The variable that beat air pressure

In the largest dataset, the barometer didn't win. Humidity did. Here's why that doesn't actually change anything.

2,658 Britons had their pain episodes logged against weather data for fifteen months — the largest study of weather and pain that exists. When the researchers later ranked the variables, humidity won. Not air pressure.

The odds ratio for humidity was 1.139 for every 10-percentage-point rise (95% CI 1.099–1.181). For air pressure it was 0.962 for every 10 mbar rise (95% CI 0.937–0.987) — weaker, and in the direction you'd expect: higher pressure, slightly lower odds of a pain day.

Converted to standard deviations, the difference is even clearer. Humidity was associated with 12% higher odds of a pain day. Air pressure with about 4% lower. Three times the difference. Humidity was also more strongly associated with osteoarthritis than with any of the study's other diagnoses.

The same pattern in the hand

The same pattern showed up in a French cohort of 377 patients with hand osteoarthritis. Humidity above 80% was associated with the number of spontaneously tender joints (p = 0.02); at 79–85% humidity, the odds of tender joints were 2.67 times higher than below 68% humidity. Air pressure had an inverse association with the number of pressure-tender joints (p = 0.01) — but the authors found no dose-response pattern, and concluded themselves that the weather overall doesn't appear to affect symptoms to any notable degree.

The pattern even shows up in migraine research. In a Boston cohort, a 26.5-percentage-point rise in humidity was associated with 28% higher odds of a migraine attack during the warm season.

Why Aneroid still measures pressure, not humidity

So why build an app around air pressure, when the data itself points to humidity as the stronger variable? The answer isn't that the numbers are wrong. The answer is about where you are when they're measured.

The humidity outside your door doesn't say much about the air around you if you're sitting indoors. Heating, ventilation, a shower, a pot of boiling water — all of that changes the relative humidity in a room far more than the weather does. Most people spend most of the day indoors. A weather app's humidity figure is therefore a number for a place you rarely occupy.

Air pressure behaves differently. A house isn't sealed against pressure the way it's sealed against rain and damp — windows, doors and ventilation equalise pressure differences within minutes. The pressure in your living room is, for practical purposes, the same as the pressure outside. That makes air pressure the variable a phone can actually measure something meaningful with, whether you're outdoors or in.

That's a technical point, not an excuse. If humidity could be measured reliably where people actually are, the sums might look different. It just can't, yet — and air pressure can.

So no, pressure isn't necessarily the strongest variable in the literature. It's just the one that still means something once you go indoors.