather walk on white sand or dark sand on a sunny day in your bare feet? Why could it be cooler?? 38. Many plants and animals cannot tolerate uncontrolled temperature fluctuations caused by direct sunlight exposure. Based on the various measurements you took in this exercise, suggest at least three (3) ways that desert plants and animals might reduce their skin heating due to direct sunlight. (HINT: which measurements had the lowest temperatures?). Sky 39. Which was warmer, cloud or clear sky? Clouds can have an important effect Earth's surface temperature. Based on your measurements, do you think the surface would be warmer at night under clouds or clear sky? Explain your reasoning. 40. Clouds have a major effect on planetary temperatures by influencing the amount of sunlight absorbed and reflected by the planet and through an analog with the so-called greenhouse effect. The greenhouse effect traps heat in a planet's atmosphere. From the surface, the greater the greenhouse effect, the warmer the sky appears. Based on this information and your measurements, do you think that more clouds would increase or decrease the planetary greenhouse effect? Explain your reasoning:. 41. What was the difference in degrees between the surface air temperature (#3) and the cloud base temperature (#18)? Under dry conditions, rising air cools at about 10 o C per kilometer (5.5 o F per 1000 ft). Based on this information, how high would the surface air have to rise to match the cloud base temperature that you measured? km_ Show your work below. (HINT: subtract cloud temperature from surface air temperature and divide by 10 o C.) Show how you calculated the answer 7