Dew Point Calculator

How Dew Point Calculator Works

Td = (b × γ(T,RH)) / (a − γ(T,RH)) where γ = (aT/(b+T)) + ln(RH/100), a = 17.27, b = 237.7

T

Air Temperature — The current air temperature in degrees Celsius.

RH

Relative Humidity — The relative humidity percentage (0-100).

γ

Intermediate Gamma Value — The combined temperature-humidity parameter.

a, b

Magnus Constants — Empirical constants: a = 17.27, b = 237.7 (for temperatures above 0°C).

Td

Dew Point — The dew point temperature in degrees Celsius.

How to Use

1. Enter the current air temperature in degrees Fahrenheit.
2. Enter the relative humidity percentage (0-100%).
3. The calculator converts to Celsius, applies the Magnus formula, and converts the dew point back to Fahrenheit.
4. Review the comfort level description and HVAC relevance.
5. A dew point below 55°F is comfortable; above 70°F feels oppressive.

Understanding the Result

Frequently Asked Questions

Why is dew point better than relative humidity for comfort?

Relative humidity changes with temperature — warm air can hold more moisture than cold air. A cold day with 100% humidity feels comfortable, but a hot day with 60% humidity can feel oppressive. The dew point is an absolute measure of moisture content and does not change with temperature, making it a much better indicator of how humid the air actually feels.

What is a comfortable dew point?

Most people find dew points below 60°F comfortable. Between 60-65°F, the air starts to feel slightly humid. Above 70°F, the air feels uncomfortable and oppressive. Above 75°F, conditions become miserable, and above 80°F, they can be dangerous — especially when combined with high temperatures.

Why does the Magnus formula use different constants below freezing?

The Magnus formula constants a=17.27, b=237.7 are optimized for temperatures above 0°C (32°F). For sub-freezing conditions, different constants (a=21.875, b=265.5) are used because the saturation vapor pressure over ice differs from that over liquid water. This calculator uses the standard constants for conditions typically above freezing.

What is the relationship between dew point and HVAC?

HVAC systems use dew point to control dehumidification. When the dew point is high, air conditioners must work harder to remove moisture. The recommended indoor dew point for comfort is between 45-55°F (corresponding to 30-50% relative humidity at 70°F). High indoor dew points can lead to mold growth and dust mite proliferation.

How does dew point relate to fog formation?

Fog forms when the air temperature drops to within about 4°F of the dew point. As the air cools overnight, it approaches the dew point temperature, at which point the air becomes saturated and water vapor begins to condense into tiny suspended water droplets — fog. The type of fog depends on how the cooling occurs: radiation fog forms on clear, calm nights when the ground radiates heat away and cools the air above it; advection fog forms when warm, moist air moves over a cooler surface (common in coastal areas like San Francisco); and upslope fog forms when moist air is forced up a mountainside and cools adiabatically. The most dangerous fog for driving is radiation fog, which often forms in low-lying areas near rivers and lakes just before dawn and can reduce visibility to near zero in patches. Pilots use the "temperature-dew point spread" (the difference between the air temperature and dew point) to forecast fog — when the spread is 4°F or less with light winds and clearing skies, fog is likely to form overnight. For aviation, the dew point is also used to calculate the cloud base height using the formula: cloud base (feet) = (temperature - dew point) × 400, which gives the altitude at which cumulus clouds will form.

Sources

• Wikipedia - Dew Point