A hook echo is a radar signature which is associated with imminent tornadic activity. Hook echoes indicate either a tornado on the ground or a funnel cloud which has the potential to become a tornado. They are charactarized by a hook-shaped radar echo, which is caused by the rain shaft spiraling into the tornado. In doppler radar, which can detect wind direction and speed, hook echoes are even easier to spot. Hook echoes are used by storm chasers and the NWS to determine which storms are tornadic and this technology has greatly aided in increasing the warning time in tornado warnings.