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While pilots – especially those who often fly in hot environments – are quite familiar with the concept of density altitude, the term is a vague one for most skydivers. We hear the term tossed around, particularly in the summer months; but few jumpers really understand what it means – or what it means for them, beyond a longer takeoff roll and a lengthier climb to altitude in the back of a hot, stuffy airplane.
What is density altitude?
Density altitude is pressure altitude corrected for nonstandard temperature. It is the altitude at which the wing “feels” like it is flying. Density altitude at a given landing area is affected by field elevation, air temperature, barometric pressure, and humidity. Let’s break this down.
As temperature and altitude increase, and as barometric pressure decreases (e.g. as a low pressure system moves through an area), the gaseous molecules in the air spread further apart*, causing air density to decrease. In addition, humid air is less dense than dry air because humid air has a greater amount of water molecules (H2O) in proportion to the other gases which make up our atmosphere; and H2O is a smaller, lighter molecule than O2, N2, and CO2. Decreased air density = increased density altitude.
*their relative proportions remain the same
Calculating density altitude
The simplest formula for density altitude doesn’t account directly for humidity because humidity’s effect on density altitude is fairly minor; however, humidity does affect the current station barometric pressure, which is a piece of the equation below.
Density altitude in feet = pressure altitude in feet + (120 x (outside air temperature in C - standard temperature in C)). Standard temperature is 15 C at sea level and decreases about 2 degrees per 1000 feet of altitude above sea level. Pressure altitude = ((standard pressure - current station pressure) x1000) + station elevation (std pressure =29.92 Hg)
For example, on the hottest day of the 2016 summer in Eloy, the temperature maxed out at 114.8 °F, with a max dew point of 26.6 °F at the time the temperature hit its high, and a barometric pressure of 29.78”Hg*. While Eloy only sits at 1515 feet ASL, the conditions resulted in a density altitude of 5735 feet late that afternoon. In other words, if you were flying your parachute during the hottest part of the day on 6/19/16, you were landing at an effective altitude 4200 feet higher than Eloy’s actual altitude. If it felt like you were coming in hot… well, you were!
*You can get detailed historical weather data for just about any day on wunderground.com
Calculation: Pressure altitude = (1000*(29.92-29.78))+1515 = 1655 feet Standard temperature, Eloy = 15-(2*1.5) = 12 °C 12 °C ~ 54 °F (53.6 °F). Density altitude = 1655 + (120*(46-12)) = 5735 feet
If you don’t have this formula handy and you need a quick, rough estimate, you can use Barry Schiff’s tips: for every degree C higher than standard temperature for a given elevation, increase density altitude by about 105 feet; every degree F above standard elevates density altitude by about 60 feet. If the altimeter setting is above or below 29.92 inches mercury, add another 100 feet of density altitude for each tenth of an inch below 29.92 or subtract 100 feet for each tenth of an inch above 29.92.
Using this, given Eloy standard temperature of ~54 °F and the temperature that day ~115 °F, and a barometric pressure ~0.15 lower than standard pressure: take 1515 feet and add (115-54=61, 61x60=) 3660 feet for temperature and 150 feet for pressure = 5325 feet.
How is wing performance influenced by density altitude?
So what does all this mean for YOU? Why is it important that you as a parachutist understand density altitude?
- As density altitude increases and the air becomes less dense, there are fewer large molecules (O2, N2, and CO2) in a given space to create friction (air resistance) between the canopy and the air.
- As density altitude increases, airspeed increases by almost 5% per 3000 feet up to 12,000 feet MSL, and by more than 5% per 3000 feet above 12,000 feet MSL.
- As density altitude increases, a ram-air canopy pilot can expect a higher stall speed, a faster forward speed, a faster descent rate, and higher opening forces. The canopy will also lose more altitude in a turn. Finally, with fewer molecules in the surrounding air to help slow the parachute down, the canopy will have less stopping power (a less-effective flare).
*** for every 1000 feet of elevation gained – real or virtual (induced by density altitude) – your canopy will lose about 4% of its flare performance. So going back to our example of 6/19/16, if you’d been flying when the density altitude peaked, your canopy would have had about 20% less flare power than on a day with no modifications to Eloy’s density altitude.
- If you are at the point in your canopy progression where you are considering a downsize, and the weather has just started really heating up (this is typically sometime in April in Eloy), it might be wise to hold off on getting that new, smaller wing until the temperature starts dropping again in the autumn, and spending the summer managing the increased airspeed and decreased flare power on your current wing instead.
Finally, note that the air temperature used to calculate density altitude is usually based off thermometers kept in the shade. The air temperature in the sun over the landing area may be significantly higher!
Sources:
USPA SIM Section 5-5 (you can download a current copy of the SIM for free from USPA’s website)
https://www.aopa.org/news-and-media/all-news/2007/july/01/proficient-pilot-(7) (Barry Schiff’s article)
AXIS Flight School states what the current density altitude is at SDAZ here: http://axis.tools/tool_Cond.php