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| Earth's Highest Clouds Now is the time to start searching for noctilucent clouds. Darlisa Black (Starlisa Black Photography, atmos optics) captured these over Mount Adams, Washington, USA. ©Darlisa Black, shown with permission. The NLC 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 - as here - further south. In the U.S. 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. |
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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. Their crystals are much too small to produce the colourful diffraction effects of stratospheric nacreous clouds. |