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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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