The oldest forecast still works
Long before satellites, radar, and computer models, sailors and farmers read the weather from a single instrument: the barometer. A falling barometer meant wind and rain on the way; a rising one meant clearing skies. The physics behind that folk wisdom is simple and still sound. Air pressure measures the weight of the air above you. High pressure means air is sinking, which suppresses cloud formation and brings calm, clear weather. Low pressure means air is rising, cooling, and condensing into clouds and rain. But the pressure value alone tells only half the story. The trend — whether pressure is rising or falling, and how fast — tells you which way the weather is heading. A pressure of 1013 hectopascals that is falling fast means something very different from the same pressure rising fast, and the calculator on this page combines both numbers into a single weather score so you can read the trend at a glance.
The score: value plus trend
The calculator's weather score runs from 0 to 100 and starts at 50 for standard pressure (1013 hPa) with no change. Every hectopascal above 1013 adds 2 points, every hectopascal below subtracts 2, every hectopascal of rise in the last three hours adds 8 points, and every hectopascal of fall subtracts 8. A pressure of 1025 hPa rising 3 hPa in three hours scores 98: fair weather, confidently. A steady 1013 hPa scores 50: mixed or unremarkable. A pressure of 990 hPa falling 4 hPa scores 0 after clamping: unsettled, deteriorating weather. The weighting is deliberate: trend matters more than absolute value, because a high pressure that is collapsing fast often brings worse weather than a moderate pressure that is steadily rising. The three-hour window is the standard used by weather stations and forecasters, because it is long enough to show a real trend and short enough to react to. Always read the score together with the sky, the wind, and a proper forecast; the barometer is a trend instrument, not a crystal ball.
| Pressure | 3-hour change | Score | What it usually means |
|---|---|---|---|
| 1025 hPa | +3 hPa | 98 | Fair, settled, clearing |
| 1020 hPa | +1 hPa | 72 | Fine, improving slowly |
| 1013 hPa | 0 hPa | 50 | Mixed, no strong trend |
| 1005 hPa | -1 hPa | 26 | Clouding over, rain possible |
| 995 hPa | -4 hPa | 0 (clamped) | Stormy, deteriorating fast |
Reading a barometer properly
Most home barometers are aneroid instruments: a sealed metal cell expands and contracts with pressure and moves a needle. They need two things to be useful: correct calibration to sea level, and a habit of checking the trend, not just the reading. If your barometer is not adjusted for your altitude, it will read low by roughly 1 hPa per 8 metres of elevation, which makes its absolute value meaningless even though its trend remains valid. Many digital weather stations and smartwatches report both station pressure (raw) and sea-level pressure (adjusted); use sea-level pressure for the calculator, because the score's 1013 baseline is a sea-level figure. The old-fashioned habit of tapping the glass before reading is not superstition: it frees a sticky mechanism so the needle settles on the true value. Record the reading morning and evening, or let a weather station log it, and the trend will emerge from the noise of small fluctuations.
Why falling pressure means wind and rain
Low-pressure systems are engines of rising air. Warm, moist air converges at the surface, rises, cools, and condenses into cloud, and the condensation releases heat that drives the system further. The pressure gradient around the low drives wind: the steeper the gradient, the stronger the wind, which is why the fastest pressure falls accompany the strongest storms. High-pressure systems do the opposite: air sinks from above, warms as it descends, and evaporates cloud, producing clear skies and light winds. The boundary between a high and a low is where fronts live, and a front's passage is marked by a sharp pressure change: falling steadily ahead of a warm front, dipping to a minimum as it passes, then rising behind a cold front, often with a burst of rain and a wind shift. Reading the barometer is, in effect, reading the approach and passage of these systems hours before they arrive overhead.
Pressure, altitude, and false alarms
- Altitude changes: driving up a mountain drops the pressure reading without any weather change. Only compare readings taken at the same place.
- Temperature: extreme cold can stiffen an aneroid cell slightly, but modern instruments compensate well. Do not chase fractions of a hectopascal.
- Indoor heating: a barometer near a heater or in a sealed, heated room can read oddly. Keep it in a stable, representative spot.
- Storm surge of noise: a single three-hour fall can be a passing shower, not a storm. Look for a sustained trend over six to twelve hours before acting.
- Sea-level adjustment: if your station reports station pressure, add about 1 hPa per 8 metres of elevation to get sea-level pressure for the calculator.
Pressure and the human body
People often blame pressure changes for headaches, joint pain, and migraines, and the evidence is mixed but not imaginary. Rapid pressure falls are associated with increased migraine reports in some studies, possibly because pressure changes affect the inner ear and blood vessels, though the effect is modest and confounded by the weather changes that accompany the pressure fall. Divers and mountaineers care about pressure for harder physical reasons: pressure determines decompression risk underwater and oxygen availability at altitude. For most people, the practical health point is simpler: falling pressure often brings the weather that keeps you indoors, and rising pressure brings the weather that gets you out. If you track pressure alongside symptoms in a diary, a pattern may emerge that is genuinely useful for planning, even if the mechanism remains debated.
Combining the barometer with a real forecast
The barometer's great strength is locality: it measures the air above your house, while a forecast model averages over kilometres. Its great weakness is that it cannot see what is coming from hundreds of kilometres away, which is exactly what models do well. The smart approach uses both: check the forecast for the synoptic picture (fronts, systems, timing), then watch your barometer for local confirmation and timing. If the forecast says rain this afternoon and your pressure is falling fast, bring the washing in now. If the forecast says rain but your pressure is rising steadily, the rain may arrive later than forecast or miss you entirely. Farmers, sailors, and hillwalkers have always worked this way, and modern weather apps have not made the habit obsolete: they have just added a screen to the wall instrument. The calculator gives that habit a number, but the judgement remains yours.
A short history of the barometer
The barometer was invented in the 1640s by Evangelista Torricelli, who discovered that a column of mercury in a sealed tube rose and fell with the weather, proving that air has weight. For two centuries the mercury barometer was a scientific instrument and a gentleman's curiosity. The aneroid barometer, invented in the 1840s, made the instrument portable and domestic, and by the Victorian era a barometer hung in most middle-class halls, often engraved with optimistic labels like Rain, Change, and Fair that oversimplified the trend it measured. FitzRoy, the captain of HMS Beagle and founder of the UK's first weather service, championed barometers for storm warnings at sea and is credited with saving many lives. Today's digital sensors in phones and watches are the same idea miniaturised: a tiny pressure sensor, logged continuously, giving you a trend chart in your pocket that FitzRoy would have envied.
Using the calculator day to day
Take a reading now and a reading from three hours ago (your weather station's history, a smartwatch log, or a note on the fridge). Enter both, read the score, and check the sky: high, thin cloud thickening from the west with falling pressure is the classic rain warning, while clearing skies with rising pressure after rain is the classic improvement. Log the score morning and evening for a week and you will start to recognise your local patterns: the pressure range that means a settled week, the fall rate that means a storm, the rise that follows a cold front. Over time the number becomes intuitive, and you will find yourself glancing at the barometer the way sailors always have: not to replace the forecast, but to know, before the forecast updates, which way your own sky is heading.
Pressure is the weight of the sky, and its trend is the sky's direction of travel. Read both, and you are reading tomorrow's weather today.