A unique cloud formation was observed Wednesday night in the sky at dusk -- a faint blue and silver shimmer showing the presence of noctiluscent clouds.
Noctiluscent clouds, also known as polar mesospheric clouds, are rare for a few reasons. First, they are clouds located in the mesosphere of the atmosphere. The mesosphere sits above both the troposphere (the first 10 km of our atmosphere that contains much of our weather) and the stratosphere, which reaches a height of approximately 50 km and tends to be the vertical limit in which instruments sent up on weather balloons reach. That places the height of these clouds at approximately 80 km.
The clouds themselves are so high up and comprised of ice crystals so tiny that they are not visible during the day. The shimmer of these clouds only occurs at dusk due to the setting sun passing below the horizon, but with remaining rays striking at a high enough angle to reflect of those ice crystals producing a visible shimmer.
Despite being observed since the late 1800s, their formation process is still not completely understood. The formation of the clouds requires both dust and water vapour, both of which are rarely present in the mesosphere.
It is thought that the dust needed to act as a seed for the icy crystal formation is provided by micrometeors or even volcanic eruptions. Water vapour may occasionally rise to the troposphere, but is also a component of rockets used to reach orbit, and noctiluscent cloud formation has in the past been observed following passage of a space shuttle and, more recently, private company rocket booster launches.
I want to thank those keen observers last night for being quick on the camera and sharing the sight with us.
