Saturation Vapour Pressure
The maximum water-vapour pressure air can hold at a given temperature (Tetens formula).
Formula
Variables
Example
At 25°C, eₛ ≈ 3.17 kPa
How it works
This formula estimates the equilibrium water-vapour pressure above a flat surface of pure water at a given air temperature. It rises very quickly with temperature because warmer air molecules and water molecules have more energy, so more liquid water can evaporate before the vapour becomes saturated. The constants in Tetens’ formula are an empirical fit to measured vapour-pressure data, making it a practical approximation for weather and humidity calculations.
When to use it
Use it when you need the saturation vapour pressure for a temperature, such as when finding relative humidity, dew point, vapour pressure deficit, or estimating condensation risk. It is common in meteorology, agriculture, HVAC, and environmental science.
Worked example, step by step
Find the saturation vapour pressure at 18°C. Step 1: Substitute T = 18 into the formula: e_s = 0.6108 exp((17.27 × 18)/(18 + 237.3)) Step 2: Calculate the exponent: (17.27 × 18)/(255.3) = 310.86/255.3 ≈ 1.218 Step 3: Evaluate the exponential: exp(1.218) ≈ 3.380 Step 4: Multiply by 0.6108: e_s ≈ 0.6108 × 3.380 ≈ 2.06 kPa Final answer: e_s ≈ 2.06 kPa
Common mistakes to avoid
- Using Kelvin instead of °C in the formula
- Forgetting that the result is in kPa, not Pa
- Treating saturation vapour pressure as the same as actual vapour pressure or relative humidity
Did You Know?
Warm air holds exponentially more moisture — roughly 7% more water vapour per °C.
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