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Density body

Air rising and falling in a house

Figure A: Air Circulation Relative to Density


The technical definition of density is mass per unit volume.  Generally, density describes how tightly packed something is.  An object with a lot of material in a small space is more dense than an object that has lots of air space included.  In the atmosphere, gas that is less dense has a lower concentration of molecules per volume than a denser gas and will tend to rise  compared to the air around it. For example, gasoline vapor is heavier than air molecules and so tends to stay near the ground when you fill up your gas tank, which can lead to a fire hazard if the lingering vapor encounters a spark.

Warm air is less dense than cooler air. Air density varies with the relative humidity (amount of water vapor molecules in the air) along with temperature. Water vapor molecules (H2O in the gaseous phase) are composed of Hydrogen (H) and Oxygen (O) molecules. Hydrogen has a molecular weight of 1.01 g/mol. Dry air is composed mostly of Nitrogen (N) molecules since Earth's atmosphere is 78% Nitrogen and 21% Oxygen. Nitrogen has a molecular weight of 14.0 g/mol. In the atmosphere, the density of air particles decreases with height, with more gas particles remaining near the surface of Earth. When only taking into account humidity, dry air is more dense than moist air because of molecular weights of the gases. Note that gases, such as hydrogen and nitrogen, do have weights, but the weight cannot be felt just by moving through the air. Special equipment and procedures are used to measure the weights of gases. When only considering the temperature of air, which does not all heat up at the same rate, dry warm air tends to rise compared to areas of cool air around it.  You can notice the effects of this if you live in a two-story house as the upstairs tends to be warmer than the downstairs. Areas of warm rising air often result in the development of clouds and even precipitation in the regions where the vertical movement of air is strongest.

A hot air balloon is a good example of how people work with density (Figure B). Hot air balloons use the properties of density in order to float. In the base of the hot air balloon, there is a torch that heats up the air inside of the balloon. When the air inside the balloon becomes warmer than the surrounding air, the balloon will begin to float. The person controlling the hot air balloon can add more heat to the balloon to reach the desired height. The air inside the balloon needs to cool in order for the balloon to land.

Hot air balloons use the properties of density to float in the sky

Figure B Figure C: Atmospheric Density
http://www.boston.com/bostonglobe/ideas/brainiac/hot%20air%20balloons.jpg Image from Utah State University

Last modified date: Wednesday, June 5, 2013 - 3:21pm