Dust picked up locally from the surface of the earth and blown about in clouds or sheets. It is classed as a lithometeor and is encoded BLDU as an obstruction to vision in an aviation weather observation (METAR). (Encoded as BD in SAO observation format. ) Blowing dust may completely obscure the sky; in its extreme form it is called a duststorm. A layer of stable air aloft tends to stop the vertical transport of dust by eddies. There is then a sharply defined upper limit to the dust layer.