Funnel clouds form when wind directions are different in layers at different heights generate rotation. A vortex develops as warm moist air is carried upwards by convection into cooler air above. Large circulating systems, called wall clouds, under the right conditions may spawn one or more smaller faster rotating tornadoes.
The energy source that drives a tornado is the release of latent heat from the condensation of rising water vapor. As the radius of a tornado reduces the angular velocity increases due to the conservation of angular momentum. Wind speeds in excess of 400 km/h are possible in the funnel walls.
Shelf clouds are not associated with wall clouds and with cumulonimbus (storm clouds). This small shelf was at the leading edge of a rampaging Bangkok thunderstorm.