Electronics Puzzler Solution: Greenhouse GasBy: Forrest M. Mims III
Measuring the most important greenhouse gasEarth would be considerably cooler without water vapor in the atmosphere. This is why water vapor is the number one greenhouse gas. Science teacher Bob Celsius wants to show his students how to measure the total amount of water vapor overhead with the help of a simple, very useful instrument that is ordinarily used to measure temperature.
Water vapor strongly absorbs and then re-emits infrared radiation. This makes a moist sky much warmer than a dry sky. Therefore, Bob chose a Jameco infrared (IR) non-contact thermometer to conduct his demonstration.
Bob then took his class outdoors and pointed the IR thermometer at the base of a low cumulus cloud. The thermometer indicated a temperature of 60°F, which was close to the dew point that day. He then pointed the thermometer straight overhead at the clear zenith sky. This time the thermometer indicated a much, much colder temperature of 10°F. This indicated that the clear sky was much drier than the moist cloud.
Bob assigned a team of students to repeat measurements of the sky directly overhead when no clouds were present. The students did this for a series of very dry to very moist days. Bob then retrieved the total water vapor data closest to the measurement times from the nearest National Oceanic and Atmospheric Administration's (NOAA) GPS water vapor receiver in the GPSMet Network.
Bob then entered the clear sky temperatures and the total water vapor data into a spreadsheet program. He used the spreadsheet to make an xy scatter graph of the data and found the equation that best fit the data. The resulting equation allowed his team to estimate the total water vapor in the sky simply by pointing the IR thermometer at the zenith at any time (day or night) when no clouds were overhead.
The graph below is a plot of a few years of water vapor data I've measured using an IR thermometer. You don't need nearly this much data to get started. Just try to take measurements on a series of days ranging from very dry to very moist.
While the equation on this graph allows the total water vapor to be estimated by entering the zenith temperature indicated by the IR thermometer, it's also possible to estimate the water vapor simply by visually finding the intersection of the red curve fit with the zenith temperature on the x axis and the total water vapor on the y axis.
This Jameco Electronics Puzzler has practical applications. Amateur weather watchers and science fair students can use an IR thermometer to keep a daily log of total water vapor and study seasonal long term trends.
While the readout of an IR thermometer indicates temperature, the number is also proportional to the infrared the sensor receives. This is important when an IR thermometer is pointed at the sky, for the temperature displayed by the readout cannot be assigned to any particular layer of air. For this reason, it's best to think of the number in the display as being proportional to the total infrared being received from the sky instead of the "temperature" of the sky.
Since February 1990, I've been measuring total water vapor on most days using a sun photometer pointed at the sun. For the past several years I've also been making these measurements using several different IR thermometers. After collecting two years of data, I worked with NASA's Lin Hartung Chambers and David R. Brooks of the Institute for Earth Science Research and Education to write the paper "Measuring Total Column Water Vapor by Pointing an Infrared Thermometer at the Sky" for the Bulletin of the American Meteorological Society (October 2011, pp. 1312-1320). This paper provides full details on how to measure total water vapor over your head by means of an inexpensive IR thermometer.
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