# Re-Visiting Acoustic Sounding to Advance the Measurement of Optical Turbulence  
[PERSON]  
1 Department of Engineering Physics, Air Force Institute of Technology, Wright Patterson Air Force Base, 2950 Hobson Way, Dayton, OH 45433, USA; [EMAIL_ADDRESS] (S.B.-P.); [EMAIL_ADDRESS] (K.K.) [PERSON]  
1 Department of Engineering Physics, Air Force Institute of Technology, Wright Patterson Air Force Base, 2950 Hobson Way, Dayton, OH 45433, USA; [EMAIL_ADDRESS] (S.B.-P.); [EMAIL_ADDRESS] (K.K.) [PERSON]  
1 Department of Engineering Physics, Air Force Institute of Technology, Wright Patterson Air Force Base, 2950 Hobson Way, Dayton, OH 45433, USA; [EMAIL_ADDRESS] (S.B.-P.); [EMAIL_ADDRESS] (K.K.) Footnote 1: [[https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci)]([https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci))  
2 Applied Research Solutions, 51 Plum Street, Suite 240, Beavercreek, OH 45440, USA  
Footnote 2: [[https://www.mdpi.com/journals/article/pdf/1](https://www.mdpi.com/journals/article/pdf/1)]([https://www.mdpi.com/journals/article/pdf/1](https://www.mdpi.com/journals/article/pdf/1))  
up-to-the-minute mission briefs [4] and current-mission system settings. The latter advance forecasts are built on a combination of increasingly sophisticated laser propagation and numerical weather prediction models. By emphasizing the high frequency measurement of the time of flight of an acoustic pulse transmitted between two probes, 3D sonic anemometers were originally conceived, and subsequently extensively applied, to assess temperature gradients and, thus, the temperature structure constant. However, limiting one's analysis to temperature gradients can lead to a sub-optimal assessment of optical turbulence strength during, for example, diurnal quiescent periods. During these periods, temperature gradients in all directions diminish to near zero--the evening quiescent period ends because the ground (due to radiative losses to space) cools the air near it and creates a vertical temperature gradient, and as the night goes along, horizontal gradients are established because of different cooling rates of different surfaces. The morning quiescent period breaks down because of the opposite effect of the sun heating the ground. At night (in stable conditions), waves along the stable layers--caused by wind and large object movement--create a spatial and temporal index of refraction fluctuations that create \"optical turbulence\" effects on laser/optical propagation. During the day, the sun heating the ground creates convection that supports more statistically characterize-able advecting turbulent eddies (e.g., Kolmogorov turbulence) that produce a spatial and temporal index of refraction fluctuations that, in turn, effect electromagnetic--especially optical and near infrared--wave propagation in the classical \"optical turbulence\" way. While temperature gradients are, in general, the largest contributor to the index of refraction gradients, humidity gradients and pressure perturbations also contribute. Calculating \(C_{n}^{2}\) from \(C_{7}^{2}\), in some cases, makes quiescent periods seem far more marked than they often are as measured by an optical instrument such as a scintillometer. Arrays of sonic anemometers measure the speed of sound in air in many directions, thus, they can conceivably capture gradients in the temperature, humidity (since this affects air density and the speed of sound), and the velocity. The velocity fluctuations are created by the small turbulent eddies or waves on the stratified layers that have small circulations (e.g., vortices) associated with them, which, in turn, produce pressure/density gradients that establish an index of refraction gradients with very little temperature change. These humidity and pressure/density variations often continue to exist during quiescent periods and in adiabatic layers, keeping actual \(C_{n}^{2}\) levels significantly higher than those characterized through a consideration of \(C_{T}^{2}\) only.