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Feathery cirrus clouds have a cold metallic heart

发布时间:2017-12-01 05:01:37来源:未知点击:

ETHEREAL they may look but cirrus clouds form around metals and minerals. Clouds form when the temperature is low enough for water vapour in the air to condense into droplets or turn into ice crystals, depending on the altitude. Often this happens when the vapour collects around “seeds” such as bacteria, soot or other particles suspended in the air. This has been well studied for the low-hanging clouds, such as cumulus, but what goes on in cirrus clouds has been more of a mystery because the altitudes they form at are often higher than those at which planes fly. But this hasn’t deterred all researchers. A team headed by Daniel Cziczo of the Massachusetts Institute of Technology has compiled data from four series of research flights sampling cirrus over North and Central America and the nearby oceans (Science, doi.org/mg4). They found that 61 per cent of ice crystals form around two types of seed that account for less than 1 per cent of the particles found at the altitudes where cirrus forms. These are the fine mineral dust from sources such as deserts, and specks of metals like lead, zinc, tin,