Humidity & Dew Point Calculator — Indoor Comfort for Nebraska's Seasonal Climate Swings

Nebraska's climate swings from sweltering, humid summers (Omaha averages 74°F dew points in July) to frigid, parched winters where indoor relative humidity can drop below 10%. This calculator computes dew point temperature, absolute humidity, indoor comfort rating, and humidification/dehumidification recommendations using the Magnus–Tetens formula.

Typical Nebraska indoor range: 60–80°F (15–27°C)
ASHRAE comfort zone: 30–60% RH indoors
Used to estimate window condensation risk and winter humidification need
Nebraska July avg ~75%; January avg ~65% (but very cold = very dry absolute)

Formulas Used

Magnus–Tetens Dew Point (Lawrence 2005):

γ(T, RH) = ln(RH/100) + (b·T)/(c+T)
Tdp = c·γ / (b − γ)

where b = 17.625, c = 243.04°C (valid −40°C to +60°C)

Saturation Vapor Pressure (Buck 1981):

es(T) = 6.1121 · exp((18.678 − T/234.5) · T/(257.14 + T)) [hPa, T in °C]

Actual Vapor Pressure:

e = (RH/100) · es(T)

Absolute Humidity:

AH = (2165.0 · e) / (273.15 + T) [g/m³]

Humidex (Canadian heat-stress index, used for summer comfort):

Humidex = T°C + 0.5555 · (e − 10.0)

Window Condensation Risk (winter):
Condensation forms on a window surface when the window's inner surface temperature falls below the indoor dew point. For a standard double-pane window, inner surface ≈ outdoor temp × 0.15 + indoor temp × 0.85 (simplified U-factor model, U≈0.30 BTU/hr·ft²·°F).

Assumptions & References

  • Magnus–Tetens constants from Lawrence (2005), BAMS — accurate to ±0.1°C for typical indoor conditions.
  • Buck equation for saturation vapor pressure (Buck 1981, J. Appl. Meteorol.).
  • ASHRAE Standard 55-2020: thermal comfort zone 68–76°F, 30–60% RH for sedentary occupants.
  • ASHRAE Handbook of Fundamentals 2021: recommended winter indoor RH 30–40% to prevent window condensation in Nebraska's climate zone 5A/6A.
  • Nebraska climate data: Omaha mean July dew point ~72°F; Lincoln mean January outdoor temp ~22°F (NOAA 1991–2020 normals).
  • Window condensation model assumes standard double-pane low-e window (U≈0.30); actual performance varies by window age and type.
  • Humidex scale: <29 comfortable, 30–39 some discomfort, 40–45 great discomfort, >45 dangerous.
  • Outdoor inputs are optional; if omitted, window condensation and outdoor dew point sections are skipped.
  • All calculations are performed client-side in the browser; no data is transmitted.

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