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Forrest M. Mims III

The Global Learning and Observations to Benefit the Environment (GLOBE) Program is an international network of schools in 71 countries that monitors up to 20 environmental parameters. Recently GLOBE added a haze-monitoring program to its measurement protocols. This network has the potential of providing important data about changes in the aerosol optical depth of the atmosphere caused by weather fronts, industrial and automobile pollution, and smoke from forest and brush fires and volcanic eruptions. Initially, monitoring will be conducted with an inexpensive, single-channel (520 nm) sun photometer. Unlike conventional sun photometers that use interference filters that are subject to unpredictable and rapid degradation, the GLOBE instrument uses a common light-emitting diode (LED) as a spectrally selective detector. Annual calibrations of two LED sun photometers at Mauna Loa Observatory since 1992 show that these instruments have insignificant degradation when compared to filter sun photometers. Some 175 prototype versions of a kit LED sun photometer have been assembled and tested by students from 16 countries at the University of the Nations and by more than 130 high school teachers in various pilot studies. These studies have demonstrated that even inexperienced students and teachers can quickly assemble a sun photometer from a kit of parts and perform a reliable Langley calibration. The pilot studies have also demonstrated that sun photometery provides a convenient means for allowing students to perform hands-on science while they learn about various topics in history, electronics, algebra, statistics, graphing, and meteorology.

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Forrest M. Mims III, Lin Hartung Chambers, and David R. Brooks

A 2-yr study affirms that the temperature indicated by an inexpensive ($20–$60) IR thermometer pointed at the cloud-free zenith sky (Tz) is a proxy for total column water vapor [precipitable water (PW)]. From 8 September 2008 to 18 October 2010 Tz was measured either at or near solar noon, and occasionally at night, at a field in south-central Texas. PW was measured by a MICROTOPS II sun photometer. The coefficient of correlation (r 2) of PW and Tz was 0.90, and the rms difference was 3.2 mm. A comparison of Tz with PW from a GPS site 31 km northnortheast yielded an r 2 of 0.79 and an rms difference of 5.8 mm. An expanded study compared Tz from eight IR thermometers with PW at various times during the day and night from 17 May to 18 October 2010, mainly at the Texas site, with an additional 10 days at Hawaii's Mauna Loa Observatory. The best results were provided by two IR thermometers that yielded an r 2 of 0.96 and an rms difference with PW of 2.7 mm. The results of both the ongoing 2-yr study and the 5-month comparison show that IR thermometers can measure PW with an accuracy (rms difference/mean PW) approaching 10%, which is the accuracy typically ascribed to sun photometers. The simpler IR method, which works during both day and night, can be easily mastered by students, amateur scientists, and cooperative weather observers.

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David R. Brooks, Forrest M. Mims III, and Richard Roettger


An inexpensive two-channel near-IR sun photometer for measuring total atmospheric column water vapor (precipitable water) has been developed for use by the Global Learning and Observations to Benefit the Environment (GLOBE) environmental science and education program and other nonspecialists. This instrument detects sunlight in the 940-nm water vapor absorption band with a filtered photodiode and at 825 nm with a near-IR light-emitting diode (LED). The ratio of outputs of these two detectors is related to total column water vapor in the atmosphere. Reference instruments can be calibrated against column atmospheric water vapor data derived from delays in radio signals received at global positioning satellite (GPS) receiver sites and other independent sources. For additional instruments that are optically and physically identical to reference instruments, a single-parameter calibration can be determined by making simultaneous measurements with a reference instrument and forcing the derived precipitable water values to agree. Although the concept of near-IR detection of precipitable water is not new, this paper describes a first attempt at developing a protocol for calibrating large numbers of inexpensive instruments suitable for use by teachers, students, and other nonspecialists.

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