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Humidity Calculator

Calculate the dew point temperature from air temperature and relative humidity using the Magnus formula with Sonntag-style coefficients. The dew point tells you the temperature at which moisture in the air would begin to condense. This is essential for HVAC comfort assessment, predicting window condensation, preventing mold growth, and understanding weather forecasts.

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Formula

Magnus: alpha = ln(RH/100) + (b x T) / (c + T); Td = c x alpha / (b - alpha)

The Magnus formula relates saturation vapor pressure to temperature using empirical constants b = 17.67 and c = 243.5 (Sonntag coefficients). Given the air temperature T (Celsius) and relative humidity RH (percent), the formula first computes an intermediate value alpha, then solves for the dew point temperature Td where saturation would occur at the current moisture level.

Worked Example

Air temperature 25 C with 50% relative humidity: Step 1: alpha = ln(50/100) + (17.67 x 25) / (243.5 + 25) Step 2: alpha = -0.6931 + 441.75 / 268.5 = -0.6931 + 1.6447 = 0.9516 Step 3: Td = 243.5 x 0.9516 / (17.67 - 0.9516) = 231.71 / 16.72 = 13.9 C The dew point is approximately 14 C. Surfaces colder than 14 C in this room may develop condensation.

Frequently Asked Questions

How accurate is the Magnus formula for dew point?

The Magnus approximation with Sonntag coefficients is accurate to within about 0.4 C for temperatures between -40 C and 50 C and relative humidity above 5%. For most indoor comfort, HVAC, and weather applications, this is more than sufficient.

Does this calculator work below freezing?

The simple Magnus form is most reliable for typical comfort ranges (0 C to 50 C). Below freezing, frost point replaces dew point, and different coefficients are needed. The results below 0 C should be treated as rough estimates.

Why does dew point matter for indoor comfort?

Dew point is a better indicator of mugginess than relative humidity alone. A dew point above 20 C feels uncomfortably humid, while below 10 C feels dry. Indoor surfaces colder than the dew point (like windows in winter) will collect condensation, which can lead to mold growth.

Can I enter temperature in Fahrenheit?

This calculator requires Celsius input. To convert from Fahrenheit: subtract 32 and multiply by 5/9. For example, 77 F = (77 - 32) x 5/9 = 25 C.

What is the ideal indoor dew point?

Most people feel comfortable with indoor dew points between 10 C and 16 C (50 F to 60 F). Above 18 C feels sticky, and below 5 C feels very dry and may cause skin irritation and static electricity.

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