Why Often-Overlooked Atmospheric Pressure Is Critical for Every Pilot

No, this doesn’t address the anxiety you might feel while shooting an instrument approach to minimums or the intense pressure to keep your speed up after a slam-dunk vector to the final approach course into a busy airport. 

In this case, I am talking about atmospheric pressure. Pressure is not something we ordinarily dwell on before or during a flight. But if you fly in a location where the mountain peaks are plentiful, then you should become better acquainted with how much the altimeter setting can truly vary in mountainous areas even when the weather is tranquil.       

This Article First Appeared in FLYING Magazine

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Of all the weather elements measured by sensors at airports, atmospheric pressure is perhaps the most underappreciated, but the most important, especially to any pilot flying under IFR. In fact, pressure is the only weather element that cannot be directly observed or qualitatively sensed by a trained observer or pilot. Without it, pilots have no way to establish their height above the earth’s surface with any certainty.

Consequently, the pressure sensor is the most reliable and accurate one in the array. So, you can bet that this equipment is carefully checked for accuracy quite often.

Understanding Station Pressure

First, let’s start with what is measured at weather stations—that is, atmospheric pressure or what is referred to as station pressure. 

For the Automated Surface Observing System (ASOS), there are three pressure sensors at most towered airports—located inside the acquisition control unit, which is a climate-controlled structure, such as an observing office or air traffic control tower. There are two sensors at smaller airports. Six pressure readings are taken per minute by each sensor and a one-minute pressure value is determined. If any of the six readings are missing or if the one-minute pressure value differs by more than 0.04 inch in comparison to the other sensors, then the pressure value is set to “missing,” and the maintenance check indicator is appended to all subsequent METAR or SPECI reports. Given this redundancy, it’s very difficult to get a faulty reading.    

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In and of itself, this measurement is useless even if it is accurate and reliable. As you likely know, the air pressure decreases exponentially with increasing height. So, if we were to make a plot of actual reported station pressures at a specific time over the entire U.S., what we would really see is a terrain relief map. Essentially, there would be lower pressures always at stations with higher elevations. As you can imagine, this isn’t very practical for analyzing the surface weather patterns so a method is needed to remove the effect of the differences in station elevation. This requires that the measured atmospheric station pressure be reduced to a common reference plane called mean sea level (msl) height.   

There are two methods for achieving this. The first is the altimeter setting, and the second is referred to as a reduction to sea level pressure (SLP). The one that pilots are most familiar with is the altimeter setting. It’s typically provided on a standard briefing and may be obtained on the ground or in flight through air traffic control (ATC) or by listening to the local weather broadcast from an AWOS, ASOS, or ATIS.

According to Chapter 11 of the Federal Meteorological Handbook, the altimeter setting is defined as, the “pressure value to which an aircraft altimeter scale is set so that it will indicate the altitude above mean sea level of an aircraft on the ground at the location for which the value was determined.” While this definition is accurate, it leaves out some important details that may not be obvious to most pilots. 

The altimeter setting is calculated by a pressure reduction computation in accordance with the International Standard Atmosphere (ISA). Consequently, the altimeter setting has no temperature dependence but uses a fancy formula to try and “guess” what the air pressure measurement would be if we could dig a big hole in the ground and move the airport down to sea level elevation. Similarly, SLP is a theoretical pressure at the station if it were at sea level.  

The two computations are somewhat different in that the SLP does not assume a standard atmosphere. Instead, the computation considers the reported station temperature. In fact, it uses a 12-hour mean ambient temperature computed once each minute from the current and previous 12-hour reported five-minute average temperature. An average temperature is used to reduce any diurnal (daily) effects.  Compared to the high degree of accuracy of the measured station pressure, computing these two theoretical pressures is quite crude and requires a slightly different method to achieve the same result—that is, pressure at sea level. In the end, both methods attempt to calculate what the pressure would be at the bottom of a large hole in the earth assuming the bottom of the hole is at sea level.

To do this requires that you consider the station elevation, station pressure, and perhaps station temperature (for SLP). Bad guesses as to what this temperature profile should look like “below the ground” can result in errors in either of these calculations, especially so in regions of sharp terrain.

 Leadville, Colorado (KLVX), for instance, is one of the highest incorporated cities in the U.S. Here the absolute station pressure measured at the site is normally around 700 millibars (mb) or 20.67 inches of mercury (Hg). If the average sea level pressure is 1,000 mb, this means that nearly 300 mb must be added to obtain a reading at sea level. A mere 1 percent error in estimating the exact correction would result in a 3 mb error in SLP.

The potential errors in the altimeter setting are larger for stations at very high elevations since more correction must be applied. The factor used to reduce station pressures to altimeter settings contains an exponential term that depends on the station elevation and can become large rather quickly for high-elevation stations. 

Even if you routinely fly in designated mountainous areas such as the Rockies, you probably have not noticed that altimeter settings can vary quite a bit, even when the general pressure gradient throughout the area is rather weak.

For example, on an early August afternoon, there was an area of high pressure in west-central Colorado. All the surface stations in this region reported sea level pressures ranging from 1,023 to 1,017 mb. Not a terribly large pressure gradient between stations. But would you believe that the altimeter settings at these stations varied from 30.35 to 30.82 inches Hg?  

 Well, in this case it was true. At 2056Z, the altimeter setting at Eagle, Colorado (elevation of 6,539 feet), was 30.35 and less than 40 miles as the crow flies, and Copper Mountain (elevation of 12,073 feet) was reporting 30.82. Could it really be that much of a pressure difference in just 40 miles?

In part, there will always be small scale local variations in atmospheric pressure. However, if we add four other nearby stations to include Kremmling, Aspen, Walden, and Leadville along with their respective elevations, you’ll notice a distinct pattern. As the elevation increases, so does the altimeter setting.

Is there some direct correlation between airport elevation and altimeter setting, or could this be a sensor calibration issue? 

Given that there are multiple pressure sensors that must agree at each station and that these sensors are calibrated several times each year, the calibrations are likely fine, and the rather high altimeter readings from the high elevation stations are an artifact of the reduction-to-sea-level method used to compute the altimeter setting.

While all stations should report an altimeter setting, many do not report SLP. This is not a problem since most aircraft altimeters in the U.S. do not allow for SLP settings anyway. However, if you regularly use datalink weather such as SiriusXM, you can quickly retrieve altimeter settings for all reporting stations, even those with high elevations.

Accurate Altimeter Settings Matter

So, you might be asking, will this apparent “error” cause me to hit anything solid? Probably not. 

The important point is to always use the altimeter setting provided by ATC when flying under IFR or VFR with flight following. Even when flying VFR without ATC help, it may be tempting to pull up a surface observation or two broadcast to your datalink weather receiver and use the altimeter setting at Copper Mountain (KCCU) in central Colorado even though you are near Eagle.

Given that Copper Mountain is 40 miles to the east, that may put you off by nearly 500 feet from all those other pilots using the altimeter setting for Eagle. So, if you were flying at 16,500 feet, you could be at the same altitude as those IFR aircraft flying at 16,000 feet using the Eagle altimeter setting.

If you are flying in or out of Leadville, you should still use the altimeter setting provided by the ASOS on the field. This is because the altimeter in your aircraft also assumes a standard atmosphere and is designed to perform all the same calculations (either mechanically or digitally) as the ASOS. You may find, however, as you are switching to or from the Leadville altimeter setting to the new setting provided by ATC, you may be doing quite a bit of twisting of the Kollsman knob and adjusting your altitude.  

One last story about altimeter settings. If you are flying IFR, the latest altimeter setting is perhaps the most important piece of weather data you’ll receive prior to your departure. It almost becomes a habit to reach down and adjust the altimeter anytime you hear it on the ground or while in the air. Failing to set your altimeter could make an IFR departure an event you’ll never forget—assuming you live to tell about it.   

My “aha” moment came many years ago while departing an uncontrolled airfield. As usual, I looked down at the altimeter and cranked the Kollsman knob around to the airport’s elevation. I had to do a lot of twisting (nearly 500 feet), which seemed a bit odd at first, but the weather the last time I shut down was drastically different, so it made perfect sense. The pressure changed and the altimeter adjusted accordingly. But I made one big mistake. I turned the knob the wrong direction, so it read 1,146 feet and not the airport’s elevation at 146 feet. When you are not paying close enough attention and maybe in a little hurry, the two altitudes look quite similar.   

I did not recognize what I had done until I departed and found that my initial altitude of 3,000 feet came up quickly as I leveled off.  

“Wow, aircraft performance today is awesome,” is what went through my mind at first. I was still clueless. It was VFR conditions, and I sensed something was wrong. In the back of my mind, I kept thinking that I was a bit low. So, I switched from unicom to departure and reported level at 3,000 feet because that’s what my altimeter said…so it must be right. 

ATC came back with, “Arrow, 1234B, altimeter is 30.39, verify you are level at 3,000?” That’s when my hand hit my forehead so hard it just about knocked off my headset.  Doh! I had the altimeter set to something completely different. Fortunately, I was not departing out of a mountain airport or busy airspace. The controller caught my mistake and didn’t send me to pilot jail this time. It was certainly a wake-up call for something as ubiquitous as the altimeter setting.  

After some reflection, I added a new item to the before-takeoff checklist to be sure I always check my altimeter setting with the airport elevation. That’s right before the checklist item I added earlier to be sure I’m sitting at the proper end of the right runway. 

Yes, there’s another story for that one in the future.

This feature first appeared in the November Issue 964 of the FLYING print edition.