A skydiver's guide to how barometric altimeters work

Barometric altimeters do not measure altitude directly; rather, they measure air pressure and convert this measurement to an estimated altitude using a standardized mathematical formula. This has an effect on the accuracy of altimeter readings.

Varying pressure around the jumper

In freefall, significant pressure differences become apparent around a jumper and inside a jumper’s helmet and skydiving rig. Above the jumper is the well-known burble or wake—a low-pressure, turbulent region in which an altimeter will read high and oscillate more. Below, dynamic pressure builds, forming a thinner, smoother high-pressure region in which altimeters will read low. Such differences can amount to hundreds of feet in altitude and can cause disagreements among altimeters mounted in different locations. For wingsuiters, the differences are even more extreme. An altimeter reading low is, of course, the safer direction. The effect persists under canopy, though to a lesser degree.

Temperature affects accuracy

Pilots use the same barometric altimeter technology as skydivers, and in flight school they learn: “From high to low or hot to cold, look out below.” The mnemonic is clumsy, but the warning is real: flying into colder air or falling pressure makes a barometric altimeter read high—the dangerous direction, where you are lower than indicated.

The reason is the conversion formula mentioned above, which assumes a “standard” day: 59 °F at sea level, cooling at a fixed rate on the way up (about 3.6 °F per 1,000 feet), in perfectly dry air. The colder the day, the further below the numbers you really are—on a freezing morning, an indicated 3,000 feet can mean a true altitude closer to 2,800. On a hot afternoon the error reverses, silently awarding you extra altitude. Humidity comes into play as well, but to a lesser extent: moist air is lighter than dry air, so on a humid day you also end up a little higher than indicated. Two things are worth knowing about this error: it grows with altitude above the dropzone, and every barometric device shares it—including AADs, which work on the same principle.

Want today’s version of this ladder for your own dropzone? Our barometric altimeter error calculator estimates the gap from the current observation at any airport.

Weather drift

Barometric pressure at ground level drifts over the course of a day. For this reason, pilots must routinely update their altimeter's pressure reference scale (the Kollsman window) using the current altimeter setting provided by local weather stations or air traffic control. While most digital altimeters do this automatically, analog altimeters have to be manually zeroed before jumping. Fortunately for skydivers, the drift of pressure from takeoff to landing is generally negligible, amounting to just a few feet on a stable day. Over the course of a day, however, a front can shift surface pressure 5–10 millibars, which amounts to well over 100 feet of altitude error.

Lag

Digital pressure sensors and microcontrollers alike are electronic devices and as such have their own inherent processing lag, either due to data and signal processing techniques (which vary by manufacturer) or the speed of their processing. This causes the altimeter to read higher than the jumper actually is during descent—the dangerous direction—and the faster a jumper falls, the greater the deviation: every tenth of a second of delay is worth about 18 feet in belly-to-earth freefall, but less than 2 feet under canopy.

Assimilating this into practice

Be aware of the nature of barometric altimeter technology. Don't expect readings of altimeters in different places or on different bodies to correspond exactly. Don't expect them to correspond to GPS readings. Anticipate potential issues (e.g., wingsuits produce large stagnation-pressure regions and wakes; perhaps the center of the chest strap is not the best position for an altimeter). If you jump an analog altimeter, zero it at the dropzone and adjust it before every jump if necessary. Remember that on cold days, barometric altimeters read high (the dangerous direction), and appreciate your pilot for the secretly gifted extra altitude on those hot days.