Definition & Physics

The Dew Point (T_d) is the temperature to which air must be cooled (at constant pressure) to become saturated with water vapor. When the air temperature cools to the dew point, the relative humidity reaches 100%, and moisture begins to condense into dew, fog, or clouds.

Unlike Relative Humidity, which fluctuates wildly as the temperature changes throughout the day, the dew point remains relatively constant. This makes it a far superior metric for judging the actual amount of moisture in the air and the resulting human comfort level.

Comfort Level Guide

Human comfort relies on the evaporation of sweat to cool the body. High dew points indicate air that is already saturated, preventing evaporation and causing the heat to feel oppressive.

• < 55°F: Dry and comfortable (Refreshingly crisp).
• 55°F – 60°F: Pleasant (Noticeable humidity, but not comfortable).
• 61°F – 65°F: Sticky (The threshold where air begins to feel “heavy”).
• 66°F – 70°F: Uncomfortable (Muggy conditions typical of tropical climates).
• > 70°F: Oppressive (Dangerous heat index potential; physically draining).

Calculation Formula

This utility uses the Magnus Formula Approximation, a standard meteorological equation for saturation vapor pressure. The calculation process involves three steps:

1. Convert the input temperature from Fahrenheit to Celsius.
2. Calculate the intermediate parameter γ (gamma) using the natural logarithm of the Relative Humidity.
3. Solve for the Dew Point in Celsius and convert back to Fahrenheit.

Note: This approximation is highly accurate for temperature ranges between 0°C and 60°C.