Why does air move?
Those who have visited the Metrodome in Minneapolis may have had the experience of exiting the facility by a standard door. If you leave in that way rather than by one of the rotating doors, you will feel a terrific wind on your back. It even blows some people over.
This wind is generated by excess air pumped into the building, just likeWatson
that from a blown-up balloon. The difference in the amount of air inside vs. outside is only about two-tenths of 1 percent. This is an example of a pressure difference. Even though the temperature inside and outside the dome may be the same, the difference in the amount of air causes the excess air inside to want to equalize with the air outside.
On another occasion, you may have experienced a wind in a tunnel between a heated building and a cold one and felt a wind as strong as at an open door of the Metrodome. This is the same as the air movement you feel on a winter's day when you open a door, feeling the cold air rush in at your feet while the warm air rushes out around your head. Here, the wind is generated by density difference due to temperature, even though the air pressure is the same.
Thus, we see that air can be caused to move by two different phenomena, one where the temperature, and thus density, is different, and the other where the pressure is different.
There is a trivial cause of air movement that seldom functions in the atmosphere to cause a wind by itself. That is humdity difference, with pressure and temperature constant. Such a wind could arise, however, since humid air is a little less dense than dry air. Thus the dry air would tend to move under the more humid air. However, even slight temperature differences ordinarily have a greater effect than humidity differences, so humdity-induced winds are masked.