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Project Ideas continued

4. Measure the Temperature Across the Entire Sky.

The apparent temperature of the sky straight overhead (the zenith) is cooler than when an IR thermometer is pointed at lower angles in the sky. This is because the IR sensor in the thermometer is looking through more water vapor when the instrument is at an angle away from the vertical. Here, for example, is a scan across the entire sky from East (0 degrees) to West (180 degrees) near midnight when no clouds were present.

DATE: 03 Oct 2008
 TIME: 11:27 p.m.
SKY CONDITION: Totally clear.
ANGLE (90 is zenith)

SKY TEMPERATURE
(Celsius)
0
19.4
10
10.6
20
3.3
30
-1.7
40
-5.6
50
-7.2
60
-8.3
70
-9.4
80
-11.1
90
-11.1
100
-11.1
110
-9.4
120
-8.3
130
-7.2
140
-4.4
150
-1.7
160
3.9
170
19.4
180
18.9

Here's a graph that shows the data in the table above:

Click to view enlarged version.

The blue diamonds represent the actual temperature measurements. The blue line represents the best curve though the data points.

Project: This project is best done with two people in a place where you have a full view across the sky. You will need an infrared thermometer and an angle indicator such as the Johnson Pitch and Angle Locator.

Use a computer to make a simple spreadsheet like the one above or draw a similar spreadsheet on a sheet of paper. Place your spreadsheet on a clipboard and take it and an IR thermometer and a protractor outdoors to measure the temperature across the sky every 10 degrees.

Hold the angle indicator against the side of the infrared thermometer so that the top of the thermometer is even with the top edge of the angle indicator. You will need to experiment with the best way to hold them together.

While one person points the infrared thermometer at 10 degree intervals across the sky, the second person should write down the temperature at each interval. Some IR thermometers have a hold feature that freezes the temperature reading on the readout when a button is pressed. This feature is very handy if a partner is not available.

Enter your measurements into a computer spreadsheet to make a graph like the one above.

Study your graph. Do the two sides of the scan across the sky match fairly evenly? Why does the sky become warmer near the horizon? What happens when clouds are in the sky? Why?

Questions:

1. True or false: On the same day, the temperature of the blue sky measured by an infrared thermometer is warmer than the temperature of a cumulus cloud. Explain your answer.

2. Explain the dew point.

3. True or false: The temperature of a cumulus cloud is related to the dew point. Explain your answer.

4. True or false: A weather balloon can measure air temperature at an altitude of 30 kilometers or greater. Explain your answer.

5. When measured with an infrared thermometer, the sky temperature is coolest: (a) straight overhead, (b) at an angle of 30 degrees away from straight overhead, or (c) at an angle parallel with the ground.

Going Further:

Learn about emissivity to better understand how an infrared thermometer works. Can you provide a scientific explanation of what an infrared thermometer is really measuring when pointed at the sky and clouds?

Project Idea contributed by Forrest M. Mims III, Geronimo Creek Observatory, Texas


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