How the Sager forecast works
The line under the live readings on the front page doesn't come from a weather model. It comes from a mechanical instrument you read by hand — four dials, a four-character code, and a lookup table. Here's exactly how it reaches its answer.
Right now at the plot
The idea
The Sager Weathercaster is a forecasting instrument devised by Raymond Sager. It contains no model of the atmosphere and does no arithmetic. It encodes something older: the observation that the combination of wind direction, how that direction has shifted, air pressure and the trend in that pressure predicts the next half-day rather well in temperate latitudes.
You read four dials, combine them into a code such as C231, and look the code up.
There are 5,000 possible codes and the table answers essentially all of them.
That's the whole method.
Dial 1 · The wind
The first character carries the most information: not just where the wind is blowing from, but which way it has rotated over the last six hours. In the northern hemisphere a wind that turns clockwise usually means a weather system is clearing; one that turns anticlockwise usually means a front is arriving.
Eight compass sectors × three kinds of change = 24 letters, plus Z for calm.
The letter I is skipped, so the run is A–Z with a gap.
| Wind from | Backing | Steady | Veering |
|---|
Plus Z — calm, under 1 mph, whatever the recorded bearing.
Dial 2 · The barometer
Air pressure now, in hectopascals, in one of eight bands. High pressure settles the weather; low pressure stirs it up.
| Position | Pressure | In plain terms |
|---|---|---|
| 1 | 1029.5 hPa and above | Very high |
| 2 | 1019.3 – 1029.5 | High |
| 3 | 1012.5 – 1019.3 | Slightly above normal |
| 4 | 1005.8 – 1012.5 | Normal |
| 5 | 999.0 – 1005.8 | Slightly below normal |
| 6 | 988.8 – 999.0 | Low |
| 7 | 975.3 – 988.8 | Very low |
| 8 | below 975.3 | Extremely low |
Dial 3 · The barometer's trend
How that pressure has moved over the last three hours. This is the dial that carries the urgency: a rapid fall is the single most reliable warning of rough weather coming.
| Position | Change over 3 hours | Meaning |
|---|---|---|
| 1 | rising 1.4 hPa or more | Rising rapidly |
| 2 | rising 0.7 – 1.4 | Rising slowly |
| 3 | between −0.7 and +0.7 | Steady |
| 4 | falling 0.7 – 1.4 | Falling slowly |
| 5 | falling 1.4 or more | Falling rapidly |
Dial 4 · The sky
What the sky is actually doing at this moment — the instrument's one check against reality.
| Position | Sky | Read from |
|---|---|---|
| 1 | Clear | CAVOK, SKC, CLR, NSC, NCD |
| 2 | Partly cloudy | FEW, SCT (up to half the sky) |
| 3 | Mostly cloudy | BKN (more than half) |
| 4 | Overcast | OVC (complete cover) |
| 5 | Precipitation | any rain, snow or hail group — or rain at the plot in the last 30 minutes |
Those abbreviations are from METAR, the international format airports use to
report conditions. A report such as METAR EGBB 011920Z 32009KT CAVOK 20/07 Q1020
reads: Birmingham, 19:20 UTC, wind from 320° at 9 knots, sky and visibility OK, 20 °C,
pressure 1020 hPa.
Putting it together
The four readings are simply written side by side. Here is the code the front page is showing at the moment, taken apart:
Reading the answer
Every one of the 378 possible forecasts is built from three parts in the same order:
Fair; · Moderate to Fresh Breeze (13-24 mph) · Southwest or West winds.
| What the weather will do | 21 possibilities, from “Fair” to “Rain and turning cooler” |
| What the wind will do | 8 possibilities, from “Probably increasing” to “Hurricane” |
| Where it will blow from | 9 possibilities, including “Shifting (or variable)” |
Try the dials yourself
Every one of the 5,000 codes is in this page. Set the four dials and see what the instrument would say.
Where our readings come from
The Almanac reads the dials from two places, and it is worth being straight about which:
- From the plot itself — wind direction now, wind direction six hours ago, temperature, and whether it has rained in the last half hour.
- From Birmingham Airport, about 20 km away — the air pressure, its trend, and the state of the sky. The allotment station has no barometer, and no instrument that can see cloud.
So the wind half of the forecast is genuinely local and the pressure half is regional. For pressure that hardly matters — it varies little over 20 km. For cloud it matters more: it can be overcast at the airport and bright over the plot.
Two places we depart from the original
- Calm is measured by wind speed, not bearing. The original treats a recorded bearing of exactly 0° as “calm”. We take the wind speed instead, so a genuine northerly is never mistaken for still air.
- The pressure trend is fitted, not subtracted. Airport pressure is reported in whole hectopascals, and this dial has thresholds at 0.7. Rather than subtract two rounded numbers, we fit a line through all the readings in the window, which recovers the detail the rounding throws away.
How accurate is it?
Treat it as a well-informed hunch rather than a promise. It has no satellite, no radar and no knowledge of anything happening more than 20 km away — but wind shift and pressure trend are genuinely powerful signals, which is why the instrument was taken seriously for decades. It is at its best warning that settled weather is about to break.
It updates every ten minutes, and looks 12 to 24 hours ahead.