Pilots can't see ash clouds at night. Also, ash particles are too small to return an echo to on-board weather radars on commercial airliners. Even when flying in daylight, pilots may interpret a visible ash cloud as a normal cloud of water vapour and not a danger—especially if the ash has travelled far from the eruption site. In the image from the Chaitén volcano, the ash cloud has spread thousands of kilometers from the eruption site, crossing the width of South America from the Pacific coast and spreading over the Atlantic.