A cold air funnel is a short, narrow funnel cloud that hangs from a modest shower or weak thunderstorm growing under an unusually cold air mass aloft. Most stay well above the ground, and the rare ones that reach the surface produce only weak, brief tornadoes.
How they form
The setup does not look like classic tornado weather. A pool of unusually cold air aloft, often on the back side of an upper-level low or a cutoff low, sits over air only modestly warmed at the surface. That produces steep low-level lapse rates and just enough instability to launch shallow showers and weak thunderstorms.
Rotation comes from low-level wind shear near cloud base and from local stretching under the shallow updrafts, not from an organized mesocyclone. Because the parent cell is small and does not extend deep into the troposphere, any funnel that forms is narrow and short-lived.
How spotters recognize and report them
Cold air funnels appear most often in spring and early fall when the atmosphere is transitioning between seasons. They hang from small, unimpressive cumulus congestus or weak cumulonimbus on quiet, cool afternoons, sometimes several at a time, rather than in the middle of an active severe weather episode.
Reports still matter. Radar does a poor job of detecting these features because the parent cells are shallow and the circulations are small, so the National Weather Service asks spotters to call in sightings so warnings can be issued for the rare funnel that touches down.
What they are not
A cold air funnel is not a supercell tornado. The parent storm lacks a persistent rotating updraft, deep-layer shear is usually weak, and any tornado that does form is normally EF0.
They are also distinct from landspouts, even though both are non-mesocyclonic. Landspouts develop under towering cumulus along a boundary in a warm, humid, unstable air mass. Cold air funnels are the cool-atmosphere version of the same non-supercell physics: weaker environment, weaker outcome, and rarely a surface touchdown.