It's almost summer, so you're probably starting to think about density altitude, right? (Ok - it's not your first thought, but I bet it's crossed your mind.)
If you're a pilot, you know that a higher density altitude means less performance. And, you know that on hot days, density altitude works against you.
But, how much do temperature and pressure actually affect density altitude, and what role does humidity play? Take a look...
Density: Why It Matters
Pressure, temperature and humidity all affect air density. And you can think of air density as the mass of air molecules in a given volume. Why does that matter?
More air mass flowing over your wing allows you to generate more lift, and more oxygen mass in your cylinder allows you to burn more fuel - meaning more power. Increasing air density increases your engine, propellor and wing's performance. Decreasing air density decreases performance.
The Starting Point: Pressure
You can think of pressure as the weight of a column of air. And, as the amount of weight increases, you cram more air molecules into a given volume. So, for a given volume of air:
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- When the pressure is greater than standard, you have more air molecules in the volume than you would on an standard day, and
- When the pressure is less than standard, you have less air molecules in the volume than you would on a standard day.
In fact, increasing the pressure by one inch of Mercury (inches Hg), or 33.9 millibars (mb), decreases your pressure and density altitudes by 1000 feet - so your airplane performs like it's 1000 feet lower.
The sea level pressure usually ranges from 28.9 inches Hg to 31 inches Hg (980 mb to 1050 mb), which means that your aircraft often performs as if it's 1050 feet below or above your actual altitude.
The Summer Factor: Temperature
Temperature also affects density altitude. When you heat the air, the air molecules have more energy. And, when they have more energy, they spread farther apart. (You probably learned this in high school chemistry class...)
When the air is warmer than standard, it's less dense and performance decreases. The standard temperature at sea level is 15 degrees Celsius, or 59 degrees Fahrenheit. As you climb, the temperature decreases about 2 degrees Celsius per 1000 feet. But how much of a factor does non-standard temperature play?
Imagine you're in Miami, Florida during July, where the average temperature is 83 degrees Fahrenheit (28.3 degrees Celsius). Miami's essentially at sea level, so on average during July, the temperature's 13.3 degrees Celsius above standard. How much does your density altitude increase? It increases 1535 feet - that's on an average day!
How about Denver, Colorado? Denver International Airport sits at 5434 feet, and its average temperature in July is 88 degrees Fahrenheit (31.1 degrees Celsius). Since the standard temperature decreases 2 degrees Celsius for every 1000 feet, Denver's standard temperature is roughly 4.1 degrees Celsius. On an average day in July, Denver's temperature is 27 degrees Celsius above standard!
What does that do to Denver's density altitude? On an average July day, the temperature increases Denver's density altitude by 3012 feet to 8446 feet! That's why every one of Denver International's runways is at least 12,000' long - and one is 16,000' long.
Humidity: The Factor You Don't Compute
Did you know that humidity also plays into density altitude? Air is made up of 78% Nitrogen, 21% Oxygen and 1% other gasses. The amount of water vapor in the air varies, but it can contribute up to 1%.
In the atmosphere, Nitrogen usually exists as a N2 molecule - which means two Nitrogen atoms are bound together. Since Nitrogen's atomic mass is 7, one Nitrogen molecule weighs 14 units.
Oxygen in the atmosphere generally exists as a O2 molecule, which means two Oxygen atoms are bound together. Oxygen's atomic mass is 8, so one O2 molecule weighs 16 units.
Water molecules are made up of two Hydrogen atoms, each of which weighs one unit, and one Oxygen atom, which weighs 8 units - so a molecule of water vapor weighs 10 units.
What's with all of the chemistry?!? Water vapor weighs less than the Nitrogen or Oxygen molecules that make up the rest of the air. And, water vapor takes up about the same amount of space. So, when you have more water vapor in the air, the air has less mass - which means it's less dense.
How much of an effect does humidity have?
During July in Denver, the average daily low relative humidity is 22%, and the average daily high relative humidity is 72%. Unfortunately, factoring humidity into density altitude is complicated, but there's a great calculator for it here.
Lets use an average July day in Denver when the temperature's 31.1 degrees Celsius. Adding 22% humidity increases the density altitude by 146 feet to 8592 feet. At the average high humidity of 72%, the density altitude increases by 480 feet to 8926 feet.
You can see that humidity doesn't have as much of an effect on density altitude as temperature and pressure do, but it's something to consider. If the humidity's high, your aircraft could perform like it's several hundred feet higher.
What's It All Mean?
Density altitude's always a factor you should consider - no matter when you're flying. And when the weather's hot and you're at a high altitude, it can make a big difference in performance. And you really don't want to be caught doing this takeoff...
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