Wet-Bulb Relative Humidity Calculation Guide

Calculate RH from dry-bulb, wet-bulb and station pressure

A wet-bulb reading is the temperature reached by an appropriately ventilated wetted sensor as water evaporates. To convert a measured dry-bulb temperature and wet-bulb temperature into relative humidity (RH), also provide the air pressure at the measurement point and use a psychrometric method appropriate to the instrument. The wet-bulb value is not an RH percentage and cannot be converted correctly by a temperature difference alone.

This article is a worked calculation guide, not a live on-page calculator. It gives the inputs, equations, a checkable example and the conditions under which a field result can be trusted. For a control or safety decision, follow the applicable instrument and engineering standard rather than treating a simplified worked example as a calibrated measurement.

Collect and check the three inputs

• Dry-bulb temperature T: ordinary air temperature, shielded from direct radiation and local heat.
• Wet-bulb temperature Tw: a wetted, adequately ventilated sensor reading at the same air location and time. Verify wick condition and water supply.
• Station pressure P: actual local air pressure, not a sea-level-corrected forecast pressure.

Use °C for both temperatures and hPa for pressure in the example below. Under ordinary unsaturated conditions the measured wet bulb is no higher than the dry bulb. If Tw > T, investigate sensor, units, wick, ventilation and reading order before calculating. Near saturation the two temperatures approach one another.

A reproducible approximate calculation

The US National Weather Service publishes the following near-surface psychrometric calculation for T and Tw in °C and station pressure P in hPa. Compute saturation vapor pressure at each temperature: es = 6.112 × exp(17.67 × T / (T + 243.5)); ew = 6.112 × exp(17.67 × Tw / (Tw + 243.5)). Pressures es and ew are in hPa.

Then compute actual vapor pressure e = ew − P × (T − Tw) × 0.00066 × (1 + 0.00115 × Tw). Finally RH = 100 × e / es. Enter T, Tw and P in the stated units. Check that the computed e is physically plausible and that RH lies between 0 and 100%; out-of-range output indicates an input or method problem, not a new air state.

The numbers above are a rounded example from the published NWS equations, not a field measurement. At the same T and Tw, using a different actual station pressure changes the result. At other pressure regimes or when precise metrology matters, use the applicable ASHRAE/WMO property relationship and the psychrometer's calibration rather than assuming this one approximation covers every condition.

Do not invert an unrelated approximation

Some popular formulas estimate wet-bulb temperature from dry-bulb temperature and already-known RH. That is the opposite input direction: they cannot be copied as a direct T-and-Tw-to-RH formula. The ASHRAE Handbook explicitly treats dry-bulb, wet-bulb and pressure as a set of inputs from which humidity ratio, vapor pressure and RH are obtained. Use the correct direction and the matching unit system.

On a psychrometric chart, locate the intersection of the dry-bulb line and the wet-bulb line on a chart for the actual pressure, then read RH or intermediate vapor-pressure and humidity-ratio values. Dew point lies along the same water-vapor content state toward the saturation curve; moving along a constant dry-bulb line to find dew point would change the vapor content and is not the right construction.

Measurement errors that distort the result

An incompletely wetted wick tends to reduce evaporative cooling, making Tw approach T and biasing the derived RH upward. Poor ventilation, direct sun, sensor contact with a hot surface or reading the two thermometers at different times also changes the result. Pressure error matters at altitude and in enclosed process systems. Record the instrument, wick/water condition, airflow and pressure source with the calculated number.

A derived RH is useful for monitoring air state and estimating dew point, but it does not alone prove a crop canopy is frost-free, a product is dry, or an occupied workplace is safe from heat stress. Frost risk also depends on surface/canopy temperature and radiative conditions. Worker heat-risk assessment uses workload, clothing, acclimatization, radiant heat and air movement; NIOSH guidance should be consulted rather than treating one wet-bulb number as a universal safety cutoff.

Use the result in an engineering decision

• Record T, Tw, station pressure, time, place and operating state together.
• Validate the calculation against a trusted chart or software for the same pressure and units.
• Check sensor calibration and ventilation before using small differences to trigger a process action.
• For condensation control, compare calculated dew point with actual surface temperature; for material acceptance, use a material-specific test.

The example is a general published method and does not represent a Yakeclimate laboratory test, onsite reading or equipment performance result.

Sources

US National Weather Service: RH and dew point from temperature, wet bulb and station pressure

ASHRAE Handbook: psychrometric chart and three-input state calculation

World Meteorological Organization humidity-measurement chapter hosted by NWS

US NIOSH: occupational heat-stress context