How Is Dew Point Calculated?

The dew point is a critical meteorological parameter that indicates the temperature at which air becomes saturated with water vapor, leading to condensation. Understanding how to calculate the dew point is essential for weather forecasting, HVAC systems, and industrial processes. This article explains the formulas and steps involved in dew point calculation, along with practical applications.

What Is Dew Point?

Dew point is the temperature at which air must be cooled for water vapor to condense into dew or frost. It is a measure of atmospheric moisture and is directly related to humidity. A higher dew point indicates more moisture in the air, while a lower dew point suggests drier conditions.

Key Variables in Dew Point Calculation

To calculate the dew point, you need the following variables:

Air Temperature (T): Measured in degrees Celsius (°C) or Fahrenheit (°F).

Relative Humidity (RH): Expressed as a percentage (%).

Dew Point Formulas

Several empirical formulas can approximate the dew point. The most commonly used is the Magnus-Tetens formula, which provides a reliable estimate for temperatures between -45°C and 60°C.

Magnus-Tetens Formula

The formula for calculating dew point (T_d) in °C is:

T_d = (b × α(T, RH)) / (a – α(T, RH))

Where:

α(T, RH) = (a × T) / (b + T) + ln(RH / 100)

a = 17.27 (constant for water vapor)

b = 237.7°C (constant for water vapor)

ln is the natural logarithm

Simplified Approximation

For quick estimates, a simplified version of the formula can be used:

T_d ≈ T – ((100 – RH) / 5)

This approximation works best when RH is above 50% and temperatures are near room conditions.

Step-by-Step Calculation

Let’s walk through an example calculation using the Magnus-Tetens formula.

Example:

Given: Air Temperature (T) = 25°C, Relative Humidity (RH) = 70%

Calculate α(T, RH):
α(T, RH) = (17.27 × 25) / (237.7 + 25) + ln(70 / 100)

α(T, RH) ≈ 1.098 + (-0.3567) ≈ 0.7413

Calculate Dew Point (T_d):
T_d = (237.7 × 0.7413) / (17.27 – 0.7413)

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