Mechanism is vital in influencing how much sunlight clouds reflect – and whether or not they produce rain or snow
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Thousands of kilometres above us, clouds are quietly forming. From fluffy, cotton-wool cumulonimbus clouds to wispy cirrocumulus clouds, there are many types found in skies across the world.
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Recently, a new study has shed light on a fascinating mechanism impacting how exactly clouds form. Researchers have found that natural dust particles – swirling in from faraway deserts – can trigger the freezing of clouds in Earth’s northern hemisphere.
This mechanism might seem subtle, but in fact has far-reaching consequences. That’s because it directly influences how much sunlight clouds reflect, and how they produce rain or snow. As such, it has major implications for climate projections made by models.
To ascertain their findings, researchers – led by ETH Zurich – drew on 35 years of satellite data, focusing on mixed-phase clouds which form between -39C and 0C. These types of clouds are common in mid- and high-latitudes, particularly over the North Atlantic, Siberia and Canada.
Such clouds are known to be extremely sensitive to environmental changes, specifically to the presence of ice-nucleating particles, which stem from desert dust aerosols.
In studying these clouds, researchers found mineral dust – tiny particles swept up by the wind and carried into the upper atmosphere – can trigger the freezing of cloud droplets. The process is particularly important in northern regions, where clouds form in a temperature range just below freezing.
‘We found that where there’s more dust, clouds are much more likely to freeze at the top,’ explains post-doctoral researcher for Atmospheric Physics at ETH Zurich and lead author of the study Diego Villanueva. ‘This has a direct impact on how much sunlight is reflected back into space and how much precipitation is generated.’
The way clouds freeze directly affects how much sunlight they bounce back into space, and also how much water they release as rainfall. Importantly, these factors are vital for climate models – but until now, many models did not have a solid reference point for how cloud freezing works on a global scale.
‘It helps identify one of the most uncertain pieces of the climate puzzle,’ Villanueva continued.
Despite the findings of researchers in this study, the dust-ice link does not play out equally across the globe. For example, in the Sahara, cloud formation is sparse – with suggestions that the strong movement of hotter air may suppress the freezing process. Also, in the Southern Hemisphere, marine aerosols may replace the role of dust.
Researchers say that further studies should be carried out to better understand how other factors – such as updraft strength or atmospheric humidity – can impact cloud formation.
