Dear Tom,
While lying in bed recently, I had a south-facing window open about an inch and could
feel cold air coming into the bedroom. I wondered if warm air from inside was leaving at
the same rate. There was no wind blowing the air in and the bedroom door was closed so
there really was no cross ventilation.
Dan Oshea, Oak Lawn
Dear Dan,
Air moves in response to differences in air pressure, always moving from higher to lower
pressure. The driving force is known as the pressure gradient force and it absolutely
controls air movement. Other things being equal, the internal pressure in cold air is
greater than in warm air, and so cold air will always try to undercut warm air. The
American Society of Heating, Refrigerating and Air-Conditioning Engineers assures us
that warm air exiting your bedroom balances the cold air entering it through the open
window.
