A mesocyclone is the vortex of air—usually between 1 to 6 miles (2 to 10 kilometers) in diameter—often found within a supercell or other large thunderstorm with cumulonimbus clouds. Wind shear resulting in abrupt changes in wind direction or speed in the storm causes air to circulate in a rolling fashion parallel to the ground; an updraft can then orient the swirling air vertically, thus forming a vortex perpendicular to the ground.