Bank up

Next, heavy soft clouds, banking up till the sky becomes overcast, are recorded, easily recognised by their lowering appearance, due to the dampness of the air, which also causes them to reflect the glare of large fires or the lights of cities[…]

Then turning: "There'll be more rain," he said, "you can see the clouds banking up to the North-West, larger than these."

... ominous clouds banking up and darkening the sky behind them.