A simple presentation is made of the optics involved in the nearly horizontal propagation of short radio waves above the spherical earth. The refraction produced by various types of atmospheric layers is discussed, especially the refraction in atmospheric layers close to a water surface and the resulting wide variation in possible range of communication between low-altitude terminals. A formula showing how the refractive index of air at radio wave lengths depends on moist-air variables is reproduced. A graph for the determination of potential refractive index is presented. As examples of over-water conditions, the vertical distributions of temperature, dew point, and potential refractive index are shown for two psychrometric ascents by airplane to 1000 feet.

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1 Contribution No. 361.