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The sea level vertical refractive index gradient in the U.S. Standard Atmosphere model is −2.7×10−8  m−1 at 500 nm. At any particular location, the actual refractive index gradient varies due to turbulence and local weather conditions. An imaging experiment was conducted to measure the temporal variability of this gradient. A tripod mounted digital camera captured images of a distant building every minute. Atmospheric turbulence caused the images to wander quickly, randomly, and statistically isotropically and changes in the average refractive index gradient along the path caused the images to move vertically and more slowly. The temporal variations of the refractive index gradient were estimated from the slow, vertical motion of the building over a period of several days. Comparisons with observational data showed the gradient variations derived from the time-lapse imagery correlated well with solar heating and other weather conditions. The time-lapse imaging approach has the potential to be used as a validation tool for numerical weather models. These validations will benefit directed energy simulation tools and applications.


This is an open access article published by SPIE and distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. CC BY 3.0

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Optical Engineering