How to use a Sun Photometer:

To use the photometer to obtain water vapor observations is simple.  The following need to be recorded.

1.  Location of the observer. (Latitude and Longitude).

 

2.  Date and the Universal Time (UT) of when the observation was made. (UT is obtained by noting the local standard time and accounting for the time zone).

3.  Solar elevation angle.  (This can be computed or validated from 1 and 2).

4.  Voltage in the two "channels" of the Sun Photometer.

5.  Dark current (electrical signal produced in the detectors when it is not being pointed at the Sun). The measurement protocol sheet will be posted here when the program begins.

 

 

Where do I obtain the Sun Photometer to measure Water Vapor?

 

Sun Photometers for the GLOBE/GIFTS Water Vapor Special Measurement will be available from GIFTS Education & Public Outreach Program.  Contact us.

Can I build one?

It is indeed possible to construct a Water Vapor Sun Photometer (follow the instructions for the Haze Photometer) but use two LEDs (emitting at 920 and 950 nm).    However, if you or your school wishes to submit data, we request that you use the Photometer provided by the GIFTS EPO Program as these units will have been calibrated against the professional instruments.  SOLAR, the Education and Public Outreach Program for NASA's SAGE III Mission is utilizing Sun Photometers built by school students.  While the Aerosol and Water Vapor units are very similar, the calibration process is different. The Photometer calibration depends on specific LEDs used, and it is a fairly involved process to calibrate a unit.  Unlike the aerosol protocol where it is possible to calibrate the instrument through a Langley Plot, the Water Vapor measurement depends on a ratio of two separate measurements and given the variability of atmospheric water vapor in space and time, a Langley Plot calibration is not possible.  Given the variability in different LEDs, the calibration of the instrument cannot be generalized from one LED source to another.

If you have access to independent comparative data, you may be able to compare the data from your device and generate a calibration for your instrument.