Elevation and Barometric Readings: Why Your App Adjusts to Sea Level
Your weather app almost certainly shows sea level pressure, not the actual pressure where you are standing. The two differ by roughly 1 hectopascal for every 8 metres of elevation, which means a barometer in Denver genuinely reads around 840 hPa while the app reports something close to 1015. The adjustment exists so that a weather map can show where the highs and lows actually are, rather than a picture of the terrain. For anyone tracking symptoms against pressure, the adjustment is harmless and arguably helpful, because what the body responds to is change over time, and the adjustment is a near-constant offset that does not affect change.
The confusion this causes is common enough that it is worth walking through properly, especially if you own a home barometer or a weather station that reports something different from your phone.
Three numbers, three different jobs
There are three pressure values in routine use, and they answer three different questions.
Station pressure is the raw measurement: the actual weight of the atmosphere pressing down at the sensor's exact location and elevation. It is what a barometer in your kitchen physically measures. In Denver it is about 840 hPa. In Mexico City, at 2,240 metres, closer to 780. At sea level in Boston, around 1013. Station pressure is what an aircraft altimeter, a physiologist, or an engineer designing a pressure vessel would want.
Sea level pressure, sometimes written MSLP for mean sea level pressure, is station pressure mathematically extended downward to what it would be if the station were at sea level. This is the number on weather maps, in forecasts, and in essentially every consumer weather app. It exists for one reason: comparison. Without it, a pressure map of the United States would simply be a map of the Rocky Mountains, since every high-elevation station would show low pressure and every coastal station high pressure, and the actual weather systems would be invisible underneath the terrain signal.
Altimeter setting is a third value, close to sea level pressure but calculated differently, used in aviation so that pilots flying in the same region all set their altimeters to the same reference and therefore maintain correct vertical separation. If you have ever heard air traffic control read out a number like "two niner niner two", that is the altimeter setting in inches of mercury. It is usually within a hectopascal or two of MSLP but the two are not identical.
How the adjustment is calculated
The principle is straightforward. To convert station pressure to sea level pressure, you calculate the weight of an imaginary column of air that would fill the space between the station and sea level, and add it on.
The rough rule of thumb is about 1 hPa per 8 metres, or roughly 1 inHg per 1,000 feet, near sea level. The relationship is not linear over large distances because air is compressible and thins as you go up, so the real formula uses the barometric equation with temperature as an input.
That temperature term is the interesting part. The imaginary air column does not exist, so its temperature has to be assumed. Meteorological practice uses the average of the current temperature and the temperature twelve hours ago as a proxy for the column's mean temperature. This works acceptably well at modest elevations, and increasingly poorly as the station gets higher. At stations above about 1,500 metres the assumption strains, and at stations on high plateaus during extreme cold the sea level values produced can be genuinely questionable — the reason you occasionally see implausible record high pressure readings reported from Siberia or Greenland.
For almost everywhere people actually live, the adjustment is reliable and consistent.
Why this matters less than you would think for symptoms
Here is the practical point. If you live at 1,600 metres, your app's sea level adjustment adds roughly 200 hPa to your station pressure. That offset is not fixed to the exact hectopascal, because the temperature term moves a little, but it is close to constant across any timescale that matters for a headache.
What that means is that if the sea level value on your app falls 12 hPa over eighteen hours, the actual pressure at your house also fell close to 12 hPa over those eighteen hours. The shape of the curve, the rate, and the magnitude of the change all survive the adjustment intact. Only the baseline shifts.
This matters because the change is what the research on weather and headache has focused on. Studies looking at barometric pressure and migraine have generally examined rate and magnitude of change rather than absolute level, and several have found associations with falling pressure in particular, though the literature is genuinely mixed and effect sizes vary widely between studies and between individuals. The evidence is stronger for "some people are sensitive to pressure change" than for any universal threshold.
If you live at altitude and want to see your real station pressure, most weather station hardware will report it and some apps let you toggle it. But for tracking purposes, you are not losing information by leaving it on the default.
Where the difference does bite
Three situations where the distinction genuinely matters:
Comparing your home barometer to your app. This is the most common source of confusion. An analogue barometer on the wall measures station pressure. Many of them ship with a calibration screw on the back precisely so you can offset them to match sea level values, and if you have never adjusted yours and you live anywhere above sea level, it will disagree with your phone by a predictable amount. Neither device is broken.
Moving between elevations. If you drive from Boston to Denver, your body experiences a real drop of roughly 175 hPa in station pressure, and it is a genuine physiological event — this is the mechanism behind altitude headache. Your app will show almost nothing, because both cities are reported at sea level equivalent. A weather app is not the tool for tracking travel-related pressure exposure; it is the tool for tracking weather-related pressure change at a fixed location. The distinction becomes obvious when you look at a place like Lone Tree, Colorado, where the adjusted forecast values look ordinary while the air people actually breathe is well under 850 hPa.
Reading historical or scientific data. Research papers and raw station archives sometimes report station pressure without saying so. If you are comparing a published figure to your own record, check which value is being used before concluding anything.
A note on units
A separate and equally common confusion is units rather than reference level. Hectopascals (hPa) and millibars (mb) are numerically identical — 1013 hPa is 1013 mb — and the change in name was a change in nomenclature, not in the quantity. Inches of mercury (inHg) is used in the United States and Canada for aviation and consumer forecasts, where 1013 hPa is 29.92 inHg. Millimetres of mercury (mmHg) appears in some countries and in medical contexts. We covered this in more detail in what a millibar actually is.
A 10 hPa drop is about 0.30 inHg. Because inHg has a smaller numerical range, the same weather event looks like a smaller number, which can make pressure changes seem less dramatic than they are if you are used to hectopascals.
What to do with this
If you are tracking symptoms against pressure, three suggestions.
Pick one source and stay with it. Mixing a wall barometer reading with an app reading will produce a record with a step change in it that means nothing.
Record the change, not just the level. A single number tells you almost nothing; a value paired with "down 9 hPa in the last twelve hours" tells you quite a lot. This is why a migraine tracker app that samples continuously is more informative than checking a number once a day, since a once-a-day reading cannot distinguish a slow decline from a sharp one that already reversed.
Do not chase a magic number. People sometimes look for the threshold below which they get symptoms. Because the reported value is adjusted, and because individual sensitivity varies enormously, an absolute threshold is rarely meaningful. Rate of change and your own recorded history are far more useful than anyone else's number. Track it with Pressure Pal if you want the record built automatically rather than by hand.
FAQ
Is sea level pressure a real measurement or an estimate?
It is a calculated value derived from a real measurement. Station pressure is measured; sea level pressure is computed from it using elevation and an assumed temperature for the imaginary air column below the station. At low and moderate elevations the computation is accurate enough that the distinction rarely matters.
Why does my barometer disagree with my weather app?
Almost certainly because the barometer is showing station pressure and the app is showing sea level pressure. Check your elevation and divide by 8 to get the approximate difference in hectopascals. If the gap matches, both instruments are working correctly.
Does the adjustment hide pressure changes I would otherwise feel?
No. The adjustment is effectively a constant offset over the hours and days that weather systems operate on, so changes pass through it unchanged. A 15 hPa fall in the adjusted value corresponds to a 15 hPa fall in real pressure at your location.
Should I use station pressure for tracking instead?
You can, and if your hardware offers it there is no harm in it. But you will get the same information about changes either way, and sea level values have the advantage of being comparable to forecasts, to maps, and to what other people in other places are seeing.
What is the altimeter setting for, if sea level pressure already exists?
Aviation safety. It uses a fixed standard atmosphere rather than the observed temperature in its calculation, which makes it consistent between aircraft in a way that MSLP is not. Two pilots at different altitudes over the same airfield need to agree exactly, and the standardised calculation guarantees that.
Does high elevation mean more or fewer pressure-related headaches?
That question is about absolute pressure rather than adjustment, and the answer is complicated — we looked at it separately in does living at high altitude reduce pressure migraines. Briefly: high-elevation locations have lower absolute pressure but often a smaller day-to-day range, so the two effects push in different directions.