This is the "A River Scene, with Fishermen laying a Net" painted in 1638 by Jan van Goyen. You'll notice how the picture is almost entirely given over to a vast sky, but there's enough movement below to capture the atmosphere of a busy autumn day on the river. The series of diagonal lines sweeping across the picture creates a sense of dynamism - look at the net being hauled in by the fisherman, the sails of the boats midstream, and that tall pole in the distance.
It's interesting how van Goyen uses color to evoke the mood of the scene. There's a threat of rain from those grey clouds overhead, but then you spot this small patch of bright turquoise - it's the headscarf of a woman hunched up against the chill on board the sailing boat. It's a lovely touch that adds some warmth to an otherwise muted palette.
If you look closely across the calm waters, you can just make out the square church tower in the distance. It's thought to be the tower of the Grote Kerk in Dordrecht - a town van Goyen often depicted in his paintings.
A River Scene, with Fishermen laying a Net hangs in The National Gallery on Trafalgar Square, London — free to enter. Point your phone at any artwork there and audioguide.london plays a free audio guide in six languages — no app download needed.