Air-Shower Spectroscopy at horizons
  Horizontal and Upward air-showers are suppressed by deep atmosphere opacity and by the Earth shadows. In such noise-free horizontal and upward directions rare Ultra High Cosmic rays and rarer neutrino induced air-showers may shine, mostly mediated by resonant PeVs interactions in air or by higher energy Tau Air-showers originated by neutrino tau skimming the Earth. At high altitude (mountains, planes, balloons) the air density is so rarefied that nearly all common air-showers might be observed at their maximal growth at a tuned altitude and directions. The arrival angle samples different distances and the corresponding most probable primary cosmic ray energy. The larger and larger distances (between observer and C.R. interaction) make wider and wider the shower area and it enlarge the probability to be observed (up to three order of magnitude more than vertical showers); the observation of a maximal electromagnetic shower development may amplify the signal by two-three order of magnitude (respect suppressed shower at sea level); the peculiar altitude-angle range may disentangle at best the primary cosmic ray energy and composition. Even from existing mountain observatory the up-going air-showers may trace, above the horizons, PeV-EeV high energy cosmic rays and, below the horizons, PeV-EeV neutrino astronomy: their early signals may be captured in already existing gamma telescopes as Magic at Canarie, while facing the Earth edges during (useless) cloudy nights. 
D. Fargion