"If you pour milk into your tea and give it a quick stir... liquids mix in beautiful swirling patterns... not... merging instantaneously. ...If you look down on Earth from space, you... often see... similar swirls in the clouds... where warm... and cold air waltz around... instead of mixing directly. In Britain... they form at the boundary between cold polar air... and warm tropical air... We know these swirls as depressions or s..."