Hidden cirrus clouds make the earth warmer

Published: Saturday 15 March 2003

clouds and water vapour, hidden until now from most atmospheric sensors, are influencing global climate. They were recently detected by Junhong Wang and his colleagues from the us-based National Centre for Atmospheric Research (ncar). The clouds have little affect on daily weather forecasts. But they tend to warm the planet, by allowing sunlight to enter the atmosphere while trapping the radiation emitted from the ground.

The researchers analysed humidity data collected from the lowest few kilometres of the atmosphere by radiosondes (weather balloons). They compared readings taken by two standard sensors (commonly used on radiosondes) with data from Snow White, a high-quality sensor positioned on a satellite.

For cold air blowing 7,924 to 12,801 metres above the Earth's surface, standard sensors showed relative humidity ranging between 10 to 30 per cent. In contrast, Snow White showed pockets of moisture with relative humidity varying from 90 to 100 per cent -- a strong sign of clouds. A ground-based lidar (laser-based radar) conformed the findings.

How did these clouds escape detection? Experts say that many of them are 'sub-visible cirrus', which are so faint that both ground-based observers and satellites have trouble tracing them.

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