Which of the following best defines a thermal?

The definition of a thermal in meteorological terms refers specifically to a rising column of air that is heated from below, generally by the sun warming the Earth's surface. This heated air becomes less dense than the cooler air surrounding it, causing it to rise. In aviation, thermals are significant as they provide lift for gliders and soaring birds, who utilize these rising currents to gain altitude without the need for an engine. Understanding thermals is crucial for pilots, particularly in flight planning and performance.

The other choices describe different atmospheric phenomena: a spinning vortex created by wingtip turbulence relates to the disturbances created by aircraft at the wingtips; a type of wind shear refers to a change in wind speed or direction with altitude, which can impact aircraft performance; and a current of cold air descending, while it describes another type of atmospheric movement, does not capture the essence of what a thermal represents. Thus, the correct answer accurately encapsulates the core concept of what a thermal is in the context of atmospheric science and aviation.