Weather

If you had driven in an air-conditioned car to the Newton, Iowa, Municipal Airport for an early afternoon flight on June 14, 2010, you might have thought it would be a great day to fly. A few clouds were scattered around the sky, 2,700 feet above the airport, and the visibility was at least 10 miles.

At the airport you began to have second thoughts. Getting out of your car was like walking into a pizza oven. At 2:15 that afternoon the temperature was 95 degrees, but the humidity made it feel even hotter. You were wilted after walking a couple of hundred feet to the FBO’s office. You stay in shape by running, but not on days like this.

Weather like this saps my strength; what does it do to my airplane? you wonder. Then you recall learning about something called density altitude—which you found confusing while studying for a knowledge test. Maybe I should look into this before starting the preflight.

Heat and your body

You feel hotter on a humid day than on a dry day with the same temperature because humidity slows down the evaporation of perspiration from your body. Perspiration is nature’s air conditioning. When it evaporates, it carries heat away—which keeps your body from overheating, with potentially deadly results.

The National Weather Service calculates the “apparent temperature” (how hot it feels) using the temperature and humidity. The NWS does this as a way to alert people when the heat is particularly dangerous. In fact, NWS statistics show that heat causes more deaths each year than floods, lightning, tornadoes, and hurricanes combined.

At 2:15 p.m. in Newton, Iowa, the apparent temperature was 133 degrees, in the NWS’ “extreme danger” category. The NWS website has more information about the dangers of heat and a chart you can use to find apparent temperatures.

Heat and your airplane

As air becomes hotter and more humid, it becomes less dense, because air—like most other things—expands as it heats up.

Less dense air degrades aircraft performance, because lift depends on the mass of the gases in the air that flow over and under the wings each second. This, in turn, depends both on the number of molecules of the various gases in each cubic foot of air as well as the mass of the molecules, and how fast the air is flowing over the wing.

An airplane’s engine produces power by burning the proper mixture of fuel and oxygen from the air. As air density decreases, each cubic foot of air that the engine sucks in has less oxygen (along with less of air’s other gases); thus, power decreases as air density decreases.

The amount of thrust depends on the mass of air that a propeller or jet engine can push toward the aircraft’s rear. Decreasing air density reduces thrust. Drag also decreases as air density decreases, which helps a little, but the decreases in power, thrust, and lift more than offset the drag reduction.

Calculating performance

While the term “density altitude” is used to describe the effects of temperature and air density on aircraft performance, many airplane operating handbooks include performance charts that pilots use to find figures such as takeoff distance and rate of climb without ever using a figure for the density altitude.

For example, the takeoff distance chart in the Cessna 172M pilot’s operating handbook uses the airplane’s takeoff weight, temperature, and pressure altitude to calculate takeoff distance. The pressure altitude is the figure your altimeter reads when it’s set at 29.92 inches of mercury.

The chart shows that if you were at sea level with the standard atmospheric pressure in a Cessna 172M weighing 2,300 pounds, you would need 775 feet of runway to lift off if the temperature were 32 degrees Fahrenheit. If the temperature were 104 degrees at the same air pressure, you’d need 1,030 feet of runway before the wheels leave the pavement.

Understanding density altitude

Density altitude is defined as the “pressure altitude corrected for nonstandard temperature variations.” What does that mean? The best way to answer is to fly a little off course and examine the standard atmosphere.

This term refers to tables with the values of air pressure, temperature, and air density for each altitude listed. The table below shows standard atmosphere figures for sea level up to 4,000 feet above sea level using the units commonly used in the United States, including density in slugs per cubic foot.

You often hear density described in pounds per cubic foot, which works well enough for ordinary uses. But pounds are a measure of force, not mass. Scientists and engineers working in American units use the slug for mass. Near the Earth’s surface, a slug is approximately 32.2 pounds.

The table shows us that on a “normal” day, the air pressure at 1,000 feet is 28.86 inches of mercury, the temperature 55.4 degrees F, and the density is 0.002309 slugs per cubic foot. But, if on a particular day a higher temperature at an airport with an elevation of 1,000 feet has made the density only 0.002112 slugs per cubic foot, we would say that the density altitude is 4,000 feet, no matter what the true altitude, atmospheric pressure, and temperature at the time.

Today, of course, no one looks up values for density altitude in a table; he or she uses a computer to calculate them. An easy-to-use online computer is on the website of the El Paso, Texas, NWS office.

To find density altitude, scroll down to the Pressure Conversions box. First, select Station Pressure—you’ll need this to calculate the density altitude. Station pressure is the actual barometric reading at a location. It’s used to calculate the altimeter setting, which you use along with the field elevation to calculate station pressure at the bottom of the box.

Back to Newton, Iowa

Using the El Paso weather calculator and data from the METAR (aviation weather observations), including the temperature and dew point observations for Newton at 2:15 p.m. on July 24, 2010, we found the density altitude.

The airport elevation is 953 feet, but at 2:15 p.m. the density altitude was 4,095 feet. Even at 10 a.m., when the temperature at Newton was only 86 degrees F—but with extremely high humidity, the density altitude was 3,671 feet.

In other words, takeoff performance that day for small airplanes was more like their performance at cruise altitudes.

Performance charts like the one for the Cessna 172M described previously usually don’t allow pilots to enter a figure for humidity. While high humidity does affect performance, its role isn’t as large as those of temperature and atmospheric pressure. An advantage of online calculators is that you can factor in humidity.

If Newton’s afternoon of July 14, 2010, had been enjoying very low humidity with a dew point of 50 degrees instead of the actual dew point of 88 degrees, the density altitude would have been 3,671 feet. While this isn’t as bad as on a very humid day, an airplane’s performance would probably still be poor enough to leave a pilot wondering whether the engine is failing. In other words, high humidity lowers your personal performance more than your airplane’s.

Keep in mind that if the heat and humidity or even the heat alone is making you feel lazy, it’s doing the same thing to your airplane.