Density measures the mass of a material in a given volume. Solid density remains constant, while gas density is affected by pressure, temperature, and humidity. Air density is calculated and changes with altitude, temperature, and humidity. Higher altitude and temperature decrease air density, while humidity reduces it due to the lower molecular weight of water.
Density is the measure of the mass of a material in a given volume of space. For a solid, the density remains constant even if its environment changes. The density of a gold bar is the same at sea level, under the ocean and on top of Everest. For a gas like air, density is affected by changes in pressure, temperature, and humidity. Changes in air density can change how easily an object moves and even cause health problems.
Air is a mixture of nitrogen, oxygen and other trace gases. The molecules of these gases typically move rapidly, bouncing off each other. When the environment is a constant with no changes in temperature, pressure or humidity, the mass of air in a given volume of space will remain stable. The air density value, usually expressed in kilograms per cubic meter, is calculated rather than measured directly.
Increases or decreases in air pressure can affect air density. The two main reasons for changes in air pressure are altitude and weather systems. Of these, altitude is the most significant.
For the most part, air becomes denser as air pressure increases. The additional pressure forces the air molecules closer together, resulting in more mass in the given volume. As altitude increases, air pressure decreases—there is less pressure pushing air molecules together. This decrease in air density means that a high-altitude climber gets less oxygen when he takes a breath. The lungs contain the same volume, but the lower density of the air means there is less oxygen in the space.
Temperature is the other important factor that can have an effect on air density. As the temperature rises, the air molecules move faster and bounce farther; therefore, higher temperatures result in lower air density. When air is denser, it creates resistance to objects moving through it. For example, a golf ball hit on a hot day at a mountaintop golf course will go farther than a shot on a cold day at sea level. The high temperature and lower atmospheric pressure found at high altitudes combine to lower the density of the air.
Although a relatively minor effect, humidity or the amount of moisture in the atmosphere can also change the density of the air. Surprisingly, more moisture in the air results in lower air density. Since the number of molecules is constant, as the number of water molecules increases, the number of nitrogen and oxygen molecules must decrease. The molecular weight of water is much lower than that of dry air whose weight is dominated by nitrogen and oxygen. At high humidity, therefore, the density of the air decreases due to the reduced mass in a given volume.
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