How much mass is in each unit volume. The speed of sound in that "chunk" of air. How much lift / drag you get from that chunk of air. Whether or not you can stay conscious breathing it, or develop the bends by trying to breathe it.
Altitude has a large affect on the air pressure and air density. Air density reduces with altitude and air pressure reduces with altitude as well.
density decreases as air pressure increases
Scroll down to related links and look at "Density of air - Wikipedia" and "Atmospheric pressure - Wikipedia". The density of air is the mass per unit volume of Earth's atmosphere. Air density decreases with increasing altitude, as does air pressure. Air pressure (N/m² = Pa) is a column of air pushing on top of you. If you think of a stack of books, then the bottom has the greatest pressure and the top has the least.
= What is low air density? = well, high or low are relative concepts. Normally the standard air density of the air is defined as the density of the air at 15ºC and at sea level pressure (standard conditions), what give a value of 1,225kg/m3. This way, we can talk about high or low air density related to the standard value. Air density depends on temperature and pressure. The colder the temperature the higher the density is, and the higher the height in the atmosphere the lower the density becomes. In the Everest peak, e.g., air density can be lower than 0,5. Meanwhile in the Antarctica (at sea level) the density could reach values higher than 1,5.
Air density decreases with altitude, warmer temp and moisture
Air has both density and pressure.
Pressure affects the density of air by increase of pressure increases the density
Altitude has a large affect on the air pressure and air density. Air density reduces with altitude and air pressure reduces with altitude as well.
Air density and air pressure are both properties of the atmosphere that are related to each other. As air density increases, air molecules are packed more closely together, which also leads to an increase in air pressure. Conversely, as air density decreases, air pressure decreases as well.
density decreases as air pressure increases
Air density and air pressure are inversely related. As air density increases, air pressure also increases because there are more air molecules in a given space exerting force on the surroundings. Conversely, when air density decreases, air pressure decreases as well due to fewer air molecules exerting force.
As air pressure pressure decreases with altitude, so does the densite of air.
Air pressure varies because of the differences in air's density.
density
Scroll down to related links and look at "Density of air - Wikipedia". See there: Importance of temperature.
as the altitude increase, the effect of gravity become more insignificant on air and the air become more disperse or thinner hence it does affect the air pressure and density by decreasing both air pressure and density of the air.
When altitude rises, the air pressure and density both decrease. When temperature rises that means that more air is pushing down on it. So this means that the air pressure and density rise when temperature rises.