Measuring the refractive index gradient of the atmosphere

We recently developed a method to measure the gradient of the refractive index of the atmosphere over short paths, by using the lateral fluctuations of thin laser beams along the given path (See: C. Innocenti and A. Consortini: Refractive index gradient of the atmosphere at near ground levels, in J.of Modern Optics, vol 52, p.671-689, 2005). We showed that the horizontal and vertical instantaneous fluctuations of a ray on a screen at the end of the path are related to the horizontal and vertical components, averaged along the path, of the refractive index gradient, respectively. Our method, developed in the ray optics approximation, also called geometrical optics approximation, requires that the beams are thin enough. This means that their transversal dimensions must be not larger than the turbulence inner scale. Thin beams allow one to make local measurements, even over few meter paths. Here we report the results of a measurement campaign, that we made in 8 different days in the open air in different locations and under different conditions, by using four parallel beams. Temperature and transverse wind velocity were also measured and correlation with refractive index gradient evaluated. Some cases of strong vertical anisotropy are also shown, which can be related to the local air convective movements from the ground. This paper describes the complete original set of the experimental results, that are not published elsewhere.