Dew Point Calculator - Humidity & Temp
Calculate dew point from air temperature and relative humidity using the Magnus formula, plus frost point below freezing.
Formula
Dew Point = (237.7 × α) / (17.27 - α) where α = (17.27T)/(237.7+T) + ln(RH/100)
The Magnus-Tetens approximation calculates dew point from temperature (T in °C) and relative humidity (RH as percentage). Dew point indicates the absolute moisture content of air.
Worked Examples
Example 1: Comfortable Summer Day
Problem:Temperature is 78°F with 50% relative humidity. What is the dew point and comfort level?
Solution:Using Magnus-Tetens formula: T = 78°F = 25.6°C RH = 50% α = (17.27 × 25.6) / (237.7 + 25.6) + ln(0.50) α = 1.68 + (-0.69) = 0.99 Dew Point = (237.7 × 0.99) / (17.27 - 0.99) = 235.3 / 16.28 = 14.5°C = 58°F Dew point of 58°F = 'Pleasant' comfort level. Most people will feel comfortable.
Result:Dew point: 58°F (Pleasant)
Example 2: Oppressive Humidity
Problem:It's 88°F with 75% humidity. Is it safe for outdoor exercise?
Solution:T = 88°F = 31.1°C, RH = 75% α = (17.27 × 31.1) / (237.7 + 31.1) + ln(0.75) α = 2.00 + (-0.29) = 1.71 Dew Point = (237.7 × 1.71) / (17.27 - 1.71) = 406.5 / 15.56 = 26.1°C = 79°F ⚠️ Dew point of 79°F is EXTREMELY HUMID. • Body cannot cool effectively • High heat illness risk • Avoid strenuous outdoor activity • Stay hydrated if you must be outside
Result:Dew point: 79°F - Dangerous for exercise!
Example 3: Fog Prediction
Problem:Evening temperature is 62°F, humidity 85%. What's the fog risk overnight if temp drops to 55°F?
Solution:Current conditions: 62°F, 85% RH Dew Point calculation: T = 62°F = 16.7°C, RH = 85% α = (17.27 × 16.7) / (237.7 + 16.7) + ln(0.85) α = 1.13 + (-0.16) = 0.97 Dew Point = 14.0°C = 57.2°F Spread: 62 - 57.2 = 4.8°F If temp drops to 55°F: • Air temperature would be below dew point! • Spread = 55 - 57.2 = -2.2°F • 100% fog formation expected
Result:Fog WILL form when temp reaches 57°F
Frequently Asked Questions
What is dew point and why is it important?
Dew point is the temperature at which air becomes saturated with moisture and water begins to condense. It's the temperature to which air must be cooled (at constant pressure) for dew to form. Dew point is crucial because it's an absolute measure of moisture content - unlike relative humidity, it doesn't change with temperature. A dew point of 65°F feels humid regardless of whether it's 75°F or 95°F outside.
Why is dew point better than relative humidity for comfort?
Relative humidity depends on temperature - 50% RH at 90°F feels much worse than 50% RH at 70°F. Dew point is absolute - a 70°F dew point always feels oppressive. Weather forecasters increasingly use dew point because it directly indicates how your body can cool itself through sweating. Above 70°F dew point, sweat evaporates slowly, making it feel unbearable.
At what dew point do people start feeling uncomfortable?
Most people notice humidity when dew point exceeds 60°F. Above 65°F, most find it uncomfortable. Above 70°F is oppressive - even healthy people should limit strenuous activity. Above 75°F is dangerous and rare outside tropical regions. Conversely, dew points below 50°F feel very dry - beneficial for comfort but can dry out skin and sinuses.
How does dew point affect cooling and HVAC?
Air conditioners both cool air AND remove moisture (dehumidify). When humid air passes over cold coils, water condenses and drains away. Properly sized AC should maintain indoor dew point around 50-55°F. Oversized AC cools quickly but doesn't run long enough to dehumidify, leaving a cold, clammy feeling. This is why right-sizing HVAC matters.
What causes dew on grass in the morning?
Overnight, the ground radiates heat and cools. When ground temperature drops below the dew point, moisture from the air condenses on surfaces - this is dew. It's most likely on clear, calm nights when radiation cooling is strongest. Frost forms instead of dew when the surface temperature drops below freezing and the dew point.
How is dew point related to fog?
Fog forms when air temperature drops to the dew point. The smaller the 'spread' (difference between temperature and dew point), the more likely fog. When spread is less than 3°F, fog is highly probable. Radiation fog forms overnight when clear skies allow cooling. Advection fog forms when warm, moist air moves over cool surfaces.
What is the difference between dew point and wet bulb temperature?
Dew point is when condensation begins at a surface. Wet bulb temperature is the lowest temperature achievable through evaporative cooling alone (like a wet thermometer or sweating). Wet bulb is always between dew point and air temperature. Wet bulb temperature above 95°F is potentially fatal as the human body cannot cool itself.
Can dew point be higher than air temperature?
No, dew point can never exceed air temperature. When they're equal, relative humidity is 100% and the air is saturated. This is why fog and clouds form - the air has cooled to its dew point. If moisture is added, excess condenses immediately. The dew point rises when more moisture is added to the air.
How does dew point affect my basement?
Cool basement walls may be below the outdoor dew point, especially in summer. When warm, humid air enters and contacts cold surfaces, condensation forms - leading to mold and musty smells. Solutions: dehumidifier (keep below 60% RH), insulate walls, minimize outdoor air entry on humid days. A dew point reading in the basement helps determine condensation risk.
What dew point is best for indoor comfort?
Ideal indoor dew point is 50-55°F, which corresponds to about 45-55% relative humidity at typical room temperatures (68-72°F). Below 45°F dew point, air feels dry and can cause respiratory irritation. Above 60°F dew point indoors, mold growth becomes a concern. HVAC systems should be designed to maintain this range.
Background & Theory
Dew point is a fundamental humidity measurement that directly indicates air moisture content. Understanding dew point helps with comfort assessment, weather prediction, and moisture control in buildings.
The Science of Dew Point:
Air contains water vapor. The maximum amount (saturation) depends on temperature - warm air holds more. When air cools to its dew point, it reaches 100% relative humidity and can't hold more moisture. Any further cooling causes condensation.
Key Relationships:
Temperature → Saturation:
Higher temperature = higher moisture capacity
Dew Point = Absolute Moisture:
Doesn't change with temperature
Higher dew point = more water vapor
Relative Humidity:
RH = (actual vapor / saturation vapor) × 100%
Same moisture at different temps = different RH
Calculating Dew Point:
Magnus-Tetens Formula:
α = (17.27 × T) / (237.7 + T) + ln(RH/100)
Dew Point = (237.7 × α) / (17.27 - α)
Where T is temperature in °C and RH is relative humidity.
Comfort Scale by Dew Point:
| Dew Point | Comfort | Description |
|---|---|---|
| < 50°F | Dry | Very comfortable, may feel too dry |
| 50-55°F | Comfortable | Ideal indoor conditions |
| 55-60°F | Pleasant | Nice outdoor weather |
| 60-65°F | Somewhat Humid | Noticeably sticky |
| 65-70°F | Humid | Uncomfortable, sweat doesn't evaporate well |
| 70-75°F | Very Humid | Oppressive, high heat illness risk |
| > 75°F | Extreme | Dangerous, tropical/monsoon conditions |
Human Cooling and Dew Point:
Your body cools primarily through sweating. Sweat evaporating removes heat. But evaporation rate depends on dew point:
- Low dew point: Sweat evaporates quickly, effective cooling
- High dew point: Sweat can't evaporate, pools on skin
- Above 75°F dew point: Body can barely cool itself
This is why dew point matters more than relative humidity for heat stress assessment.
Indoor Applications:
HVAC Sizing:
AC must lower both temperature AND dew point.
Target indoor dew point: 50-55°F
Moisture Control:
Condensation occurs when surface temp < dew point
Keep basement walls above outdoor dew point
Monitor during seasonal transitions
Mold Prevention:
Keep indoor RH below 60%
Corresponds to ~55°F dew point at 70°F
Weather Applications:
Fog Formation:
Fog forms when temp = dew point
Spread < 3°F = high fog probability
Cloud Base Height:
Base (ft) ≈ (Temperature - Dew Point) × 230
Useful for pilots
Frost vs Dew:
Dew point above 32°F = dew
Dew point below 32°F = frost
Precipitation Potential:
Higher dew point = more available moisture
Geographic Variations:
Typical summer dew points:
- Desert (Phoenix): 45-55°F
- California coast: 55-60°F
- Midwest: 60-70°F
- Gulf Coast: 70-78°F
- Tropical: 75-80°F
Locations with dew points regularly above 75°F are rare - primarily Persian Gulf, Red Sea, and monsoon regions.