STORMY, RAIN, CHANGE, FAIR, VERY DRY

The words on a barometer dial are old, imprecise, and charmingly wrong. They're still there, on Aneroid's dial too. Here's why.

VERY DRY sits at 1035 hPa on Aneroid's dial. That isn't true. It has never quite been true. A pressure of 1035 hPa can easily turn up on a perfectly ordinary, overcast Tuesday. The word promises more than the physics can keep.

And yet it's there. Along with STORMY, RAIN, CHANGE and FAIR, spread across the scale, exactly as on a barometer from your grandfather's hallway.

One word for each pressure

The words are older than the can Aneroid is named after. Before Vidié's invention, air pressure was always read through a mercury tube — in stick and wheel barometers, where a hand pointed at a scale. A fixed pressure got a fixed word, and that made the instrument readable for people who couldn't have told a hectopascal from anything else.

When the Frenchman Lucien Vidié built the first workable metal barometer in 1843 — a corrugated diaphragm, held by 33 coiled springs and sealed inside an airless metal can — the mercury disappeared from the instrument. The words didn't. They still sit on aneroid barometers today. "Aneroid" means precisely "without liquid" — and that was all that got replaced.

Why it only holds approximately

A pressure of 1013 hPa doesn't mean the same thing in January as in July, and it doesn't mean the same thing in Denmark as by the Mediterranean. The same figure can signal a calm winter's day in one place and a summer shower in another. The words on the dial were written for one place, with one kind of weather, based on a rule of thumb about what usually came with a given pressure. A rule of thumb is not a law, and it never was.

Aneroid's own dial

Even so, Aneroid's dial inherits the old order: STORMY at 973 hPa, RAIN at 988, CHANGE at 1004, FAIR at 1021, and VERY DRY at 1035. Each sits at its own pressure, the way the words have sat on wall barometers for generations — not because the app is promising dry weather every time the needle reaches that figure. It's an instrument that's allowed to carry its own history. You can dress up a dial with new, accurate statistics. You can't dress it up with more truth than air pressure actually contains.

What actually counts: a different word

The honest part of a barometer was never the fixed word at a fixed figure. It was the curve — a barograph draws air pressure precisely as a line over time, because it's the shape of the line, not a single point on it, that carries the information. That's why Aneroid doesn't stop at the dial's old words. Below the dial sits another kind of text: Falling, Rising or Steady — the direction word, derived from the pressure change itself, not from a hundred-year-old rule of thumb about what a given figure usually means. That's why that word appears as plain text, not just as a needle position you have to read for yourself. It's the word that's actually true right now.