A LAYER of high, invisible cirrus clouds has been overlooked by weather balloons. If the clouds, which trap the Earth鈥檚 radiation, are more abundant than previously thought, today鈥檚 models for predicting global warming could be inaccurate.
Clouds at altitudes of around 8 to 14 kilometres are invisible to the naked eye because they鈥檙e so thin and diffuse. Researchers knew they existed but didn鈥檛 realise that standard sensors on weather balloons weren鈥檛 picking them up, so they may have vastly underestimated the amount of cloud at those altitudes.
The discrepancy was discovered by Junhong Wang of the US鈥檚 National Center for Atmospheric Research in Boulder, Colorado, when she tested newly developed 鈥淪now White鈥� sensors. These sensors rely on mirrors that are frozen to temperatures below those in their surroundings. Moisture from the atmosphere coats the mirror with frost, giving accurate readings of humidity at air temperatures as low as 鈭�40 掳C. Standard sensors fail to give reliable readings below about 鈭�4 掳C. Wang sent up weather balloons carrying Snow White sensors alongside standard sensors. When she compared the readings, Wang found that Snow White recorded air humidity levels of 90 to 100 per cent 鈥� a strong sign of clouds 鈥� while the standard sensors registered just 10 to 30 per cent.
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Last week, at the annual meeting of the American Meteorological Society in Long Beach, California, Wang said that some of her findings have been backed up by laser-based radars on the ground, the only other device capable of detecting these clouds.
The finding means that climate models used to predict the effects of global warming may need to be revised. 鈥淚 think these sub-visible cirrus clouds are widespread,鈥� says David Carlson, head of NCAR鈥檚 Atmospheric Technology Division, where Wang works. 鈥淲e need to recalibrate the old data.鈥�
He thinks this could be done through further balloon experiments around the globe over the next two years or so, again lofting Snow White sensors to high altitudes alongside their standard counterparts. The disparities in the readings could be used to re-calibrate all historic data on the clouds.
Joanna Haigh, an expert on atmospheric physics at Imperial College, London, thinks the arrival of a more accurate sensor is long overdue. 鈥淭he older instruments were probably consistently underestimating upper-tropospheric humidity,鈥� she says.
There can be no doubt surface temperatures are increasing, but the impact of high cirrus clouds on global warming can now be factored into predictions.