Relative Humidity Calculator

Relative humidity (RH) represents the ratio of the actual water vapor pressure to the saturation vapor pressure, expressed as a percentage. The calculation relies heavily on temperature ($T$) and the dew point temperature ($T_d$). Using the Magnus-Tetens approximation, we first calculate the saturation vapor pressure and actual vapor pressure. The core relationship connecting dew point and temperature is expressed as $T_d = \frac{243.04 \times \ln(RH/100) + \frac{17.625 \times T}{243.04 + T}}{17.625 - (\ln(RH/100) + \frac{17.625 \times T}{243.04 + T})}$, allowing the calculator to seamlessly solve for any missing variable when the other two are provided.

Let us walk through a practical scenario where you want to find the relative humidity when the ambient air temperature is 25°C and the measured dew point is 15°C. First, input your temperature ($T = 25$) and dew point ($T_d = 15$) into the calculator. The algorithm evaluates the saturation vapor pressure at 25°C, which is approximately 3.17 kPa, and the actual vapor pressure based on the 15°C dew point, which is roughly 1.71 kPa. Dividing the actual vapor pressure by the saturation vapor pressure ($1.71 / 3.17$) yields 0.539. Multiplying by 100 gives a final relative humidity of approximately 54%, indicating a comfortable and moderate moisture level in the atmosphere.

To get the most accurate results from your humidity calculations, always ensure your temperature sensors are shaded and shielded from direct radiant heat, which can artificially skew readings. Additionally, remember that relative humidity fluctuates wildly throughout the day as temperatures rise and fall, so take measurements at consistent times if you are tracking trends. Finally, keep hygrometers calibrated periodically against known salt standards to maintain precision in damp or highly dry environments.