Extended visual range during solar eclipses

During the solar eclipse on 21 August 2017, observed from Rexburg, Idaho, the eastern horizon for a short time period close to totality showed the contours of the Grand Teton mountain range from distances between about 80 and 90 km. In most of the partial phase before and after the eclipse, the visual range was below 80 km and the mountains could not be seen by the naked eye because of wildfire smoke in the air. We present photographic evidence of extension of the visual range during the eclipse and explain it in terms of a simple model for the daytime visual range. In this model, contrast between a black object and the sky increases whenever the ratio of nearby to distant air light reduces. As a consequence, we predict asymmetries with regard to totality and also that similar phenomena may be observed if part of the line of sight close to the observer is in deep shade of a thick cloud cover.