For climatologists, a key question isn't how high is the sky, but how hot is the sky. Understanding the temperature of various levels of the atmosphere is essential to developing global climate models. Now lidar, a photonics technology used to measure distance, is gauging atmospheric temperature up to the near reaches of outer space. Using lidar systems to monitor iron resonance lines, researchers are developing a better understanding of atmospheric temperature structures to produce better models of the global climate. Courtesy of the University of Illinois. Climate change should be most evident over the North and South poles, but measuring atmospheric temperature there has been a challenge, especially at higher altitudes, because balloon-borne instruments can reach only 30 km, or the middle of the stratosphere. Researchers from the University of Illinois, The Aerospace Corp. in El Segundo, Calif., and the National Center for Atmospheric Research in Boulder, Colo., have developed lidar systems that extend the measurements up to 110 km, into the lower thermosphere. To investigate these regions of the sky, the scientists sample trace iron atoms from meteorites. Chester Gardner, a professor at the university and the leader of the research team, explained that the study requires two lidar systems, tuned to the 372- and 374-nm iron resonance lines, respectively. The systems use diode-laser-seeded, flashlamp-pumped, frequency-doubled alexandrite lasers with an average power of 3 W and a firing rate of 30 pulses per second. The pulses of light travel up and are reflected back by the air itself at up to 80 km or by iron atoms at 80 to 110 km above the Earth. Telescopes on the ground equipped with photomultiplier tubes detect the returning ultraviolet photons. From this data, the researchers derive an atmospheric density profile and a temperature measurement. Thus far, the results obtained at the North and South poles have been in agreement with climate models, but measurements taken during May 2000 at the South Pole revealed that the lower mesosphere was 20 °C warmer and the upper mesosphere was 20 °C cooler than predicted. The cause and significance of the disparity remains unclear, and observations continue. However, Gardner noted, the work establishes the baseline temperature structure of the atmosphere against which researchers can compare future changes in the global climate.