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Polar stratospheric clouds appear over Cambridge

Image courtesy Peter Gallimore

Polar stratospheric clouds made a rare appearance over the skies of Cambridge on Monday 1 February.

Researchers in the Chemistry Department were able to view the clouds from their windows, as the clouds became visible around sunset.  

Polar stratospheric clouds (also known as nacreous clouds) are extremely rare over England because they only form when temperatures drop to extremely low levels in the stratosphere.  Stratospheric temperatures over Cambridge normally range between -50 and -70 degrees C. They must drop to -80 degrees C before these clouds can form. The low temperatures create conditions for ice crystals to form in the stratosphere, which in turn allows the clouds to form.  Reactions occurring on the clouds are a critical step leading to polar ozone loss. The last time these clouds were visible in Cambridge was in 1995-1996. 

As their name implies, the polar stratospheric clouds usually form only in polar regions as that is where the extremely cold stratospheric temperatures are present. In the first week of February, the stratospheric polar vortex which contains the extremely cold air was displaced away from the Arctic and began swinging over northern Europe.

Image courtesy Lekan Popoola