What you are talking about is the "stack effect." The pressure difference from bottom to top of a chimney, the driving force for flow of air or flue gas up the chimney increases with the height of the column of air & gas in the chimney and with the difference in densities of that column of air/gas and the air outside of the chimney. So tall stack and hotter flue gas both make for more pressure difference and thus more flow (better draft).
Now as to air leakage into/out of a house, that is much like a flue stack. You have some height over which there is a difference in density, and you have the difference in density; air at 70 F inside the house is much less dense than outside air at, say, 0 F, by about 14%. Thus you have the heavier air outside trying to force its way into the house at lower levels, via any tiny crack it can find, while the lighter inside air is trying to leak out of the upper levels through similar tiny cracks. Somewhere in between is the "neutral pressure plane." This link provides a couple of photos of this effect, where you can see the flexible temporary wind screening puckered inward down low and billowing outward up high:
http://www.greenbuildingadvisor.com...ral-questions/24286/i-need-photo-stack-effect.
Sometimes those "tiny cracks" where air leaks in a house aren't so tiny. An older house might have a rather large gap between an interior masonry chimney and the framing around it, as code requires, but completely open at attic level. Can lights in the ceiling of the top story also are notorious air leaks, even those rated AT ("Air Tight"). But cracks at ceiling-wall joints, holes in framing where wiring passes, cracks at the sill, improperly installed and flashed windows, and electrical boxes at exterior walls all contribute to the leakiness of a house. Cold outside air is exceedingly dry, compared to inside air, and excessive leakage does make for dry inside air (and makes it harder to heat the house). Just throwing humidity at the dryness isn't a good idea, as all that added moisture will be leaking out through wall cavities and the attic space, where it will contact cold surfaces and possibly lead to mold and rot over time. The solution to excessively dry inside air in winter is to tighten up the house, going after any and all cracks and holes that are accessible, even if it means an uncomfortable afternoon crawling on your belly around unfinished areas of your attic to locate and plug leaks with can foam or other sealant. Tightening up a house really does go a long way toward reducing dryness in winter, although it's unlikely you'd get to the point where an older house would become so tight that mechanical ventilation would be required, unless extreme measures had been taken. With new construction, a house built to be extremely tight does need mechanical ventilation, both to provide fresh air for the occupants and also to keep interior humidity down (yes, down). Human activity produces moisture, and if it can't leak out then it must be removed by deliberate ventilation. But all this is beyond what you asked in the first place.
