Pole­ward drift

Cloud for­ma­tion has seen an in­crease at the poles while the trop­ics have be­come clearer over the past 60 years

Down to Earth - - COVER STORY -

Clouds are pri­mar­ily made of wa­ter. Through evap­o­ra­tion and pre­cip­i­ta­tion, all the wa­ter in the at­mos­phere gets ex­changed once in 10 days At any point, around 60 per cent of the planet is cov­ered by clouds. But the to­tal cloud cover has de­clined over the years Cloud cover over oceans has re­duced by 0.04 per cent per decade dur­ing 1954-2008, while the re­duc­tion rate over land has been 0.4 per cent dur­ing 1971-2009 By trap­ping heat, clouds in­crease the earth's tem­per­a­ture by 7ÊC. But they also cool the planet by re­flect­ing sun­light. A cloud­less earth would be 12ÊC warmer

Equa­tor High-level clouds have sig­nif­i­cantly in­creased in the west of North Amer­ica Pa­cific Ocean NORTH AMER­ICA Mid­dle-level clouds have de­creased close to the poles Fewer mid­dle-level clouds are seen over South Amer­ica and the Caribbean High-level clouds have de­creased in the east of North Amer­ica At­lantic Ocean South Amer­i­can mon­soon cloud move­ment SOUTH AMER­ICA Sig­nif­i­cant in­crease in mid­dle-level clouds over South Amer­ica and Africa EUROPE AFR ICA African mon­soon cloud move­ment Cloud cover over equa­to­rial Africa has in­creased

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