Wet Bulb Temperature Calculator

Calculator

Dry-bulb temperature

°C °F

Use ambient air temperature in shade.

Relative humidity

Enter a value between 0 and 100.

Calculation method Psychrometric (iterative, uses pressure) Stull (fast approximation)

Iterative method is better for varying pressure.

Pressure

hPa kPa mmHg atm Pa

Typical sea-level pressure is 1013.25 hPa.

Altitude

Use altitude to estimate pressure

Altitude in meters above sea level.

Output precision Auto (shown to 3 decimals)

Downloads include full-precision values.

Calculate Reset

Example Data Table

Dry-bulb (°C)	RH (%)	Pressure (hPa)	Wet-bulb (°C)
30	60	1013.25	≈ 23.7
35	40	1013.25	≈ 23.9
25	80	1013.25	≈ 22.4
10	50	1013.25	≈ 6.0

Formula Used

Two options are provided. The fast option uses a widely used empirical approximation for wet-bulb temperature. The iterative option uses a psychrometric balance between vapor pressure and evaporative cooling.

• Actual vapor pressure: e = (RH/100) · e_s(T)
• Saturation vapor pressure: e_s(T) from a Magnus-type exponential relation
• Psychrometric relation (iterative): e = e_s(T_w) − A(T_w) · P · (T − T_w)
• Humidity ratio: w = 0.62198 · e / (P − e)
• Enthalpy: h = 1.006T + w(2501 + 1.86T)

Here, T is dry-bulb temperature, T_w is wet-bulb temperature, RH is relative humidity, P is pressure, and e and e_s are vapor pressures (in hPa).

How to Use

1. Enter dry-bulb temperature and choose the correct unit.
2. Enter relative humidity from a trusted sensor or forecast.
3. Provide pressure, or tick altitude to estimate it.
4. Select the iterative method for best pressure sensitivity.
5. Press Calculate to view results above the form.
6. Use Download CSV or PDF for record-keeping.

Use these outputs to plan safer outdoor activities today.

Professional Article

1) Why Wet-Bulb Temperature Matters

Wet-bulb temperature (T_w) is the lowest temperature air can reach through evaporation at a given pressure and humidity. It links physics to safety because evaporation is the body’s main cooling pathway during heat exposure and high exertion.

2) Typical Ranges and What They Mean

In dry climates, T_w can sit 8–15 °C below dry-bulb temperature, indicating strong evaporative potential. In humid conditions, the gap shrinks; a hot day at 35 °C and 60% RH can push T_w into the mid‑20s °C, meaning cooling becomes limited.

3) Evaporative Cooling and Heat Balance

The iterative method in this calculator solves a psychrometric balance using vapor pressure and a pressure‑scaled coefficient. Practically, as T_w rises, sweat evaporation slows, skin temperature climbs, and cardiovascular strain increases. This is why planners track T_w for outdoor labor, sports, and cooling-system performance checks.

4) Pressure and Altitude Effects

Pressure changes the evaporation term. At higher altitude, lower pressure slightly alters the wet‑bulb solution and derived humidity ratio. If you do not have station pressure, entering altitude lets the tool estimate pressure using a standard atmosphere approximation for everyday use.

5) Fast Approximation vs Iterative Solution

The fast option is an empirical approximation designed for speed and common weather ranges. The iterative psychrometric option uses pressure explicitly and is preferred when altitude, storms, or indoor pressurization shifts conditions. Comparing both is useful for quick sanity checks.

6) Using Results for Field Decisions

Use T_w alongside context: solar load, wind, clothing, and workload. A jump of only 2–3 °C in T_w can meaningfully reduce cooling margin, so scheduling breaks, shade, and hydration becomes more important. The dew point output helps explain whether the risk is driven by humidity or temperature.

7) Data Quality and Sensor Checks

Inputs are only as good as the measurements. Aim for temperature accuracy near ±0.5 °C and humidity accuracy near ±3% RH for routine screening. Large swings in calculated vapor pressure or VPD often signal sensor drift, wet probe issues, or direct sun heating.

8) Reporting and Traceability

The CSV and PDF exports support documentation: who measured, where, and when. Pair results with location, altitude, and method choice so teams can reproduce conditions. Consistent reporting enables trend tracking across seasons and helps validate ventilation or cooling interventions.

FAQs

1) What is wet-bulb temperature in simple terms?
It is the temperature air reaches when water evaporates into it until saturation balance is approached. It reflects how effective evaporation can be for cooling at the current humidity and pressure.

2) Which method should I choose?
Choose the iterative psychrometric method when pressure matters or when you want higher physical consistency. Use the fast method for quick checks, dashboards, or when you only need an approximate value.

3) Why can wet-bulb be close to dry-bulb?
When humidity is high, evaporation slows because the air is already moist. The wet-bulb depression (T − T_w) becomes small, meaning evaporation provides less cooling.

4) Do I need pressure to compute wet-bulb?
The fast approximation does not require pressure. The iterative approach uses pressure and is more robust across altitude changes. If you do not know pressure, enter altitude to estimate it.

5) Why does the calculator also show dew point?
Dew point summarizes moisture content and helps interpret why wet-bulb is high. Two days can share the same dry-bulb temperature but very different dew points, leading to different cooling potential.

6) What do humidity ratio and enthalpy indicate?
Humidity ratio is the mass of water vapor per mass of dry air, useful for HVAC analysis. Enthalpy combines sensible and latent heat, supporting cooling load estimates and process comparisons.

7) How should I use wet-bulb for safety planning?
Treat it as one indicator of heat stress. Combine it with workload, sun, wind, and acclimatization. Increase rest, shade, and hydration as wet-bulb rises, and document conditions using the exports.

Stay aware, hydrate, rest often, and monitor wet-bulb daily.