When the outside temperature is so low that all bodily fluids that leave the body freeze quickly. Therefore when mucus leaks from the nasal passage and freezes it forms a stalactite from the nostrils.
When Harry and Lloyd were driving to Aspen on a skooter without the necessary protective clothing for Aspens climate, snotsicles formed around their nostrils.
when yo snot be runnin' down yo schnizzle when it be hawkin' and it freezes into a stalagtite lookin' mofo!
i was out deer hunting last year. it was colder than a witch's titty. i fell asleep and woke up with a big ol' snotsicle hanging down from my nose, yo.