English (UK)
Noctilucent clouds are water-ice formations that appear in the highest atmosphere, at altitudes ranging around 85 km. They are tracers of the atmospheric conditions at that altitude. They constitute the only source of information about these layers of the atmosphere, since their altitude is too high to be reached by balloons, but too low to place there an artificial satellite in a stable orbit.
Noctilucent clouds appear as bluish patches with some white hue, that show clearer than the cobalt-blue, darker sky, that can be seen after or before sunset from observing sites with excellent atmospheric conditions, as Calar Alto. The fact that these cloud formations are clearer than the background does not imply, at all, that they emit light on their own. They appear lighter because they are placed very high, where direct sunlight still reaches them, even long times after the effective sunset, or before the effective sunrise, at sea level.
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Noctilucent cloud observed at the Leibniz Institute for Atmospheric Physics in Kühlungsborn, Germany |
These clouds have been observed for some 125 years. They normally appear close to the Polar Regions, beyond ±50° latitudes. However, the solar illumination conditions at extreme polar latitudes normally impede their observation. Their polar character explains that, when observed from space, noctilucent clouds are normally called polar mesospheric clouds. In fact, NASA has a satellite devoted only to this purpose: AIM. Also, this phenomenon displays a strong seasonal character, with a marked tendency to increase in summer, peaking ten days after the summer solstice.
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Noctilucent clouds observed from Calar Alto Observatory in June the 14th 2012 |
The German team, led by the Leibniz Institute for Atmospheric Physics (IAP), after five years of observations, has been able to locate noctilucent clouds from Calar Alto Observatory in the evenings of past 14th th and 18th of June 2012. Their calculations indicate that the clouds were placed over the Pyrenees, or slightly to the south of this mountain range separating France from Spain.
The observation of noctilucent clouds at these medium latitudes, and even before the beginning of summer, implies that extreme conditions (temperature, water vapour content, wind) are developing in the mesosphere. At the boundary between the atmosphere and space, the formation and evolution of noctilucent clouds is influenced both by atmospheric conditions and by the state of the cosmic environment of the Earth, mainly solar activity, what explains that these clouds show a periodic modulation linked to the solar activity cycles of 11 years.
In words of the researcher from IAP, Dr. Gerd Baumgarten, “There is no explanation or model prediction, why the clouds can occasionally move as far south as Spain, but we guess it is due to extreme cold temperatures, strong southward winds or increased water vapour. So one could say the clouds above Spain manifest extreme weather events in the middle atmosphere.” In fact, several studies relate the behaviour of noctilucent clouds to climate change.
The experiment of remote observation with digital cameras from Calar Alto has been working for five years, but this research team has a much longer experience in mesospheric studies. “We dedicate several lidar instruments to the observation of the clouds to measure temperature and water amount in the clouds, but in addition we also run the cameras to get a broader perspective”, says Baumgarten.
This observation sets now a new record of a low latitude observation site for noctilucent clouds. This fact, linked to their other puzzling features, poses a challenge to mesospheric science.
Images and files:
Noctilucent clouds observed from Calar Alto Observatory in June the 14th 2012
Noctilucent clouds observed from Calar Alto Observatory in June the 14th 2012
Google Earth based map showing the position estimated for the noctilucent clouds observed from Calar Alto in June the 14th and 18th 2012
Link to a Google Earth file that displays the position estimated for the noctilucent clouds observed from Calar Alto in June the 14th and 18th 2012
Lidar sytem for atmospheric studies at ALOMAR research station, in Norway. Leibniz Institute for Atmospheric Physics
Noctilucent cloud observed at the Leibniz Institute for Atmospheric Physics in Kühlungsborn, Germany
Cover of January’s 2012 issue of Geophysical Research Letters showing an outstanding image of noctilucent clouds
Link to time-lapse video composed from images taken with the IAP camera at Calar Alto in May 24th-27th 2012
© Calar Alto Observatory, July 2012