photo #8481398

Above Vilnius, Lithuania, these noctilucent clouds wave silvery-blue fabric to announce that it is noctilucent season in the Northern Hemisphere.Noctilucent clouds occur over high latitudes (between 50 and 65 degrees north and south) and at extreme altitudes – near the upper boundary of the mesosphere, at altitudes of around 50 miles (80 km), up near the fringes of Space. For either hemisphere, the time of year for observing noctilucent clouds is in the middle of summer. This is when, counterintuitively, the air up at extreme altitudes gets cold enough to support the formation of ice crystals. It is also when, over higher latitudes, the Sun’s at the right depth below the horizon during the nighttime for its light to catch these extreme-altitude clouds and make them shine at night – which, as it happens, is what the Latin name noctilucent means.