The strange phenomenon, also known as a Willy-Willy, is due to a upward spiralling, dust filled vortex of air. It occurs when the ground is dry and high surface temperatures produce strong updrafts. But this dust devil in Buntingford, Herts., on Wednesday was suspicious enough to attract police attention. East Herts Rural Police tweeted a video of the spiralling straw and said:" On patrol near Buntingford & we found this straw tornado.