Feel kind of guilty placing a non-homework physics question on Google.
 On the other hand, I am an old man who is genuinely perplexed.

My building is a 3-story concrete structure.  Used to be a can
factory.  The walls, floors, and ceilings are solid
concrete----approximately 18" thick.
(Could never build something like this today.  Would cost way too
much.)

Someone has drilled a 3" diameter hole in the floor of my third-story
office.
It goes down 18" to the ceiling of the guy below me on the second
floor. The opening in his ceiling is also 3".  Hence, the bored hole
is of uniform 3" size and connects the two floors.

When I place my hand near the 3" hole, I can detect very rapid
movement of air.
24 hours a day.  But I can't determine the direction!  Is it cold air
from my third-story office descending to the second-floor?   Or, is is
hot air from the second-story office rising?  Both?

I cannot determine either direction or temperature of the moving air
by "feel" alone...and it's driving me nuts.   The velocity is
rapid---I am guessing 25 miles per hour.

A great answer would be 3-4 sentences.   Then I can sleep tonight...

All comments greatly appreciated !

If you have airflow at 25 mph (an exaggeration, I suspect, but I
accept that there is at least a "strong" flow) through a "pipe" such
as you describe, then the flow must be unidirectional, either up or
down. Which it is depends on many factors, but I suspect that the
dominant factor is the design of the air circulation system (heating
and cooling system) in your building. A strong flow like you describe
is probably driven by a pressure difference and not just a temperature
difference, and the pressure difference is probably, in turn,
determined by the configuration of ducts and blowers. It may also
change from day to day or from winter to summer. If the flow bothers
you, block it; if you want to know which direction it’s going, try
holding a thin strip of paper or thread in the airflow and see which
way it deflects (and you can always check the temperature with a
thermometer).

[That’s already five big sentences, which exceeds my quota, but I’m a
generous soul; if you need another sentence or a few of further
discussion, just ask!]

Top-notch answer.  Thanks, Doc!  Now I can sleep better...