How it works

Twice a day the site reads the National Weather Service gridpoint forecast for Nashville, Tennessee, and passes it through a deterministic engine. The same weather always produces the same music. Nothing is chosen by hand and nothing is random. The whole mapping is below.

Governing precedence

Three rules, in this order, decide the shape of every song.

1. Condition picks genre: What the sky is doing chooses the idiom.
2. Genre defines the BPM range: Each idiom has a legal tempo window.
3. Temperature positions within it: Departure from normal moves the tempo inside that window. Temperature never breaks the range.

How the condition is decided

From the structured gridpoint numbers, not the forecast wording. The prose is for the lyrics; the numbers are for the music. A twenty percent chance of an afternoon storm is a sunny day.

Chance of precipitation under 30%: No precipitation genre is possible, whatever the words say. The sky decides.
30% to 55%: Precipitation is eligible only if sky cover is 60% or more. Otherwise the sky decides.
Above 55%: The precipitation type drives the genre.
Thunderstorm: Only at 50% chance or higher, and only when the forecast names thunder.
Light against heavy rain: Split by forecast accumulation: a quarter inch or more in the period is heavy.
Precipitation type: The one reading taken from the prose — rain, snow, sleet or freezing rain. The grids do not express it cleanly.
Fog: Only when visibility is under three miles. Patchy fog in the text is not enough.
Advisory wind: Sustained 25 mph or gusts 45 mph, measured, not described.
Sky fallback: Sky cover 0–25 clear, 25–60 partly, 60–90 mostly cloudy, 90–100 overcast. The clear branch then splits hot, mild or cold.

Key

The tonic follows the calendar: one full turn of the circle of fifths per year. Unsettled air borrows a neighbor — a rising barometer moves the tonic one step sharpward, a falling one moves it one step flatward. The step is always a single fifth, so a borrowed key sounds related rather than random. This is separate from the cadence rule below.

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Rising above +2.5 hPa: One step sharpward, toward next month's key.
Falling below −2.5 hPa: One step flatward, toward last month's key.
Otherwise: The month's home tonic.

Mode

Wind direction chooses the scale. Dead air has no mode at all.

South: Lydian
Southwest: Ionian
West: Mixolydian
Southeast: Dorian
East: Aeolian
Northeast: Phrygian
North: Phrygian dominant
Northwest: Locrian
Under 2 mph: A drone on the tonic. No mode.

Ending

The three-hour pressure trend decides how the piece stops.

Rising above +1.5 hPa: Full authentic cadence, resolving to the tonic.
Steady within 1.5 hPa: Normal resolution.
Falling below −1.5 hPa: Unresolved. Suspended on the fourth, fades without cadence.
Falling below −4.0 hPa: Ends mid phrase, cut off.

Genre and tempo range — day

Clear, 88°F and above: Desert rock / psych · 95–120
Clear and mild: Sunshine pop / folk rock · 108–132
Clear, high below 45°F: Sparse acoustic / chamber folk · 76–96
Partly cloudy: Indie pop / dream pop · 100–124
Mostly cloudy: Slowcore / trip hop · 72–96
Overcast: Shoegaze · 68–92
Fog or haze: Dark jazz / ambient · 56–76
Light rain: Emo / bedroom pop / lo-fi · 82–104
Heavy rain: Post rock / grunge · 92–116
Thunderstorm: Metal / industrial / drum and bass · 124–175
Snow: New age / neoclassical / choral · 54–74
Sleet or freezing rain: Glitch / minimal cold electronic · 88–108
Sustained 18 mph, or gusts to 35: Post punk / krautrock / motorik · 118–142
Low below 15°F, or high below 25°F: Drone / dark ambient · 48–68. Outranks everything else.

Genre — night

The night vocabulary is deliberately narrow: ambient, downtempo, slowcore, dream pop, dub, nocturnal jazz, neoclassical. Hard cap of 96 BPM, whatever the weather. The condition still colors it.

Stormy night: Dub with heavy low end.
Clear cold night: Solo neoclassical piano.
Foggy night: A single held saxophone in a large empty room.
Clear mild night: Dream pop.
Overcast night: Slowcore.

Tempo position

Departure from the 30-year normal, as a U curve rather than a ladder. Both ends are high energy — but heat is loose and sprawling while cold is tight and sharp. Extreme cold is tense, not slow.

Within 5°F of normal: Lower middle of the range. Measured.
5–15°F above: Upper middle. Loose.
More than 15°F above: Top of the range. Feverish.
5–15°F below: Upper middle. Taut.
More than 15°F below: Top of the range. Severe and sharp.

Reverb and space

Dew point sets how wet the room is.

Below 40°F: Bone dry. Close mic, no tail.
40–55°F: Natural room.
55–65°F: Plate reverb, wide.
65–70°F: Large hall. Long tails, blurring.
Above 70°F: Everything drowning. Smeared, no separation.

Instrumentation

Dew point also chooses the instrument bodies, not only the room. Moisture is the one thing that meaningfully differs between two otherwise identical calm mild afternoons. Dry air is struck and plucked; humid air is bowed and breathed.

Below 40°F: Struck and plucked. Piano, acoustic guitar, marimba, pizzicato. Every note has a beginning and an end.
40–55°F: Mixed acoustic. Guitar, upright bass, light strings. Attacks still clear.
55–65°F: Sustain enters underneath. Electric piano, organ, bowed strings under the attacks.
65–70°F: Bow and breath dominant. Horns, pads, swells. Attacks softened away.
Above 70°F: Pure sustain. Everything is a swell. No transient survives.

Timbre and register

Sky cover sets the frequency balance.

0–25%: High register present. Bright transients, shimmer.
25–60%: Full spectrum, balanced.
60–90%: Rolled off highs. Midrange forward, felted.
90–100%: Low register dominant. Dark, no air.

Percussion

Genre sets a floor and a ceiling; sustained wind positions the kit between them. A thunderstorm on a dead calm day is still not rubato, and a gale in falling snow is still not a drum solo. The gust factor — the spread between sustained wind and peak gust — sets how erratic the rhythm is.

0–4 mph: None, or a single pulse. Rubato, no grid.
5–11 mph: Light kit. Brushes and shakers.
12–19 mph: Full kit, driving.
20–29 mph: Busy. Fills and syncopation.
30 mph and up: Chaotic. Breakbeat or blast beat, by genre.
Thunderstorm floor: Never below busy with fills.
Heavy rain and wind floor: Never below a full driving kit.
Light rain and sleet floor: Never below a light kit.
Snow and fog ceiling: Never above a light kit.
Clear and cold ceiling: Never above a full driving kit.
Extreme cold ceiling: No percussion at all.
Night: Never above a full driving kit, whatever the floor.

Dynamic arc

Chance of precipitation sets the shape. Precipitation amount sets the low end weight.

Under 20%: Flat and static.
20–50%: Gentle build, no payoff.
50–80%: Clear build to a peak in the final third.
Above 80%: Breaks open early and stays open.

Vocal

Visibility sets how far away the voice is.

Under 3 miles: Buried, processed, distant. Partly unintelligible.
3–8 miles: Present but softened, set back in the mix.
Above 8 miles: Dry, forward, clear.

Duration

Each song lasts as long as the period it describes, scaled between two and five minutes. Nashville's shortest day is 9.54 hours and its longest 14.46. One formula serves both songs: the day song is handed daylight hours, the night song hours of darkness. Sunrise, sunset and moon phase are computed locally from latitude and longitude — no external service.

Both songs: 120 + ((hours − 9.54) ÷ 4.92) × 180 seconds, held between 120 and 300
Summer solstice: Day song 5:00. Night song 2:00.
Winter solstice: Inverted.
Equinoxes: Both near 3:30.

Moon

The moon appears only in the night song, where it gates melodic presence. Cloud cover gates the moon: a full moon behind total overcast gets the new-moon treatment, because you cannot see it.

New: Near silent. Drone and texture only, no melody.
Waxing crescent: A single quiet, incomplete motif.
First quarter: The motif takes shape, harmony underneath.
Waxing gibbous: Fuller. Countermelody, wordless voice.
Full: Peak luminosity. Real melody, real vocal — the most song-like night of the month.
Waning: The arc reverses. Elements withdraw in the order they arrived.

Memory

The engine also looks backward, at the last 7 and 30 days. These facts are what the lyric is written about — the lyric describes change, not conditions.

Streaks: Consecutive days of the same dominant condition.
Droughts: Days since the last precipitation, days since the last dry day.
Records: Whether today is the warmest or coldest in thirty days.
Seasonal firsts: First freeze, first 90, first snow.
Swing: Magnitude of the change from yesterday.
Monotony: Raised when the last five days sit inside a six-degree band.

The page itself

The site takes its appearance from the same numbers. Background luminance comes from sky cover, hue from temperature — cold blues through to warm ambers — motion speed from wind, and blur and grain from dew point. No one designs the variants.

Every scored day is stored permanently, raw forecast included, so the archive is a real record and any day can be recomputed from source.