Humidity and Dew Point Calculator
ANA›Life Services Authority›National Calculator Authority›Humidity and Dew Point Calculator
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Humidity and Dew Point Calculator
Calculate relative humidity, dew point temperature, and absolute humidity from dry-bulb temperature and either a wet-bulb reading or a known relative humidity value.
### Input Method
Calculation Mode
Known Relative Humidity → Dew Point & Absolute Humidity Dry-Bulb + Wet-Bulb → Relative Humidity & Dew Point
Temperature Unit
Celsius (°C) Fahrenheit (°F)
### Temperatures
Dry-Bulb Temperature
°C
Wet-Bulb Temperature
°C
Relative Humidity
%
Elevation (for wet-bulb mode)
m
Calculate
### Results
// ── helpers ────────────────────────────────────────────────────────────────── function humToggleMode() { const mode = document.getElementById('hum_mode').value; const unit = document.getElementById('hum_unit').value; const label = unit === 'C' ? '°C' : '°F'; document.getElementById('hum_unit_tdb').textContent = label; document.getElementById('hum_unit_twb').textContent = label; document.getElementById('hum_row_twb').style.display = mode === 'wb' ? '' : 'none'; document.getElementById('hum_row_rh').style.display = mode === 'rh' ? '' : 'none'; document.getElementById('hum_row_elev').style.display = mode === 'wb' ? '' : 'none'; document.getElementById('hum_result').style.display = 'none'; document.getElementById('hum_error').style.display = 'none'; } humToggleMode(); // init
// Saturation vapour pressure (hPa) via Buck equation // es(T) = 6.1121 * exp((18.678 - T/234.5) * T / (257.14 + T)) T in °C function humEs(T_C) { return 6.1121 * Math.exp((18.678 - T_C / 234.5) * T_C / (257.14 + T_C)); }
// Dew point from actual vapour pressure e (hPa) — Magnus inversion // Td = 243.04 * ln(e/6.1121) / (17.368 - ln(e/6.1121)) function humDewPoint(e_hPa) { const lne = Math.log(e_hPa / 6.1121); return 243.04 * lne / (17.368 - lne); }
// Absolute humidity (g/m³) // AH = 216.7 * e / (273.15 + T) e in hPa, T in °C function humAbsHumidity(e_hPa, T_C) { return 216.7 * e_hPa / (273.15 + T_C); }
// Atmospheric pressure from elevation (hPa) — barometric formula // P = 1013.25 * (1 - 2.25577e-5 * h)^5.25588 function humPressure(elev_m) { return 1013.25 * Math.pow(1 - 2.25577e-5 * elev_m, 5.25588); }
// Psychrometric constant γ (hPa/°C) // γ = 0.000665 * P function humPsychrometric(P_hPa) { return 0.000665 * P_hPa; }
function humFmt(val, dec) { return val.toFixed(dec); }
function humRow(label, value, unit) { return '' + label + '' + value + '' + unit + ''; }
// ── main calculation ───────────────────────────────────────────────────────── function humCalc() { const errEl = document.getElementById('hum_error'); const resEl = document.getElementById('hum_result'); errEl.style.display = 'none'; resEl.style.display = 'none';
const mode = document.getElementById('hum_mode').value; const unit = document.getElementById('hum_unit').value;
const tdbRaw = parseFloat(document.getElementById('hum_tdb').value); if (isNaN(tdbRaw)) { humErr('Please enter a valid dry-bulb temperature.'); return; }
// Convert to °C for all calculations const tdb_C = unit === 'C' ? tdbRaw : (tdbRaw - 32) * 5 / 9;
if (tdb_C 60) { humErr('Dry-bulb temperature must be between −60 °C and 60 °C.'); return; }
let RH, e_hPa, Td_C, AH, rows = '';
if (mode === 'rh') { // ── Mode 1: RH given ────────────────────────────────────────────────── const RH_in = parseFloat(document.getElementById('hum_rh_in').value); if (isNaN(RH_in) || RH_in 100) { humErr('Relative humidity must be between 0 % and 100 %.'); return; } RH = RH_in; const es = humEs(tdb_C); // saturation vapour pressure e_hPa = (RH / 100) * es; // actual vapour pressure Td_C = humDewPoint(e_hPa); // dew point AH = humAbsHumidity(e_hPa, tdb_C);
const Td_out = unit === 'C' ? Td_C : Td_C * 9 / 5 + 32; const unitLabel = unit === 'C' ? '°C' : '°F';
rows += humRow('Dry-Bulb Temperature', humFmt(tdbRaw, 1), unitLabel); rows += humRow('Relative Humidity', humFmt(RH, 1), '%'); rows += humRow('Saturation Vapour Pressure (es)', humFmt(es, 3), 'hPa'); rows += humRow('Actual Vapour Pressure (e)', humFmt(e_hPa, 3), 'hPa'); rows += humRow('Dew Point Temperature (Td)', humFmt(Td_out, 1), unitLabel); rows += humRow('Absolute Humidity', humFmt(AH, 2), 'g/m³');
// Frost point warning if (Td_C ⚠ Dew point is below 0 °C — frost conditions possible.'; }
} else { // ── Mode 2: Wet-bulb given ──────────────────────────────────────────── const twbRaw = parseFloat(document.getElementById('hum_twb').value); if (isNaN(twbRaw)) { humErr('Please enter a valid wet-bulb temperature.'); return; } const twb_C = unit === 'C' ? twbRaw : (twbRaw - 32) * 5 / 9;
if (twb_C > tdb_C + 0.001) { humErr('Wet-bulb temperature cannot exceed dry-bulb temperature.'); return; } if (twb_C 60) { humErr('Wet-bulb temperature must be between −60 °C and 60 °C.'); return; }
const elev = parseFloat(document.getElementById('hum_elev').value) || 0; if (elev 9000) { humErr('Elevation must be between 0 and 9000 m.'); return; }
const P = humPressure(elev); // atmospheric pressure const γ = humPsychrometric(P); // psychrometric constant const es_wb = humEs(twb_C); // sat. vp at wet-bulb const es_db = humEs(tdb_C); // sat. vp at dry-bulb
// Sprung formula: e = es(Twb) - γ * P * (Tdb - Twb) // Simplified: e = es(Twb) - A * P * (Tdb - Twb) where A = 0.000665 e_hPa = es_wb - γ * (tdb_C - twb_C); if (e_hPa db)', humFmt(tdb_out, 1), unitLabel); rows += humRow('Wet-Bulb Temperature (Twb)', humFmt(twb_out, 1), unitLabel); rows += humRow('Elevation', humFmt(elev, 0), 'm'); rows += humRow('Atmospheric Pressure (P)', humFmt(P, 2), 'hPa'); rows += humRow('Psychrometric Constant (γ)', humFmt(γ, 6), 'hPa/°C'); rows += humRow('Sat. Vapour Pressure at Twb', humFmt(es_wb, 3), 'hPa'); rows += humRow('Sat. Vapour Pressure at Tdb', humFmt(es_db, 3), 'hPa'); rows += humRow('Actual Vapour Pressure (e)', humFmt(e_hPa, 3), 'hPa'); rows += humRow('Relative Humidity (RH)', humFmt(RH, 1), '%'); rows += humRow('Dew Point Temperature (Td)', humFmt(Td_out, 1), unitLabel); rows += humRow('Absolute Humidity', humFmt(AH, 2), 'g/m³');
if (Td_C ⚠ Dew point is below 0 °C — frost conditions possible.'; } }
// Comfort index let comfort = ''; if (RH ' + comfort + '';
document.getElementById('hum_result_body').innerHTML = rows; resEl.style.display = 'block'; }
function humErr(msg) { const el = document.getElementById('hum_error'); el.textContent = msg; el.style.display = 'block'; }
#### Formulas Used
1. Saturation Vapour Pressure — Buck Equation (1981)
es(T) = 6.1121 · exp[(18.678 − T/234.5) · T / (257.14 + T)]
T in °C, es in hPa. Valid from −40 °C to +60 °C.
2. Actual Vapour Pressure from RH
e = (RH / 100) · es(Tdb)
3. Actual Vapour Pressure from Wet-Bulb — Sprung Formula
e = es(Twb) − γ · (Tdb − Twb)
where γ = 0.000665 · P (psychrometric constant, hPa/°C) and P is atmospheric pressure.
4. Atmospheric Pressure from Elevation — Barometric Formula
P = 1013.25 · (1 − 2.25577×10⁻⁵ · h)5.25588
5. Dew Point — Magnus Inversion
Td = 243.04 · ln(e / 6.1121) / [17.368 − ln(e / 6.1121)]
6. Absolute Humidity
AH = 216.7 · e / (273.15 + Tdb) [g/m³]
#### Assumptions & References
- Absolute humidity formula derived from the ideal gas law for water vapour (Mw = 18.015 g/mol).
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