Local forecasting
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183
server/src/forecast.mjs
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183
server/src/forecast.mjs
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// forecast.mjs
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import { queryHourly, getCoords } from './influx.mjs';
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import { getCelestialForecast } from './celestial.mjs';
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// ── Physics Helpers ───────────────────────────────────────────────────────────
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function linearTrend(values) {
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const n = values.length;
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if (n < 2) return 0;
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const xMean = (n - 1) / 2;
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const yMean = values.reduce((a, b) => a + b, 0) / n;
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const num = values.reduce((s, v, i) => s + (i - xMean) * (v - yMean), 0);
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const den = values.reduce((s, _, i) => s + (i - xMean) ** 2, 0);
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return den === 0 ? 0 : num / den;
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}
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function dewPoint(tempC, humidity) {
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const a = 17.27, b = 237.7;
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const gamma = (a * tempC) / (b + tempC) + Math.log(humidity / 100);
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return (b * gamma) / (a - gamma);
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}
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function heatIndex(tempC, humidity) {
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const T = tempC * 9 / 5 + 32;
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if (T < 80) return tempC;
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const HI =
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-42.379 + 2.04901523 * T + 10.14333127 * humidity
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- 0.22475541 * T * humidity - 0.00683783 * T * T
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- 0.05481717 * humidity * humidity + 0.00122874 * T * T * humidity
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+ 0.00085282 * T * humidity * humidity - 0.00000199 * T * T * humidity * humidity;
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return (HI - 32) * 5 / 9;
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}
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function vaporPressureDeficit(tempC, humidity) {
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const svp = 0.6108 * Math.exp((17.27 * tempC) / (tempC + 237.3));
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return Math.round(svp * (1 - humidity / 100) * 1000) / 1000;
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}
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function absoluteHumidity(tempC, humidity) {
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const svp = 0.6108 * Math.exp((17.27 * tempC) / (tempC + 237.3));
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return Math.round((humidity / 100 * svp * 2165) / (tempC + 273.15) * 1000) / 1000;
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}
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// ── Pressure Tendency → WMO Weather Rule ─────────────────────────────────────
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function pressureRule(trend3h, currentHpa) {
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if (trend3h < -2.0) return { label: 'Storm likely', code: 211, precipChance: 0.85 };
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if (trend3h < -0.8) return { label: 'Rain likely', code: 501, precipChance: 0.65 };
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if (trend3h < -0.3) return { label: 'Cloudy', code: 803, precipChance: 0.35 };
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if (trend3h > 1.5) return { label: 'Clearing', code: 801, precipChance: 0.05 };
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if (trend3h > 0.3) return { label: 'Improving', code: 800, precipChance: 0.10 };
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if (currentHpa < 1000) return { label: 'Unsettled', code: 802, precipChance: 0.30 };
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if (currentHpa < 1013) return { label: 'Partly cloudy', code: 802, precipChance: 0.15 };
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return { label: 'Fair', code: 800, precipChance: 0.05 };
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}
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// ── Solar Heating Curve ───────────────────────────────────────────────────────
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// Uses sun_elevation from celestial.mjs — max ~6°C gain at solar noon, clear sky
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function solarTempDelta(sunElevation, cloudFraction) {
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if (sunElevation <= 0) return 0;
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return Math.sin((sunElevation * Math.PI) / 180) * (1 - cloudFraction) * 6;
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}
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// ── Nocturnal Cooling (Stefan–Boltzmann approximation) ────────────────────────
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// Clear nights lose more heat via longwave radiation
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function nocturnalCooling(cloudFraction) {
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return (1 - cloudFraction) * 1.5; // °C/hr max clear-sky cooling
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}
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// ── Humidity Forecast ─────────────────────────────────────────────────────────
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// As temp rises, relative humidity drops (conserving absolute humidity)
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function forecastHumidity(absHumidity, forecastTempC) {
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const svp = 0.6108 * Math.exp((17.27 * forecastTempC) / (forecastTempC + 237.3));
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const rh = (absHumidity * (forecastTempC + 273.15)) / (svp * 2165) * 100;
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return Math.min(100, Math.max(0, Math.round(rh * 10) / 10));
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}
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// ── Cloud Cover Estimate ──────────────────────────────────────────────────────
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// Derived from humidity + pressure tendency (Oktas-style heuristic)
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function estimateClouds(humidity, pressureTrend) {
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let base = humidity / 100 * 0.8;
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if (pressureTrend < -0.3) base = Math.min(1, base + 0.2);
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if (pressureTrend > 0.3) base = Math.max(0, base - 0.15);
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return Math.round(base * 100) / 100;
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}
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// ── Wind Forecast ─────────────────────────────────────────────────────────────
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// Pressure gradient approximation — steeper falls = stronger winds
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function forecastWind(currentWind, pressureTrend) {
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const boost = pressureTrend < -1 ? 1.3 : pressureTrend < -0.5 ? 1.1 : 1.0;
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return Math.round(currentWind * boost * 10) / 10;
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}
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// ── Main 24hr Forecast ────────────────────────────────────────────────────────
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export async function get24HourForecast(currentSensors) {
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const now = new Date();
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const sixAgo = new Date(now - 6 * 3600 * 1000);
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const coords = await getCoords();
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const history = await queryHourly(sixAgo.toISOString(), now.toISOString());
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// Extract trend series from history
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const pressures = history.map(r => r.pressure_hpa).filter(Number.isFinite);
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const temps = history.map(r => r.temperature).filter(Number.isFinite);
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const humidities = history.map(r => r.humidity).filter(Number.isFinite);
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const winds = history.map(r => r.wind_speed).filter(Number.isFinite);
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const pressureTrend = linearTrend(pressures); // hPa/hr
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const tempTrend = linearTrend(temps); // °C/hr
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const humidityTrend = linearTrend(humidities); // %/hr
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const pressure3h = pressureTrend * 3; // 3hr tendency for WMO rules
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// Seed from current sensors
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const seedTemp = currentSensors.temperature ?? temps.at(-1) ?? 20;
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const seedHumidity = currentSensors.humidity ?? humidities.at(-1) ?? 60;
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const seedPressure = currentSensors.pressure_hpa ?? pressures.at(-1) ?? 1013;
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const seedWind = currentSensors.wind_speed ?? winds.at(-1) ?? 0;
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const seedAbsHum = currentSensors.humidity_abs
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?? absoluteHumidity(seedTemp, seedHumidity);
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const weather = pressureRule(pressure3h, seedPressure);
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// Build 24 hourly slots
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const slots = Array.from({ length: 24 }, (_, i) => {
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const time = new Date(now.getTime() + (i + 1) * 3600 * 1000);
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time.setMinutes(0, 0, 0);
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// Pressure decays toward mean over time (damped trend)
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const pressureDamping = Math.exp(-i * 0.08);
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const pressure = seedPressure + pressureTrend * (i + 1) * pressureDamping;
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// Cloud cover from humidity + pressure tendency
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const clouds = estimateClouds(seedHumidity + humidityTrend * i, pressureTrend);
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// Temperature: solar heating + nocturnal cooling on top of background trend
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// We use a placeholder sun_elevation here — getCelestialForecast will enrich it
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// so we do a two-pass: first build slots, then apply celestial, then fix temp
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const bgTemp = seedTemp + tempTrend * (i + 1);
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const temperature = Math.round(bgTemp * 10) / 10;
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const humidity = forecastHumidity(seedAbsHum, temperature);
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const wind_speed = forecastWind(seedWind, pressureTrend);
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return {
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time: time.toISOString().slice(0, 16), // "YYYY-MM-DDTHH:MM"
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temperature,
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humidity,
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humidity_abs: absoluteHumidity(temperature, humidity),
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dew_point: Math.round(dewPoint(temperature, humidity) * 100) / 100,
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heat_index: Math.round(heatIndex(temperature, humidity) * 100) / 100,
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vapor_pressure_deficit: vaporPressureDeficit(temperature, humidity),
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pressure_hpa: Math.round(pressure * 100) / 100,
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wind_speed,
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clouds,
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precip_chance: Math.round(weather.precipChance * (1 - i * 0.02) * 100), // confidence decays
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label: weather.label,
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code: weather.code,
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};
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});
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// Pass through getCelestialForecast to enrich sun/moon data
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const enriched = getCelestialForecast(coords.latitude, coords.longitude, slots);
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// Second pass — apply solar heating now that we have sun_elevation per slot
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for (const slot of enriched) {
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const solar = solarTempDelta(slot.sun_elevation ?? 0, slot.clouds);
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const cool = slot.daytime ? 0 : nocturnalCooling(slot.clouds);
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slot.temperature = Math.round((slot.temperature + solar - cool) * 10) / 10;
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slot.humidity = forecastHumidity(seedAbsHum, slot.temperature);
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slot.dew_point = Math.round(dewPoint(slot.temperature, slot.humidity) * 100) / 100;
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slot.heat_index = Math.round(heatIndex(slot.temperature, slot.humidity) * 100) / 100;
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slot.vapor_pressure_deficit = vaporPressureDeficit(slot.temperature, slot.humidity);
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slot.humidity_abs = absoluteHumidity(slot.temperature, slot.humidity);
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}
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return enriched;
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}
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