Abstract
The enhancement of cirrus cloudiness by orographic effects along the periphery of a large-scale thin cirrus cloud system is illustrated. From radiometric measurements and comparison with a nearby sounding, it was found that the cloud tops coincided with the tropopause. Their high emissivity suggests cloud condensation in liquid form, which quickly freezes to form long ice-crystal plumes. These crystals may have seeded a supercooled liquid-droplet low cloud to cause snow at the ground.