Black cloud of doom

There are several things I don't like to see as I stand underneath an electricity pylon, preparing for my son's game of football. Clouds like this are one of them.

It was like something out of the Lord of the Rings: the Winged Steeds of the Nazgûl preparing to attack. Fortunately, as quickly as it formed (and it formed very quickly), it also dissipated. And ten minutes later, it was as if nothing had happened. (See extra.)