Public Release: 

Puzzling height of polar clouds revealed

British Antarctic Survey

Scientists have discovered why icy clouds found at the edge of space are higher at the South Pole than at the North. The answer to this puzzle is that the intensity of solar radiation at the South Pole is six percent higher than at the North Pole during the austral summer, as the Earth comes closer to the sun. New research from British Antarctic Survey and University of Illinois is reported in this month's Geophysical Research Letters (online 29 January 2004). This research helps understand the role of these clouds as indicators of climate change.

Polar mesospheric clouds form at an altitude of 52 miles at the summertime polar caps when temperatures in the mesosphere fall below -125 degrees Celsius. Scientists were puzzled why clouds at the South Pole were on average consistently two miles higher than those found in the North. To confirm these geographic differences, measurements were taken at British Antarctic Survey's Rothera Research Station, 1500 miles from the South Pole, at the same latitude as measurements made in the northern hemisphere (68°). Using a laser radar (LIDAR) to bounce light pulses off the clouds and measure their distance from earth, the researchers demonstrated that even though the clouds were slightly lower at Rothera than at the South Pole, they were considerably higher than at similar latitudes in the northern hemisphere.

Since the Earth's orbit is not exactly circular, solar radiation at the South Pole is six percent higher that at the North as the Earth orbits the Sun. Using a model to explore temperature and vertical wind distribution, the researchers concluded that this increased solar input heats the polar ozone and creates a vertical upwelling that forces the clouds up higher than in the north.

Polar mesospheric clouds have brightened by approximately 15% over the last twenty years demonstrating a cooling of the mesosphere. This cooling intensifies as the atmosphere near the Earth's surface warms, so polar mesospheric clouds may be an indicator of long-term global climate change.

Pat Espy, scientist at the British Antarctic Survey explains: "The growing brightness of polar mesospheric clouds is attributed to increasing levels of atmospheric carbon dioxide and methane, which in the upper atmosphere lead to cooler temperatures. By understanding more about how and where these clouds form scientists can use them as a measurement of long-term global climate change."

Espy and his team took measurements using a LIDAR (Light Detecting and Ranging System), which transmits a light beam up to 52 miles into the mesosphere.


Issued by British Antarctic Survey. Press enquiries to BAS Press Office:
Athena Dinar Tel: 122-322-1414, mobile: 774-082-2229,
For more information contact Dr Pat Espy, Tel: 122-322-1255, mobile: 775-319-5089,

Lidar Observations of Polar Mesospheric Clouds at Rothera, Antarctica by Xinzhao Chu, Graeme Nott, Patrick Espy, Chester Gardner, Jan Diettrich, Mark Clilverd and Martin Jarvis is published in Geophysical Research Letters, Vol 32, online 29 January 2004.

Research on solar variability is part of the BAS core science programme 'Geospace - Atmosphere Transfer Functions', which examines the Antarctic upper atmosphere to understand global upper atmospheric circulation, temperature balance, short-term variability, long-term changes and how these changes may be linked to human activity.

Polar Mesospheric Clouds are the highest on Earth and are only seen during the summer solstices near the North and South poles. They are created when cloud particles grow in size and mass and they slowly fall to lower altitudes where the combined effects of increasing atmospheric density and the upwelling air mass provide sufficient buoyancy to cause them to collect in thin layers.

The team at University of Illinois made the first measurements at the North Pole in 1999 and at the Amundsen-Scott South Pole Station at the South Pole over the 1999-2000 and 2000-2001 summer seasons. The lidar is now operational at British Antarctic Survey's Rothera Research Station. In 2003, scientists confirmed that clouds measured at Rothera were higher than at the northern counterparts even though stationed at the same latitude. Scientists from University of Illinois and the High Altitude Observatory of the National Center for Atmospheric Research in Boulder, Colorado compared the cloud measurements with predictions from a global circulation model of the upper atmosphere.

British Antarctic Survey is responsible for most of the UK's research in Antarctica. It is a component of the Natural Environment Research Council. More information about the work of the Survey can be found on the website at

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