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What is Density Altitude?

by NK Amy on August 9th, 2007, 4:37 pm

This is another commonly asked Kestrel question. Typically, if you don't know what Density Altitude is, you probably don't need to worry about it! But just in case, here's a quick explanation of Density Altitude:

If you think about how your body feels at the beach versus standing on top of a 10,000-foot mountain, there is a big difference in the air that you breathe – it’s thinner and there is less oxygen per breath that you take. That’s why mountaineers need to “acclimatize” to new altitudes as they climb mountains, allowing their bodies to adjust to the new air. Additionally, weather factors – such as temperature, pressure, dewpoint and humidity – can affect the weight of a cubic volume of air, in essence making the atmosphere “feel” like it does at a higher or lower altitude. Density altitude is a calculation that adjusts your actual geographic altitude to the altitude that it “feels” like based on the environmental conditions of the atmosphere.

Increases in temperature and humidity can cause a reduction in air density- hot and humid conditions can make the density altitude significantly higher than the actual geometric altitude. Since water vapor is lighter than any other gas in the air, the more vapor in the air, the lighter it feels. (Even though liquid water would be heavier than oxygen or nitrogen, the gaseous form of water is lighter.)

So why is density altitude an environmental condition to watch? Denser air slows down objects because the air is heavier and the object has more resistance or “drag” moving through the air. That’s why it’s easier to hit a home run in Denver (where the ball park is a mile above sea level) than at a lower altitude park.

Density altitude is essential for pilots to monitor because it affects the lift, propeller efficiency and power output of the engine. Depending on the weather, an airplane may need a longer runway to take off and reach the same altitude. Similarly, a helicopter’s rotors have a harder time grabbing the air depending on the air density. Race car drivers and pit crews also monitor the density altitude because the car’s high performance engine will perform differently depending on the density altitude. Some long range shooters also consider the density altitude, as it can influence the bullet's performance.

The Kestrel Meter uses a formula to calculate the density altitude. And since density altitude takes station pressure into account instead of barometric pressure, there's no need to set a reference for an accurate density altitude reading. (Station pressure is the pressure reading assuming that you are at sea level, and barometric pressure is the pressure reading adjusted to your geographic altitude.)