Lore, on a switch
Every forager has a grandfather’s rule.
Four days after the thunder. The week before the full moon. The morning the glass drops. These rules have been handed down in Swedish forests for as long as people have picked in them, and they are half the pleasure of going out.
So they are here, as three switches you hold: storm, moon, pressure. Each one off until you turn it on, each one showing you the real measurement underneath it — including the actual lightning SMHI located over your forest, with its time and its position, drawn on your map.
Start with the lightningThe cues
EnrichedStorm
Last thunder 6 days ago
Moon
Waxing gibbous · full in 3 days
Pressure
Falling · 9 hPa over 3 days
What a switch does
A shade on the forecast, capped at 0.12.
Switch a factor on and it shades the favourability — enough to colour a decision between two even days, and held at ±0.12 so the decision stays yours. The forecast carries an enriched badge for as long as a switch is on, and it shows the arithmetic that got it there.
Lore shades how good the ground looks. Confidence answers to measured data alone — the same station, the same laser scan, the same ten days, whatever the moon is doing. Turn all three off and the forecast is the measured one, to the digit: a thing you can check rather than a thing we say.
The storm rule, taken seriouslyA worked example · Söderåsen, a September Thursday
Storm
24 mm of rain six days back
+0.05
up to +0.05
Pressure
the glass down 9 hPa over three days
+0.04
up to ±0.06
Moon
waxing gibbous, 78 per cent lit
+0.03
up to ±0.05
All three, this day
+0.11
Three folk cues all pointing the same way on the same day, and the total still sits inside the cap. That is the cap doing its job: on the rare day everything lines up, lore gets to tint the call and the measurement keeps it.
Wherever a read appears
The measured verdict stays the primary one and the enriched verdict rides alongside it, so you can see the exact moment lore tips a call from building to moving — and decide for yourself whether you buy it. When the band holds, it says so: lore nudging up +0.05, holding the same call.
Storm · the real lightning
The strikes that actually fell on your forest.
SMHI runs the national lightning archive: every located strike over Sweden with a position, a time to the nanosecond and an uncertainty ellipse. We fetch the last ten days over your ground and draw each one where it landed, sized by how precisely it was fixed and brightest when it is newest.
That turns the oldest mushroom rule in the country into something you can look at. The strike is day zero. Days four to nine are shaded — the window the rule points at — and the marker sits where the lightning genuinely puts you, because it is driven by when the lightning genuinely fell.
Scrub forward through the ten-day forecast and the strikes age with you. A bolt that fell six days ago is inside its window this morning and has aged out of it by Thursday, and you watch it happen on the map.
Storm to flush · Söderåsen
24 mm fell with the thunder six days ago. By the old rule you are in the window now, and it stays open for three more days.
Scrub forward · the same strike, ageing
Now
+1
+2
+3
+4
+5
+6
+7
+8
+9
Gold days are the days that strike is still inside its window. Push past them and the dot fades to its own age — the lore expires on schedule, the way the rule always said it did.
What could be underneath it
The storm rule is worth taking seriously, and the interesting part is knowing exactly which half of it is measured. A Swedish summer thunderstorm arrives with heavy convective rain; that rain raises litter humidity and soil moisture at the 7–28 cm depth that matters; and mycelium that has taken up water builds fruit bodies over the following days, which is a lag documented across fungal phenology and modelled in the forecast on its own two clocks.
So the chain from rainfall to flush is established science, and it is already forecast from the rain alone. Where folk observation takes over is the count of days — four to nine is a forager’s number, handed down and remembered — and the idea that thunder in particular marks the start, rather than the same water arriving quietly overnight. There is lab work on electrically stimulating cultivated shiitake into fruiting; whether anything like that reaches a spruce forest in Skåne is an open question, and squarely folk territory for now.
Read the storm rule in fullMeasured, and shown as measured
- The strike: position, time and location uncertainty, from SMHI’s national archive.
- The rain that came with it, in millimetres, from the nearest active station.
- Soil moisture and litter humidity in the days after, which is what the forecast already runs on.
Folk observation, and badged as folk
- The four-to-nine day count, held as a window rather than a date.
- That thunder marks the start better than plain rain does.
- That a big storm is worth more to a season than the sum of its millimetres.
Honest about the feed too: SMHI’s daily files land about a day behind, so you see strikes for the last ten days with the newest timestamp on the card, and a day with zero strikes reads as zero. The strikes are evidence for you to weigh, kept architecturally separate from the number — what shades it is the measured rainfall, capped like everything else.
Moon
Phase, illumination, and days to full.
The moon is the one cue that needs no feed at all: its phase is arithmetic, exact anywhere, available on the dullest February afternoon. The disc draws the real terminator, the strip counts you down to full, and it tells you which day the lore window opens.
Plenty of foragers time their week around the full moon, and there is a long literature of gardeners and old almanacs behind them. Treat it as what it is: a pleasing rule with a thin evidence base, on a switch, worth ±0.05 at its very furthest. It also makes the almanac something to open in the dead months, when the map has little to say and the calendar still turns.
The moon and the mushroomTonight
Waxing gibbous · 78% lit
Full moon in 3 days.
−3
−2
−1
Now
+1
+2
Full
At 78 per cent lit this reads +0.03 on the favourability, and it says so on screen.
Pressure
The glass, over three days.
Barometric pressure comes off the same SMHI stations as the temperature, and the trend across three days is the version the old rule actually cares about: a glass falling steadily ahead of a front, rather than an hour of wobble.
A falling barometer before rain is the classic bringing-on cue, and it has the virtue of arriving early — the pressure moves before the weather does, which is why it was worth watching in a house with no forecast. Falling shades the forecast up, to a furthest ±0.06, with the hectopascals on the card so you can see what it is responding to.
Back to the forecastPressure · nearest active station
Falling · 9 hPa over 3 days
+0.04
Steady or rising, the same switch reads the other way and tells you plainly that the cue is quiet today.
Three switches, yours
All three off, and you have the measured forecast back exactly.
The lore layer is opt-in, per factor, off on the day you install it. With every switch down the engine takes a zero and the forecast is the measured one to the last digit — which is a test you can run in ten seconds rather than a promise you have to take.
And when you do switch one on, you have the badge, the cap, the arithmetic and the real lightning behind it. That is what we think folklore deserves: kept, shown properly, and handed to you with the controls.
Lore
The storm rule, taken seriously
What could be behind four-to-nine, and what the lightning data lets you check.
Lore
The moon and the mushroom
A long history, a plausible-sounding mechanism, and thin evidence — worth understanding properly.
The forecast
The ground at one metre, the weather ten days out