On the morning of October 11, 2025, in a bothy on the south side of Beinn Eighe, the walker waking up at six checked her altimeter. The display read 720 metres. She had gone to sleep at 11 p.m. with the altimeter reading 705 at the same location. The bothy had not moved. The pressure had.
A drop of fifteen metres on a barometric altimeter, with the instrument and the walker stationary, corresponds to a pressure decrease of roughly 1.8 hectopascals. Over seven hours, that is a noticeable trend. It is the leading edge of a low-pressure system, and it is information the walker can use.
Most barometric altimeters do this calculation in reverse. They measure pressure, assume a standard atmosphere, and report elevation. The standard atmosphere assumption is what makes the altimeter useful as an altitude indicator and, paradoxically, also what makes it useful as a weather indicator. When the walker is stationary, any apparent change in altitude is in fact a change in pressure at a fixed elevation.
The relationship is approximately one hectopascal per eight metres of indicated altitude, near sea level. At higher elevations, the relationship changes slightly, but for trail-walker purposes the standard ratio is close enough.
Reading pressure trends from an altimeter requires three small disciplines.
The first is taking the reading at a known elevation. The walker should calibrate the altimeter at a benchmark elevation, ideally at the start of the day or at a known landmark, and then check it at every rest stop where the elevation is also known.
The second is interpreting the difference. If the altimeter, at a known elevation, reads higher than the true elevation, the pressure has dropped since the last calibration. If it reads lower, the pressure has risen. A useful rule: an apparent altitude increase of fifteen metres at a fixed location indicates roughly a 1.8 hectopascal drop in surface pressure.
The third is interpreting the trend over time. A drop of one to two hectopascals in twelve hours is unremarkable. A drop of three to four hectopascals in twelve hours suggests an approaching low-pressure system. A drop of more than five hectopascals in twelve hours is a meaningful weather event and, in many ranges, presages a storm within twenty-four hours.
Some walkers find it easier to track the trend by recording the altimeter reading at a fixed location at fixed times. The bothy walker on Beinn Eighe had recorded 705 at 11 p.m. and 720 at 6 a.m. The fifteen-metre difference was unambiguous; she did not need to consult a forecast to know that the day would not be the day the forecast had described the previous evening.
Rising pressure indicates the opposite, and is generally good news for walkers. A reading at a known elevation that drifts lower over a day or overnight indicates pressure rising and weather likely improving. A walker who falls asleep in a low-pressure trough and wakes to a rising altimeter trend has a reasonable expectation of a better day than the one that ended.
Some practical applications.
Multi-day trips: on a hut-to-hut or backcountry traverse, the pressure trend is often more useful than the forecast, which is increasingly stale as the trip continues. A walker on day four of a five-day route, without cell signal and without a hut warden's radio, can still make informed decisions about the next day's exposure by tracking the altimeter at fixed elevations.
Overnight in exposed bivouacs: a falling pressure trend overnight at a high camp is the strongest signal a walker is likely to get of incoming weather. Many alpine guides have a rule that a drop of more than three hectopascals overnight at a high camp means descending in the morning rather than continuing.
During storm conditions: a barometric altimeter is often the first instrument to indicate that a storm has reached its lowest pressure and begun to ease. The pressure minimum often corresponds to the windiest and most intense phase. A walker waiting out a storm in a shelter can use the altimeter to identify when the worst has passed.
There are limitations to be honest about. Pressure trends do not reveal what kind of weather is approaching, only that weather is approaching. A walker who tracks pressure and ignores cloud, wind, and the regional forecast is using a fraction of the available information. Pressure is one input. It is the input that works without cell signal, without a forecast, and without anything other than the small machine on the walker's wrist.
Modern GPS-equipped watches often combine GPS altitude with barometric altitude in a calculated reading that can mask the pressure trend. Walkers who want to use the altimeter as a weather instrument should check whether their watch has a setting to lock the barometric reading or to display raw pressure in hectopascals. Most do. The setting is often buried two menus deep and is worth finding before leaving the trailhead.
Mechanical altimeters, of the sort still made by Thommen and a few small Swiss workshops, do not have the calibration ambiguity. They read pressure directly, converted to elevation by a simple aneroid mechanism. Walkers who carry mechanical altimeters often develop a faster intuition for pressure trends, because the mechanism does not hide the underlying physics behind GPS smoothing.
Either way, the discipline is the same. Note the reading at a known location. Note it again at the same location or at another known location. Compare. Decide.
The walker on Beinn Eighe left the bothy at seven, walked the four kilometres back to her car, and drove to a friend's flat in Kinlochewe. The storm arrived at noon. The forecast issued the previous evening had not mentioned it. The altimeter had, in its quiet way, mentioned it at six in the morning.
A small instrument used attentively is often more useful than a sophisticated one used carelessly. The altimeter on a walker's wrist is one of the most attentive instruments in the pack. It rewards the small habit of reading it, twice, at known locations, and asking what the difference between the two readings is saying.
Astrid Pereira teaches navigation at a Welsh outdoor centre. She owns a thirty-year-old Thommen altimeter that has outlasted four GPS watches and intends to outlast a fifth.
