Abstract
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.
Highlights
The effect of atmospheric refraction on light has been known and studied since antiquity.[1]
The faster, random motion is due to turbulence while the slow, vertical drift is due to changes in average refractive index gradient along the path
The U.S Standard Atmosphere model provides a single value for the atmospheric refractive index gradient
Summary
The effect of atmospheric refraction on light has been known and studied since antiquity.[1]. An imaging approach to measure the variations in the path-averaged refractive index gradient is described here. The imagingbased quantification of refractive bending can help to validate and improve fine-scale numerical weather prediction models by providing quantification of the vertical temperature lapse rate in the lower atmosphere. The vertical temperature lapse rate in the lower atmosphere is strongly influenced by the radiative properties of the surface, it can change significantly with cloudiness, terrain, and changes in ground cover (e.g., snow) yet it is only directly measured approximately twice a day with radiosonde balloon launches that are widely spaced geographically. If refractive bending observations were conducted with low-cost digital photography along with standard surface weather observations, the deduced temperature lapse rate could be used to increase forecast accuracy in the lower atmosphere
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