Funnel clouds or 'tuba' are extending, spinning fingers of cloud that reach towards the ground, but never touch it. When they do reach the ground they become a tornado.
A funnel cloud is a cone-shaped cloud which extends from the base of a cloud towards the ground without actually reaching the surface. In the UK they often look like thin dangling bits of rope, hanging from the cloud above. But in hotspots such as tornado alley in the USA, funnel clouds can sometimes be thicker and much more intense.
A rotating column of wind draws in cloud droplets, making a region of intense low pressure visible. They are formed in the same way as a tornado building around this localised area of intensely low pressure and are typically associated with the formation of cumulonimbus thunderclouds. For more information on the way tuba and tornadoes form see 'How tornadoes are formed'.
Cumulonimbus clouds are almost always the host cloud from which tuba form, meaning that heavy rain, hail, thunder and lightning can all be expected. If a funnel cloud does make contact with the ground and produce a tornado, very strong winds can be expected in the immediate vicinity of the vortex potentially causing severe damage.
Crucially, a funnel cloud does not reach the earth's surface, at the point it reaches land it becomes a tornado, or if it reaches a body of water it becomes a waterspout. In a typical year the UK sees around 30-35 tornadoes each year, though it is very rare that are they strong enough to cause any significant damage
Last updated: 12 August 2016