What is dew point?
Dew point is the temperature at which air becomes saturated with water vapor, causing condensation to form. When the air temperature falls to the dew point, water vapor begins to condense into liquid water — forming dew on surfaces, fog in the air, or frost if the temperature is below freezing.
Unlike relative humidity, which changes throughout the day as temperature fluctuates, dew point is a direct measure of the amount of moisture in the air and remains relatively constant. This makes it the preferred metric for meteorologists and HVAC professionals when assessing comfort and humidity.
How is dew point calculated?
The dew point formula used in this calculator is the Magnus formula, widely used in meteorology:
T_d = (b × α(T, RH)) / (a - α(T, RH))
Where:
- α(T, RH) = (a × T) / (b + T) + ln(RH/100)
- a = 17.27
- b = 237.7°C
- T = air temperature in °C
- RH = relative humidity in %
The formula is accurate for temperatures between -40°C and 50°C (-40°F to 122°F).
Step by step: how to calculate dew point
- Measure the air temperature in °C (or convert from °F)
- Measure the relative humidity as a percentage (0-100%)
- Calculate α using the formula: α = (17.27 × T) / (237.7 + T) + ln(RH/100)
- Calculate dew point using: T_d = (237.7 × α) / (17.27 - α)
- Round the result to the nearest whole number
- Look up the comfort level in the chart below
Example calculation
Air temperature: 25°C, Relative humidity: 60%
α = (17.27 × 25) / (237.7 + 25) + ln(0.60)
α = 431.75 / 262.7 + (-0.511)
α = 1.644 + (-0.511) = 1.133
Dew point = (237.7 × 1.133) / (17.27 - 1.133) = 269.3 / 16.137 ≈ 17°C
Dew point chart
| Temp (°C) | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|
| 0 | -21 | -17 | -14 | -11 | -9 | -7 | -5 | -3 |
| 5 | -16 | -11 | -8 | -5 | -3 | -1 | 1 | 3 |
| 10 | -11 | -6 | -3 | 1 | 3 | 5 | 7 | 9 |
| 15 | -6 | -1 | 3 | 7 | 9 | 11 | 13 | 14 |
| 20 | -1 | 4 | 9 | 12 | 15 | 17 | 19 | 20 |
| 25 | 4 | 10 | 14 | 17 | 20 | 22 | 24 | 25 |
| 30 | 9 | 15 | 19 | 22 | 25 | 27 | 29 | 30 |
| 35 | 14 | 20 | 24 | 28 | 30 | 32 | 34 | 35 |
Color coding in the calculator: blue = very dry (< 0°C), green = comfortable (0-15°C), yellow = slightly humid (15-20°C), red = humid/muggy (> 20°C)
Dew point comfort levels
| Dew Point | Comfort Level | Description |
|---|---|---|
| Below 0°C (32°F) | Very Dry | May cause skin irritation, dry eyes, and respiratory discomfort. Consider using a humidifier. |
| 0-5°C (32-41°F) | Dry | Air is dry but comfortable for most people. Some may experience minor skin dryness. |
| 5-10°C (41-50°F) | Comfortable | Air feels fresh and comfortable. Ideal conditions for most activities. |
| 10-15°C (50-59°F) | Pleasant | Air feels pleasant and comfortable. Good conditions for physical activity. |
| 15-20°C (59-68°F) | Slightly Humid | Air feels slightly humid. Some people may start to notice moisture. |
| 20-24°C (68-75°F) | Humid | Air feels muggy and uncomfortable. Sweating becomes less effective. |
| Above 24°C (75°F) | Very Humid | Oppressive conditions. High risk of heat-related illness. Limit outdoor activities. |
Dew point vs relative humidity
| Feature | Dew Point | Relative Humidity |
|---|---|---|
| What it measures | Absolute moisture content | Moisture relative to capacity |
| Changes with temperature | No (stays constant) | Yes (fluctuates throughout the day) |
| Best for | Assessing comfort and mugginess | Assessing current conditions |
| Preferred by | Meteorologists, HVAC professionals | Weather reports |
| Range | Limited by air temperature | 0-100% |
How humidity affects your body
Your body cools itself through perspiration — heat is removed as sweat evaporates from your skin. When the dew point is high:
- The air already contains a lot of moisture
- Sweat evaporates more slowly
- Your body's natural cooling system becomes less effective
- You feel hotter and more uncomfortable
- Risk of heat-related illness increases
This is why a 30°C day with a dew point of 24°C feels much worse than a 30°C day with a dew point of 10°C.
FAQ
What dew point is considered comfortable?
A dew point between 10°C and 15°C (50°F to 59°F) is considered comfortable for most people. Below 5°C, the air may feel too dry for comfort, causing skin and respiratory irritation. Above 20°C, the air starts to feel oppressive and muggy.
Can dew point be higher than air temperature?
No, dew point can never exceed the air temperature. When the dew point equals the air temperature, relative humidity is 100% and condensation begins. This is the point at which dew, fog, or frost forms.
Why do meteorologists prefer dew point over relative humidity?
Dew point directly measures the amount of moisture in the air and stays relatively constant throughout the day, while relative humidity fluctuates with temperature. A dew point of 20°C feels equally muggy at 25°C or 35°C air temperature, but the relative humidity would be very different. Dew point gives a more consistent and intuitive picture of how the air actually feels.
How does dew point relate to fog formation?
Fog forms when the air temperature drops to or below the dew point, causing water vapor to condense into tiny water droplets suspended in the air. This typically happens at night when temperatures drop, or when warm, moist air moves over a cold surface. A small spread between air temperature and dew point (2-3°C) indicates high fog potential.