Landscape photographers know that they will get particularly clear air the day after a low-pressure depression has gone through. Rain will have washed out dust particles so that atmospheric clarity is just about as good as it gets — long-distance visibility is very good indeed and from the right vantage point, it is possible to see objects very clearly at distances approaching 50 km. On the Met Office scale used by weather observers, this is Grade 9 visibility, the highest possible.
My two snapshots show the same point on the Northumberland coast on two consecutive days in late March 2009. The photos were taken more or less 24 hours apart. The first shows high seas under the influence of strong northerly winds. The second shows calm, clear conditions associated with the ridge of high pressure that followed the depression system.
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