APOD: Noctilucent Clouds over London (2014 Jul 10)

[APOD Robot]

Noctilucent Clouds over London

Explanation: This scene from the early morning hours of July 3 looks out across the River Thames from the Westminster Bridge. Part of a luminous timelapse video (vimeo), the frame captures a sight familiar in London, the nighttime glow of the London Eye. But a not-so-familiar sight is shining in the still dark sky above, widespread noctilucent clouds. From the edge of space, about 80 kilometers above Earth's surface, the icy clouds can still reflect sunlight even though the Sun itself is below the horizon as seen from the ground. Usually spotted at high latitudes in summer months the diaphanous apparitions are also known as polar mesospheric clouds. The seasonal clouds are understood to form as water vapor driven into the cold upper atmosphere condenses on the fine dust particles supplied by disintegrating meteors or volcanic ash. NASA's AIM mission provides daily projections of the noctilucent clouds as seen from space.

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[Jasja]

Cool! The following night I witnessed some very bright noctilucent clouds in Amsterdam, and was able to make a timelapse video as well. Not as nice as this one...

[zbvhs]

The view is to the north. The clouds are illuminated by sunlight coming over the North Pole during the summer months. Even though the Sun has set on the ground on the Arctic Circle, sunlight still extends to high altitudes far south of the Arctic Circle.

[neufer]

The view is to the north. The clouds are illuminated by sunlight coming over the North Pole during the summer months. Even though the Sun has set on the ground on the Arctic Circle, sunlight still extends to high altitudes far south of the Arctic Circle.

The view is to the north-EAST.

The clouds are illuminated by sunlight from no further than ~1000 km away to the NE (over Oslo?).

<<Night clouds or noctilucent clouds are composed of tiny crystals of water ice at a height of about 76 to 85 km

Noctilucent clouds form predominantly during summer when, counter-intuitively, the mesosphere is coldest, therefore they can't be observed (even if they are present) inside the Polar circles because the Sun is never low enough under the horizon at this season at these latitudes. Noctilucent clouds form mostly near the polar regions [at latitudes between 50° & 70°] because the mesosphere is coldest there.

Climate models predict that increased greenhouse gas emissions cause a cooling of the mesosphere, which would lead to more frequent and widespread occurrences of noctilucent clouds. A competing theory is that larger methane emissions from intensive farming activities produce more water vapour in the upper atmosphere. Methane concentrations have more than doubled in the past 100 years.>>