The tense is the gerundive, the ongoing present: breeding, mixing, stirring, covering. This is a feature common to all weather reports. Listen out for it the next time you hear the shipping forecast or watch the weather section of the news. They never use an active verb, never say ‘it will rain’, ‘winds will blow’: it is always ‘becoming windier’, ‘clearing up’,  ‘moving westward’, ‘growing brighter’. I asked Alex Hill [head of London weather centre] why this was. ‘Simple’, he said. ‘You can only describe weather as being in transition from a point A to point B. In transition between – never at.’ In dramatic terms, it is called in medias res: in the middle of things. 
McCarthy, T ‘Meteomedia; or, Why London’s Weather Is in the Middle of Everything’ in Kerr, J and Gibson, A (eds) (2003) London From Punk to Blair London: Reaktion