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fix(weather): derive clear-sky peak from latitude and date, use current time for sun-angle

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Replace the hardcoded peakSolarRadiation constant with solarNoonPeak(), which
scales the atmospheric ceiling (950 W/m²) by sin(solar elevation angle). Elevation
is computed from the current latitude and solar declination (function of day-of-year),
so the normalization is accurate across seasons and locations rather than assuming
equatorial/summer-solstice conditions.

Also switch the sun-angle time calculation from d.LastUpdated to time.Now() so
that a stale cache entry (e.g. after a Starlink connectivity gap) doesn't cause the
wrong time-of-day to be used when inferring conditions.
This commit is contained in:
Adriano Caloiaro 2026-04-11 19:13:49 -06:00
parent 5dae01b00f
commit 7099961ba3
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1 changed files with 32 additions and 6 deletions
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38
weather/condition.go
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@ -1,6 +1,9 @@
package weather
import "math"
import (
"math"
"time"
)
// Condition is an inferred weather condition with a human label and emoji.
type Condition struct {
@ -8,8 +11,22 @@ type Condition struct {
Emoji string `json:"emoji"`
}
// peakSolarRadiation is the approximate clear-sky maximum at sea level (W/m²).
const peakSolarRadiation = 950.0
// atmosphericCeiling is the clear-sky solar radiation (W/m²) with the sun
// directly overhead at sea level. This is the physical upper bound; actual
// noon peaks are lower everywhere outside the tropics and scale with sin(elevation).
const atmosphericCeiling = 950.0
// solarNoonPeak returns the approximate clear-sky solar radiation (W/m²) at
// solar noon for the given latitude and date. It accounts for both seasonal
// declination and latitude so the normalization in Infer stays accurate year-round.
func solarNoonPeak(lat float64, t time.Time) float64 {
declinationDeg := 23.45 * math.Sin(2*math.Pi/365*float64(t.YearDay()-81))
elevationDeg := 90.0 - math.Abs(lat-declinationDeg)
if elevationDeg <= 0 {
return 0
}
return atmosphericCeiling * math.Sin(elevationDeg * math.Pi / 180)
}
// twilightSolar is the solar radiation threshold (W/m²) below which the sun is considered
// low in the sky, producing dawn or dusk conditions.
@ -54,7 +71,11 @@ func Infer(d Data, lat, lon float64) *Condition {
}
// Estimate local time from longitude (rough UTC offset).
utcMinutes := d.LastUpdated.Hour()*60 + d.LastUpdated.Minute()
// Use the current wall-clock time rather than d.LastUpdated so that a stale
// cache entry (e.g. after a connectivity gap) doesn't cause the sun-angle
// calculation to reflect an hour from earlier in the day.
now := time.Now().UTC()
utcMinutes := now.Hour()*60 + now.Minute()
localMinutes := utcMinutes + int(math.Round(lon/15))*60
localMinutes = ((localMinutes % 1440) + 1440) % 1440
fraction := sunFraction(localMinutes)
@ -83,8 +104,13 @@ func Infer(d Data, lat, lon float64) *Condition {
}
// Normalize solar radiation against the expected clear-sky max for this hour.
// Clamp to [0, 1] to absorb sensor noise.
normalized := math.Min(solar/(peakSolarRadiation*fraction), 1.0)
// Clamp to [0, 1] to absorb sensor noise or unusually bright readings (e.g.
// cloud-edge reflection).
peak := solarNoonPeak(lat, now)
if peak == 0 {
return &Condition{"Clear night", "🌙"}
}
normalized := math.Min(solar/(peak*fraction), 1.0)
if normalized > 0.85 {
return &Condition{"Sunny", "🌞"}