What is Density Altitude?
According to the Pilot’s Handbook of Aeronautical Knowledge, “Density altitude is pressure altitude corrected for non-standard temperature.”
Increasing temperature and higher elevations decrease the air density and increases the density altitude. Aircraft typically do not perform as well in hot and humid temperatures. This is why pilots need to be cautious when flying at higher altitude airports on hot days. Due to the higher density altitude, aircraft performance is reduced resulting in longer required takeoff distances and reduced climb performance.
Colder temperatures will lead to lower density altitudes. Aircraft will typically see a boost in performance in colder temperatures. Aircraft performance increases as the density of air increases. This is called low density altitude.
As air density decreases it affects aircraft performance in three ways:
- 1. Power is reduced because the engine takes in less air.
- 2. Thrust is reduced because the propeller is less efficient in thin air.
- 3. Lift is reduced because the decrease in air molecules exerts less force on the airplane.
Let’s assume we were in Seattle which is at sea level. At sea level the air is denser and there is higher air pressure.
If we were able to visually see air molecules moving in the air (see figure 1 below), there would be more air molecules at sea level and the molecules would be densely packed together.
At higher elevation airports like Denver International (about 1 mile above sea level) or Telluride Regional (9,070 feet above sea level), there are less air molecules due to the higher altitude. The thinner air and reduced oxygen levels at higher elevations is why mountain climbers often use oxygen when climbing Mt. Everest, which is more than 29,000 feet above mean sea level! Due to the increased air density, aircraft performance is better at sea level than it would be at a high elevation airport.
Standard Air Pressure and Temperature Lapse Rates
Atmospheric pressure is measured in inches of mercury abbreviated HG. The standard altimeter setting in atmospheric pressure at sea level is 29.92. As you can see from the chart below air pressure and temperature decreases as altitude increases.
The standard air pressure lapse rate is 1 inch of mercury decrease for every 1,000 foot gain in altitude up to 10,000 feet.
Density altitude is greatly affected by temperature changes as well. The standard temperature lapse rate is 2 degrees Celsius per 1000-foot change in altitude up to 36,000 feet according to the Pilot’s Handbook of Aeronautical Knowledge. Put another way, temperature will decrease 2 degrees Celsius for each 1,000 -foot increase in altitude.
What is Pressure Altitude?
Pressure altitude is defined as the height above the standard datum plane. The standard datum plane is a theoretical level at which the pressure of the atmosphere is 29.92 inches of mercury, and the weight of the air is 14.7 pounds PSI according to the Pilot’s Handbook of Aeronautical Knowledge.
The altimeter is essentially a sensitive barometer calibrated to indicate altitude in the standard atmosphere at the standard altimeter setting of 29.92, when the indicated altitude is equal to the pressure altitude. Barometric pressure affects the altitude readings on an altimeter, so this is why you always get the altimeter setting when listening to the ATIS before taxi and takeoff.
Pressure altitude can be determined in three ways:
- First is setting the altimeter to 29.92 and reading the indicated altitude shown on the altimeter.
- Second is by applying the correction factor to the altimeter setting
- Third is to use an electronic flight computer such as an E6B calculator
Phrases to Remember How Pressure and Density Altitude Work
Suppose we took off from an airport with a field elevation 5,000 feet above sea level. The air pressure at 5,000 feet is about 24.89 inches of mercury. If we could turn our barometric pressure down to 24.89, the altimeter would read 0 even though we were 5,000 feet above mean sea level. Thus, our indicated altitude would be lower than our actual altitude above mean sea level. If we flew to 10,000 feet above mean sea level, our altitude would indicate 5,000 if our altimeter was set to 24.89. If we then flew to an area at sea level and did not change our altitude, our altimeter would read 5,000 feet even though we were actually 10,000 feet above sea level. Again, our indicated altitude would be lower than our actual altitude above mean sea level. Remember the phrase, “low to high, look to the sky.” This means if you fly from an area of low pressure to high pressure without adjusting the altimeter, your indicated altitude would be lower than your actual altitude.
Conversely, the opposite is true. If a pilot flies from an area of high pressure to low pressure without adjusting the altimeter, the indicated altitude on the altimeter would be higher than the actual altitude. Remember the common phrase in aviation, “high to low, look out below.” This means if you fly from an area of high pressure to low pressure without adjusting the altimeter, your indicated altitude would be higher than your actual altitude. It may seem a bit confusing but after some practice you should start to get the hang of atmospheric pressure and how altimeters work.
Thank you for reading. We hope you now have a better understanding of atmospheric pressure and the difference between density altitude and pressure altitude.