What is a hectopascal?
1013.25. The number appears on every weather chart in the world. Here's where it comes from, and why your grandfather's barometer showed something completely different.
1013.25. No unit, just the number, and yet anyone who has looked at a weather chart has a rough idea what it means: normal pressure, calm weather, a barometer needle sitting in the middle of the scale. It's the standard atmosphere at sea level, and it's the reference point every other pressure reading is measured against.
Same number, two names
A pascal is the SI unit for pressure — one newton spread over one square metre. That's a very small pressure, so meteorologists use hectopascals, hPa, a hundred pascals at a time.
Before hPa became the standard, weather services around the world used millibars, mbar. The two units aren't just related, they're identical: one millibar is exactly one hectopascal. When meteorology officially switched to the SI unit hPa, every number on every weather chart could stay exactly the same. Only the letters changed.
The pressure of the standard atmosphere at sea level, in hectopascals. It's not an average of the weather in any particular place — it's a fixed reference value that instruments, aircraft altimeters and weather charts across the world calculate from.
WMO, Guide to Meteorological InstrumentsMercury in inches, air in pascals
Your grandfather's barometer probably didn't show hPa. Older barometers were literally columns of mercury, and so they were read in the unit you could see with your own eyes: millimetres or inches of mercury. That habit still lingers in parts of the world — American aviation still uses "inches of mercury" on altimeters today, even though there isn't a drop of mercury left in the instrument.
The move from a physical column of liquid to a plain number on a dial is a story in itself. It's about an empty metal can that made mercury unnecessary.
What all these units have in common, whether they're called inches, millimetres, millibars or hectopascals, is that they measure the same physical quantity: the weight of the air sitting above the spot where the instrument is. Switching units is just bookkeeping. The pressure they describe is the same.
Why Aneroid's dial stops at 965 and 1040
Aneroid's dial runs from 965 to 1040 hPa. That's not an arbitrary choice of tidy round numbers. It's the range ordinary weather actually moves within: deep low-pressure systems passing through with wind and rain push the number down towards the lower limit, while solid high pressure with calm, clear weather pushes it up towards the top. Go further in either direction and you're into the extreme — the pressure values that belong to storms, not to an ordinary week.
A scale covering everything from the theoretical lowest to the theoretical highest pressure on Earth would leave most of its space unused. A scale built for the pressure you actually experience shows every real change clearly instead.
Put 1013.25 in the middle of that range, and there's room on both sides for a proper change in the weather, without the needle running off the scale at the first sign of a storm.
This article explains a physical unit, not health advice. Aneroid shows pressure in hPa because it's the world standard — the app is not medical equipment and does not predict or treat anything.