Optical instability of the earth's atmosphere
  Meteorological data have been used to calculate refractive index fluctuations - the indicator of optical instability of the Earth's atmosphere. The calculations were made for standard pressure levels of the atmosphere in winter and summer. They are presented as distributions over the Earth's surface. The findings enabled us to determine preferred areas for astronomical observations as well as to compare astroclimate conditions of the world's largest observatories. 
Pavel G. Kovadlo, Olga S. Kochetkova
astro-ph.IM