Measuring the Earth’s Water Vapor Blanket – Page 4

Procedure continued

Date
Time
Sky
Temp (F)
8-Sep-08
12:35:00 PM
46
4.63
19-Sep-08
12:33:02 PM
25.0
20-Sep-08
12:41:28 PM
2.67
21-Sep-08
12:12:17 PM
37.0
3.02
22-Sep-08
12:59:05 PM
32.0
2.84
23-Sep-08
12:54:37 PM
42.0
4.52
24-Sep-08
11:44:14 AM
3.8
25-Sep-08
12:31:08 PM
19.0
2.185
26-Sep-08
12:31:45 PM
2.35
27-Sep-08
08:48:48 AM
2.28
28-Sep-08
12:40:11 PM
24.0
1.83
29-Sep-08
12:32:40 PM
13.0
1.71
30-Sep-08
12:33:32 PM
22.5
2.18
1-Oct-08
12:29:36 PM
20.0
2.425
2-Oct-08
12:35:00 PM
20.0
2.37
3-Oct-08
12:34:19 PM
30.0
3.34
4-Oct-08
10:08:33 AM
24.0
2.33
5-Oct-08
12:45:58 PM
32.0
3.38
6-Oct-08
01:18:52 PM
40.0
4.39
7-Oct-08
12:19:26 PM
23.0
2.76
8-Oct-08
12:34:04 PM
8.0
1.175
9-Oct-08
12:31:18 PM
10.0
1.35
10-Oct-08
12:29:25 PM
27.0
3.095
11-Oct-08
12:17:46 PM
33.0
3.84
12-Oct-08
13-Oct-08
14-Oct-08
01:01:23 PM
38.0
5.6
15-Oct-08
27.0
4.345
16-Oct-08
03:36:00 AM
17-Oct-08
12:15:55 PM
17.0
2.78
18-Oct-08
12:27:53 PM
11.0
1.71
19-Oct-08
09:59:38 AM
8.0
1.59
20-Oct-08
12:32:25 PM
16.0
1.85
21-Oct-08
12:28:14 PM
19.0
2.315

Notice that some days have missing data. That’s OK if you have a dozen or more days with good data.

After your data are entered in the spreadsheet, you’re ready to make a calibration graph. First, place the cursor over “Sky” in the top row. Press down the left mouse button (on a PC) and then scroll down to the end of the column and the column headed “GPS” until all the data are highlighted.

Select “Scatter” from the spreadsheet’s graph options. Select a scatter graph composed of unconnected markers. A graph will appear on your spreadsheet. The graph below was made using Excel and is similar to a graph made using Open Office 2.4.

Place the cursor on one of the markers in the graph and press the right mouse button. In Excel, select “Add Trendline.” The select “Linear Trendline,” “Display Equation on chart,” and “Display R2 value on chart.” (Similar instructions are available in Open Office 2.4.) The graph will now resemble this:

Click to view enlarged version.

Your calibration graph is ready to use. The equation at upper left is the calibration. R2 indicates the statistical correlation of the sky temperature and the PW (total water vapor). A perfect correlation would be 1. The R2 shown here is acceptable, particularly since the GPS site was 30 km north of the site where the sky temperature was measured. Better results might have been obtained had the measurements been made closer to the GPS site.

(During the measurements that were made while preparing this project, data was collected from an adjacent sun photometer designed to measure the column water vapor. A comparison of these data with the infrared sky temperature yielded an R2 of 0.88.)

You can make a columnar chart as shown below, or you can jump to the next step and begin using the calibration formula.

Your calibration formula can now be used to find the PW when you know the zenith sky temperature. You can use a calculator or a spreadsheet to solve the formula. The formula above is: y = 0.094x + 0.5217, where y is the y axis (GPS Total Water Vapor or PW) and x is the x axis (Zenith Sky Temperature or T).

The formula can also be written: PW = 0.094T + 0.5217.

Here’s a spreadsheet version of the formula for spreadsheet cell B5 that assumes the sky temperature will be entered in cell A5: =A5*0.094+0.5217

Here’s a table of results for temperatures ranging from -5F to 50F.

Enter T to find PW:
T
PW
-5
0.0517
0
0.5217
5
0.9917
10
1.4617
15
1.9317
20
2.4017
25
2.8717
30
3.3417
35
3.8117
40
4.2817
45
4.7517
50
5.2217

Your calibration formula will differ from the one derived from the measurements above. That’s OK. Your main goal is for the R2 to be as close to 1 as possible, preferably above 0.7.

It’s important to understand that your calibration might be affected by shifts in the Earth’s temperature over time. Thus, a calibration made during summer might not work during winter. How to study possible calibration changes is among the projects listed below.

5. Going Further

After you have calibrated your infrared thermometer, you’re ready to measure the atmosphere’s water vapor, the main greenhouse gas.

Science Project Ideas:

You can learn much about the water vapor overhead using an infrared thermometer calibrated as described above. Here are some projects you can try:

1. Measure Total Water Vapor from Different Locations.

Take an infrared thermometer that you have calibrated on a trip or vacation. Measure the column water overhead at various locations. Ideally, make measurements from very dry sites (desert and mountain locations) and very moist sites (southern and eastern US during summer). You will see dramatic differences in total water vapor.

Warning: Be sure that the laser pointer in the thermometer is deactivated during your measurements. If necessary, place tape over the laser opening. Never point a laser pointer at a person, car, truck or airplane!

2. Measure Total Water Vapor at Day and Night.

Data from NOAA’s Ground-Based GPS-IPW project web site shows that the total water vapor does not necessarily change at night. What happens when you use a calibrated infrared thermometer to measure total water vapor during both day and night? How do you explain any differences? Hint: The infrared radiated toward the ground by water vapor is related to the infrared released by the Earth.

3. Does the Water Vapor Calibration of an Infrared Thermometer Change with the Season?

The infrared radiated toward the ground by water vapor in the sky is related to the infrared released by the earth. Does this mean that the calibration of an infrared thermometer changes with the seasons? How well do the water measurements measured by a calibrated infrared thermometer compare with a nearby GPS location when snow covers the ground? If the calibration does not work well when conditions are different from when you made your calibration, can you find a way to correct the calibration using the temperature of the air or ground?

Questions

1. The purpose of NOAA’s Ground-Based GPS-IPW project is to measure _________________________.

2. True or false: Without water vapor in the atmosphere, the oceans would freeze.

3. When using an infrared thermometer to measure the precipitable water vapor, the thermometer should be pointed (a) at a cloud (b) straight up or (c) at a 45-degree angle.

4. True or false: The precipitable water vapor (PW) is the same as the total column water vapor.

5. The most important natural greenhouse gas is _____________________________.

Going Further:

Do a web search to learn how precipitable water vapor can be measured by a sun photometer during daylight hours. A well calibrated sun photometer designed to measure water vapor can be used to calibrate an infrared thermometer.

This science project contributed by Forrest M. Mims III, Geronimo Creek Observatory, Texas.

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