Sometimes, while lighting a fire in a wood stove in a basement, the smoke does not exit through the chimney like it normally does. Rather, a large amount of the smoke seems to get "pushed back" into the room instead of exiting through the chimney.
Why does this occur?
Assuming there's not a mechanical problem (debris in the way, damper partially or completely closed, chimney cap damaged/missing and allowing wind to blow down the chimney, etc) then the phenomenon is as follows:
At a given pressure, cold air is more dense than warm air, and tends to fall. Warm air is less dense than cold air and tends to rise. The same phenomenon on a much larger scale is the cause of wind. A chimney is too small for both to happen at once. Either your house will draw cold air down the chimney and vent the excess air through the slight drafts that all houses have, or it vent air out of the chimney and draw cold air through those drafts. The intended function is the latter, but it's hard to engineer a passive chimney that's open on both ends that only does one or the other.
You can feel this: with the stove safely off and cold, open the damper and put your hand or a piece of tissue paper near the flue. You will feel or see a breeze flowing either up or down the chimney.
If the hot air from the fire rises up the chimney, as intended, it will rapidly warm the whole air column in the chimney, which will then rise, carrying the smoke out of the house.
However, if the hot air from the fire escapes out into the room instead, the cold air outside will flow down to fill the low pressure region. Wind will blow following the pressure gradient, keeping the chimney cold and the hot air from the fire (plus smoke) blowing out into the room.
To prevent this from happening (again assuming there's not a mechanical problem), you can use a hand-held electric heater or use tongs to hold a bit of burning kindling inside the flue opening after opening the damper to pre-warm the air column in the chimney and start the airflow moving in the desired direction before lighting the fire.
Could be one of two things:
Cool air being forced down the chimney taking smoke with it - that is, we know that cold air sinks (and hot air rises), and so a column of cold air sinking can force smoke back down into the room.
It could also be simply a matter of winds blowing downward pushing smoke back inside.
Airflow around buildings and over walls can compress air at the top of the roof at higher pressure and bring low pressure at the sides of the house and the shielded zones.
If the chimney has very cold conditions at the top, and compressed air on the roof, the chimney can draft backwards, especially if the house has leaks towards the lee side of the wind, where the pressure is low.
