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Meteorological Calculation

Ventilated Dry & Wet Bulb Humidity

Estimate humidity from a ventilated water-covered wet bulb, dry bulb and station pressure.

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Relative humidity (%)
62.9904
Vapor pressure (hPa)
19.9053
Introduction & principle

e=es(Tw)−0.00066(1+0.00115Tw)P(T−Tw); RH=100e/es(T), P in hPa. This is an empirical ventilated water-bulb estimate. Use 0–50°C, Tw≤T, 500–1100 hPa. Natural ventilation, ice coating, dirty wick and instrument coefficients can change the result.

Worked example

25°C dry bulb, 20°C wet bulb and 1013.25 hPa give approximately 63% RH. Equal bulb temperatures imply saturation.

Detailed usage guide
  1. 1Confirm a clean water-covered wick and suitable ventilation; do not use this formula for an ice bulb.
  2. 2Enter simultaneous dry and wet temperatures and local station pressure, not sea-level pressure.
  3. 3Read RH and vapor pressure; impossible combinations are rejected rather than clipped.
Common problems & FAQ

Which instrument conditions are assumed?

A water-covered clean wick, adequate forced ventilation and the displayed empirical coefficient are assumed. Use instrument-specific instructions where available; this is not an ice-bulb calculator.

Methods & references

Method reviewed November 4, 2026. These tools support learning and recording, not safety-critical decisions.

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