Harold one of the questions ALWAYS asked on the 2nd year apprentice test is how can a trap loose its seal?
Of the 6 ways one answer is always “Oscillation” due to wind effect.
What is happening to LuAnn is a common problem.
As wind goes down the vent terminal it causes a negative pressure and back syphons the trap seal until the trap seal is gone (water empties out)
Now the height of the vent terminal has actually NO BEARING as some codes state
A roof for weather protection may have the vent terminal terminate at least 24″ above the roof.
Besides on a boiler vent one could always use a “SWISS CAP” to prevent down drafts as this diverts the downward flow to outside the chimney plus on “plumbing” we don’t use a diverter on vent piping as we can’t have openings like we do in heating
Now the code also states that a promenade deck shall have the roof terminal extend 7 Feet above the roof. (Old code use to be 5 feet for all vent piping except acid systems.)
To stop this phenomenon all that is required is either install a return bend (180 degree fitting) Or a J. R. Smith 1748 Vent cap that allows the vapor out BUT prevents down drafts and it will not allow hoar frost to close the vent opening in the winter months.
Back syphonage is also caused by poor design when for example a toilet on the 47th floor of a building is flushed and it causes a negative pressure sucking out the traps on the lower floors along this soil stack.
The installing plumber most lightly did a soil stack vent set up (twined the vent into the soil) rather then a soil stack tie in to the vent thus causing all down drafts to effect the soil line and thus causing he trap seals to oscillate back and forth until the seal is gone or the very least weakened.
Wow plumbing can be so educational at times HUH Harold?
ILL bet hydro would just cap off the vent with a coffee can with a few holes punched into causing all the great sewer gases to enter the home and make everyone SICK But hey its CHEAP fix and no license required