Weather, Humidity and Dew Point Guide

📚 Overview

Relative humidity changes all day because warm air can hold more moisture than cold air, so the same amount of water in the air gives a different percentage at noon and at dawn. Dew point does not have that problem. It is the temperature at which the air becomes saturated and dew or fog begins to form, and it tracks how much moisture is really in the air. The calculators here turn a temperature and a humidity reading into a dew point and a plain comfort description. Results are estimates from standard formulas. Official forecasts and weather warnings always come first.

📐 Magnus Dew Point Formula

T is the air temperature in °C and RH is the relative humidity in percent. The formula comes from the Magnus approximation of saturation vapor pressure and works well from about −45 °C to 60 °C. Fahrenheit readings are converted to Celsius first, and the result can be shown in either scale.

🔢 Worked Example

Scenario: A warm afternoon has an air temperature of 30 °C and a relative humidity of 70%, and you want to know how muggy it feels.

• Air temperature: 30 °C
• Relative humidity: 70%

1. γ = ln(0.70) + 17.625 × 30 ÷ (243.04 + 30) = −0.3567 + 1.9366 = about 1.5799
2. Dew point = 243.04 × 1.5799 ÷ (17.625 − 1.5799) = about 23.9 °C
3. In Fahrenheit: 23.9 × 9 ÷ 5 + 32 = about 75.1 °F

💡 Pro Tips

• Compare dew points, not humidity percent, to judge how muggy two days or two places feel.
• Surfaces colder than the dew point collect condensation, which is why cold glasses and windows fog up.
• When the air temperature falls to the dew point overnight, expect dew, fog, or frost.
• A dew point near the air temperature means the humidity is close to 100%.

🧮 Weather & Climate Calculators (1)

Put the formulas above into practice with these free tools:

❓ Frequently Asked Questions