High and Fast: Understanding Density Altitude

How many of you have muttered, “Whoa, that was faster than I expected!” (or a more forceful, colorful variation) after blazing in for landing on a hot, humid summer’s day? Maybe you paused for a moment afterward to ponder why the landing was so much faster than anticipated, or maybe you just shrugged it off and blamed it on a gust of wind and then rushed to pack for the next load. Pondering the “why” is a worthwhile exercise, actually, because although a fast landing can be a rush if you know what you’re doing and are anticipating the speed, unexpectedly landing more quickly than normal can cause a bruised ego, broken bones or worse. And this is especially true for less-experienced skydivers.

A number of variables affect the speed with which a canopy flies and lands. If you take a poll around the drop zone, you’ll probably hear jumpers cite wind speed, the canopy’s flight path relative to the wind (i.e., whether it’s flying downwind, crosswind or into the wind) and various pilot inputs to the toggles, risers and harness as the significant factors that affect flight speed. However, if you ask jump pilots about factors that affect the speed of a wing’s flight, they will probably mention an additional variable: density altitude.

WHAT IS AIR DENSITY?
Air density describes the mass of air that occupies a given volume of space. Since each molecule has a specific mass, we can easily convert the total mass of molecules into the total number of molecules, and it’s the number of molecules squeezed into a given volume of space that we really care about. Whether air molecules are tightly packed (high density) or thinly spread out (low density) affects everything from the power and efficiency of the jump plane’s engine to the flight characteristics of your parachute. In general, lower density air causes a canopy to fly faster because a more rarified airflow generates less lift and because reducing the density of a fluid reduces drag.

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The density of air is affected by four main factors: altitude, weather systems, temperature and moisture content. As skydivers, most of us know that air pressure and density decrease in a predictable way with height. After all, this is why our altimeters (which are really just pressure sensors) reliably estimate altitude and why we have to suck on oxygen before high-altitude jumps. However, we tend to be less familiar with the roles that temperature, weather patterns and humidity play in determining density.

Enter density altitude, which combines all four factors’ effects on air density into one convenient measure and relates it to something all aviation enthusiasts intuitively understand: altitude. The formal definition of density altitude is, “pressure altitude corrected for nonstandard temperature and humidity,” which is a fancy way of saying it’s the altitude at which an airplane or your canopy feels like it’s flying.

“Feels like” is a bit ambiguous when talking about a piece of fabric, but don’t worry, understanding density altitude doesn’t involve determining your parachute’s emotional state. Instead, on any given day, the density altitude tells you at what altitude, under standard temperature and pressure conditions, your parachute performs as if it were flying.

Understanding “standard temperature and pressure conditions” is key; density altitude is really just a comparison between the actual conditions at your location and standard conditions that would be expected at your location’s altitude. Since temperature and pressure both decrease predictably with height (they decrease at the “standard temperature lapse rate” and the “standard pressure lapse rate,” respectively), a standard temperature and a standard pressure can be assigned to any given altitude.

For instance, standard temperature at sea level is 15 degrees Celsius (59 degrees Fahrenheit), and standard pressure is 1013 millibars (29.92 inches of mercury). If you jump at a drop zone on the coast and the temperature and pressure are any different from those numbers, your density altitude will be different from your actual altitude, and your parachute will perform as if it were flying correspondingly higher or lower in a “standard atmosphere.”

BY THE NUMBERS
Calculating density altitude requires several steps, but thanks to modern technology, you can skip the math and instead take advantage of online calculators (like this one from the National Weather Service) that require you to do nothing more than enter the temperature, dew point and pressure at your location. Play around with the calculator. Enter some average numbers for winter jumping at your home DZ, then enter some average summer numbers. Check the current conditions at your home DZ, then plug in the ones for a destination DZ you’ve wanted to visit. See how the density altitude (and therefore your canopy’s expected performance) changes under different conditions.

Actually understanding density altitude requires a bit more effort than using a plug-and-play online calculator, but not much more. Let’s go back to those four main factors (altitude, weather systems, temperature and moisture content) that affect air density and look at the effects of each one.

  • Altitude—This one is simple. The higher you climb in the atmosphere, the lower the density of the air around you. For instance, in Denver, Colorado, which has an elevation of 5,300 feet, the average air density is almost 20 percent lower than at sea level.
  • Weather systems—Almost everyone has heard a local meteorologist refer to high- and low-pressure systems moving through the region. Air density is directly proportional to air pressure, so when pressure goes down, density goes down by the same factor. If a low-pressure weather system moves into your area, the air density will decrease, and your canopy will “feel like” it’s flying at a higher altitude.
  • Temperature—From a physics perspective, temperature is just the movement of molecules. The higher the temperature, the faster the molecules move, and the faster they move, the farther apart from each other they spread. So, when air temperature goes up, air density goes down. On a dry, 95-degree Fahrenheit day, your canopy will perform as if it were flying 2,300 feet higher in the atmosphere than on a “standard” 59-degree Fahrenheit day.
  • Humidity—Water vapor weighs 40 percent less than dry air. When you add humidity (water vapor) to a parcel of air, you actually decrease the mass of that parcel because you are, in essence, replacing a heavier dry air molecule, like nitrogen or oxygen, with a lighter water molecule. Since density is defined as mass per unit volume, if you decrease the mass while holding the volume constant, you decrease the density.

Altitude and temperature have the greatest effect on density altitude, with pressure fluctuations due to weather systems and changes in humidity contributing much less. If you normally jump at sea level but travel to the mountains of Colorado for a boogie, you’ll almost certainly feel like you’re coming in hot, but if the dew point rises by 10 degrees, it’s unlikely that you’ll notice a change in your canopy’s performance (you might notice it going into the deployment bag more easily when you pack, though).

The bottom line is that high and hot and humid = lower density = higher density altitude = faster canopy descent. That higher density altitude and its correspondingly more rarified air also means that the jump plane’s engine and propeller aren’t going to operate as efficiently. No, the pilot isn’t just being a jerk and making you swelter in the back of the boat in July; that Cessna really won’t climb any faster. So, if it’s hot or you’re at a high field elevation, expect to have a slooow ride up, but a fast ride down under canopy thanks to density altitude.

About the Author
FEATURE20143-10Megan Walker-Radtke, C-41168, grew up in the central U.S. staring at the swirling, thunderous skies of Tornado Alley, learning meteorology and chasing storms. She received a bachelor's degree in physics from the University of Tulsa and a master's degree in atmospheric science from Purdue University. She now runs Blue Skies Meteorological Services, a weather and climate consulting company, and lives and jumps in sunny Florida.

 

 

 

Some things to consider about your canopy flight when jumping at a location with a density altitude that is higher than you are used to (e.g., on a hot and humid day or at a high-altitude drop zone):

  • Plan to land in as large an area as possible to allow you to land with a level wing even if you make errors in accuracy.
  • Don’t downsize to a smaller canopy until you become very proficient with your current canopy at that particular density altitude. At very high density altitudes, you may even want to upsize to a larger canopy.
  • Understand that your canopy will use more altitude to return to straight and level flight after making a turn, and turn conservatively.
  • Understand that your canopy will stall at a higher forward speed, and your feet will touch down sooner than you expect. Use extra care while managing the landing flare, and focus on flying the canopy all the way through the touchdown point and until the forward energy stops.

Jim Crouch |  D-16979
USPA Director of Safety & Training

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Bryan Burke
Sat, 04/12/2014 - 15:18

An accurate density altitude calculation is hard to do without tables or conversion programs, but here's a broad rule of thumb that will get you fairly close. For each increase of 1,000 feet in altitude or each 10 degree (F) increase in temperature, there will be about a 3% change in performance. Thus if your typical jump is in San Diego, sea level and 75 degrees, and then you go to the Lost Prairie Boogie and 3,500 feet higher and ten degrees warmer, you'll experience an approximate change of 3% for temperature and a little over 10% for altitude. Round that up for safety to 15%.

You have just effectively downsized by 15%, so your 150 square foot canopy will now fly like a 130. Everything is going to happen about 15% faster: descent, forward speed, loss of altitude in turns, and you'll have a corresponding loss of flare power. That's why at high dzs (or hot ones, like Arizona in summer) people from cooler, lower dzs tend to come in really hot, overshoot, and not get the flare they are used to.

This is a physical truth that doesn't care how many jumps you have. Experts are just as likely to experience problems as intermediates, more so if they are already at the practical limit of wing loading at their home dz before they travel to a high or hot dz. Don't underestimate density altitude! Think of it exactly like downsizing your canopy and plan accordingly.

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