The Dutch artist Piet Mondrian is best known for the squares and rectangles of his abstract De Stijl paintings from the 1920s onwards. But this earlier seascape that he painted in 1909 features the very un-rectangular forms of a Cumulus congestus, purple against the pale gold sky, throwing its shadow over the lapping waves. Cumulus congestus is the towering form of Cumulus, when the cloud is on its way to developing into a fully fledged Cumulonimbus storm. No wonder Mondrian’s one looks like it wants to throw a punch, waving its lilac fist in the air.