Thursday, January 31, 2013

Noctilucent Clouds

These rare, mystifying clouds are referred to as “night-shining” clouds, or noctilucent clouds. They are formed under very restrictive conditions and are only seen in the summer, at latitudes north of 50 degrees.They originate in the layer called the mesosphere; making them the highest cloud formations in our atmosphere.

Normally, they are far too faint to be seen, but they can be visible when illuminated by sunlight below the horizon. As with normal clouds, ice crystals within noctilucent cloud systems need two things to grow; water and something for the water to adhere to, usually dust. While ordinary tropospheric clouds get their source of dust from things like desert storms, this is not a viable medium in the mesosphere where this dust simply cannot reach. Accordingly, it is speculated that these cloud formations utilise dust particles from outer space.

As for the source of the water vapor necessary to produce clouds at such extreme altitudes, upward winds during the summertime are capable of carrying water droplets from the moist lower atmosphere toward the mesosphere. That's why noctilucent clouds only appear during the warm summer months.

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Picture courtesy of Science Photo Library.

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