Funnel clouds, usually short-lived, that develop from relatively small showers or thunderstorms when the air aloft is very in cold. Cold air funnels may touch down briefly, but in general are less violent than most other types of tornadoes.
A funnel cloud or (rarely) a small, relatively weak tornado that can develop from a shower or thunderstorm when the air aloft is unusually cold (hence the reference to "cold air").
A funnel cloud that can develop from a small shower or thunderstorm when the air aloft is usually cold. Cold air funnels rarely become tornadoes, and when they do, they are almost always weak tornadoes.