Dew Point Calculator - Humidity & Temp
Calculate dew point temperature from air temperature and relative humidity. Uses Magnus formula for accurate results. Shows frost point for sub-zero temps.
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:\nT = 78°F = 25.6°C\nRH = 50%\n\nα = (17.27 × 25.6) / (237.7 + 25.6) + ln(0.50)\nα = 1.68 + (-0.69) = 0.99\n\nDew Point = (237.7 × 0.99) / (17.27 - 0.99)\n= 235.3 / 16.28 = 14.5°C = 58°F\n\nDew point of 58°F = 'Pleasant' comfort level.\nMost 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%\n\nα = (17.27 × 31.1) / (237.7 + 31.1) + ln(0.75)\nα = 2.00 + (-0.29) = 1.71\n\nDew Point = (237.7 × 1.71) / (17.27 - 1.71)\n= 406.5 / 15.56 = 26.1°C = 79°F\n\n⚠️ Dew point of 79°F is EXTREMELY HUMID.\n• Body cannot cool effectively\n• High heat illness risk\n• Avoid strenuous outdoor activity\n• 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\n\nDew Point calculation:\nT = 62°F = 16.7°C, RH = 85%\nα = (17.27 × 16.7) / (237.7 + 16.7) + ln(0.85)\nα = 1.13 + (-0.16) = 0.97\nDew Point = 14.0°C = 57.2°F\n\nSpread: 62 - 57.2 = 4.8°F\n\nIf temp drops to 55°F:\n• Air temperature would be below dew point!\n• Spread = 55 - 57.2 = -2.2°F\n• 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.