Earth's Highest Clouds
50-53 mile high noctilucent clouds seen over Somogy, Kaposfő, Hungary 15th June '12 by Rafael Schmall. Canon EOS1000D, 3-10s exposure at f/3-5.6, ISO 400-200. ©Rafael Schmall, shown with permission
Now is the time to search for noctilucent clouds.

Their visibility window extends from mid May to mid August in the Northern Hemisphere. They are best seen at high latitudes of 50 - 65° but are increasingly noticed further south - as here in Hungary.  In the New World they have been seen as far south as Utah and Colorado.

Look to the north in a non light polluted sky an hour or more after sunset or before sunrise. They can be distinguished from lower (and dark) tropospheric clouds by their much slower motion and sharp skein-like structure.  Binoculars help.
Atmospheric
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The clouds are composed of very small ice crystals floating a few km below the immensely cold mesopause. Temperatures below -123 Celsius are needed. They shine by scattered high altitude sunlight. The crystals are much to small to produce the colourful diffraction effects of stratospheric nacreous clouds.