A cloud weighs 500 t

Clouds fly with the wind, but they are not by any means weightless – 1 cm3 contains about 0.5 g of vapour. Hence, the total mass of even a small altocumulus cloud is close to 500 t. The reason why the clouds are still "flying" is that they are lighter than the air below them, just like a ship is lighter than the water below it.