How does the surface type and moisture content affect the surfaceʼs temperature?

Transcription

1 How does the surface type and moisture content affect the surfaceʼs temperature? Introduction In addition to the output of and the distance from the energy source, surface type (surface reflectivity, specific heat, etc.) and moisture content affect the absorption of incident radiation and how incident energy changes the temperature of a surface. In this lab you will examine the effect of various types of surfaces on the time rate of change of temperature of those surfaces when they are exposed to incident sunlight. Recall that an object s change in temperature, ΔT, is related to the object s mass, M, the specific heat of the object, c sh, and the energy the object absorbs and uses to change its temperature, ΔE, i.e., ΔT = ΔE / ( c sh M ). Both surface reflectivity and moisture content impact the amount of incident radiation that is used to change the surface s temperature, i.e., these two properties contribute to determining ΔE in the above relationship. For example, if the surface were highly reflective, then little of the incident radiation would be absorbed and used to warm the surface. Mathematically, we can express this relationship for a dry surface as ΔE = (1 - Albedo ) * (Incident radiation), where Albedo is defined as the fraction of incident energy that is reflected. Note that if the surface is non-reflective, i.e., has zero albedo, then ΔE equals the incident radiation. Thus, a more complete expression that relates the change in temperature for a dry object can be calculated as follows ΔT = ( 1 - Albedo ) * (Incident radiation) / ( c sh M). Likewise, if the surface is wet, then much of the absorbed radiation will be used to evaporate water rather than warm the surface. Recall that the Bowen Ratio, BR, is defined as the ratio of the sensible heating to the latent heating that the surface experiences when radiation is absorbed by the surface, i.e., BR = ΔE / LH. Also, if we neglect conduction away from the surface, the absorbed radiation is equal to the sum of the latent heat and the sensible heat, i.e., the absorbed radiation equals LH + ΔE. After some

2 algebra, the amount of absorbed radiation available to warm a wet, non-reflective surface can be calculated as ΔE = ( BR ) * (Incident radiation) / (1 + BR). Note if BR is zero, i.e., all the absorbed radiation is used to evaporate water, then ΔE would be zero and thus, ΔT would be zero; if BR is very large, i.e., almost all the absorbed radiation is used to warm the surface, then BR / (1 + BR ) become very close to one. Thus, ΔE nearly equals the absorbed radiation. Therefore, for a wet, non-reflective surface a more complete expression that relates the change in temperature for a surface with sunlight shining on it can be calculated as follows ΔT = ( BR ) * (Incident radiation) / (1 + BR ) * ( c sh M). Part I. Effect of Albedo Experiment In this experiment, the mass and specific heat of the two cups of dry sand will be the same since they contain the same amount of sand in identical cups, only the sand s color is different. Place about 1.5 inches of sand in two white styrofoam cups, one with white sand and the other with black sand. Both quantities of sand should be dry. Measure the temperature of the sand initially. Use both a thermometer and the Raytek infrared gun to measure the temperature. The infrared gun remotely senses the temperature of the surface optically by collecting and focusing the emitted, reflected and transmitted radiation onto a detector. The gun s electronics translate the collected information into a temperature that is displayed on the screen on the rear of the device. The sand in each cup should be about the same temperature at the start of the experiment. Be careful with the thermometers! They are very fragile. Do not shake the mercury down the tube (as one might do with an oral thermometer) as the snapping action of your wrist may break the glass. Next, place an energy source (a bright reflector lamp) equal distance (as close as practical) above the two cups. Recall that the distance between the source and the object has a large affect on radiation received. For this experiment, it is very important that the two cups receive the same intensity of

3 incident radiation. Thus, align the cups such that the center of the lamp is directly above where the two cups are touching each other. Now turn on the lamp. Again using both the thermometer and the infrared gun, measure and record the temperatures of the sand in each of the two cups at three-minute intervals for fifteen minutes. Take the readings by turning off the lamp while you are taking the temperature of the sand. Place a thermometer in the sand in each cup, i.e., you need two thermometers. The thermometer should be inserted until all of the bulb is below the surface of the sand. Be sure the thermometer has come to thermal equilibrium with the sand before recording the temperature. Do not leave the thermometers in the sand between measurements; insert the thermometer in the sand for each reading. Temperature White Sand Black Sand Time Thermometer Infrared Gun Thermometer Infrared Gun Initial 3 minutes 6 minutes 9 minutes 12 minutes 15 minutes Did the temperature readings from the two methods of measuring agree? If not, speculate why they may not agree.

4 Why did the instructions tell you to turn off the lamp while taking the readings? Why did the instructions tell you not to leave the thermometers in the sand between readings? Explain your results based on the lecture discussing albedo. Part II. Effect of Soil Moisture Experiment This portion of the experiment will demonstrate the affect of soil moisture or surface water content on how surfaces use incident radiation. Recall the discussion of the Bowen Ratio above. Again the mass and specific heat of the two cups of sand will be the same since they contain the same amount of sand in identical cups, only the sand s moisture content will be different. Place about 1.5 inches of sand in two white styrofoam cups, one with dry black sand and the other with damp black sand. Use water at room temperature to dampen the sand. Again using both the thermometer and the infrared gun, measure the temperature of the sand initially. They should be about the same temperature at the start of the experiment. Again, place the reflector lamp equal distance (as close as practical) above the two cups. As above, it is very important that the two cups receive the same intensity of incident radiation so align the cups as above. Now turn on the lamp. Read and record the temperatures of the sand in each of the two cups at three-minute intervals for fifteen minutes. Take the readings by turning off the lamp while you are taking the temperature of the sand. Place a thermometer in the sand in each cup, i.e., you need two thermometers. The thermometer should be inserted until all of the bulb is below the surface of the sand. Do not leave the thermometers in the sand between measurements; insert the thermometer in the sand for each reading. Be sure the thermometer has come to thermal equilibrium with the sand before recording the temperature.

5 Temperature Wet Sand Dry Sand Time Thermometer Infrared Gun Thermometer Infrared Gun Initial 3 minutes 6 minutes 9 minutes 12 minutes 15 minutes Explain your results based on the lecture discussing latent heat and soil moisture.

6 Part III. Real World Measurements Using the Raytek infrared gun, go outside and measure the temperature of five (5) different surfaces, such as concrete, asphalt, a grassy surface in the open, a grassy surface under a tree. Record the temperature readings in the Table below. Surface Description Temperture Speculate about why the temperature of each surface type is different from the other surfaces you measured.