Dew Point Calculator —
air temperature, humidity & condensation.

Weather science guide

Dew point, humidity, and condensation: explained.

The dew point is the temperature to which air must be cooled, at constant pressure and water vapor content, for condensation to begin. It is one of the most meaningful numbers in meteorology and everyday weather: more informative than relative humidity for describing how the air actually feels, and critical for understanding fog, frost, aircraft icing, building science, and agricultural frost warnings.

What is dew point?

Air can hold a finite amount of water vapor, and that capacity increases with temperature. When air is cooled to its dew point, it becomes saturated (relative humidity reaches 100%) and water vapor begins to condense into liquid water on any available surface: a blade of grass, a window pane, a cold drink, or water droplets in the air itself (fog or clouds). Below 0°C (32°F), the equivalent process deposits water vapor directly as ice crystals, which is called the frost point.

Unlike relative humidity, which changes with temperature even when the actual moisture content of the air stays the same, dew point is an absolute measure. The same parcel of air has the same dew point at 6 AM and noon, even though the relative humidity may have dropped from 90% to 40% as the temperature rose. This is why meteorologists and pilots prefer dew point over relative humidity for assessing atmospheric moisture.

The Magnus-Tetens formula

The standard approach for calculating dew point from temperature and relative humidity uses the Magnus-Tetens approximation, also known as the August-Roche-Magnus equation. It is a simplified but highly accurate form of the Clausius-Clapeyron relation, which describes how saturation vapor pressure changes with temperature.

The formula (Alduchov & Eskridge, 1996 refinement, accurate to within 0.1°C for typical weather conditions):

Where T is air temperature in °C, RH is relative humidity in percent (0–100), and T_d is the dew point in °C. For Fahrenheit inputs, convert to Celsius first, compute, then convert the result back.

The formula relies on the approximation that saturation vapor pressure follows an exponential relationship with temperature, which is nearly exact over normal atmospheric temperature ranges (−40°C to 60°C). More precise results require the Buck equation or full numerical integration of the Clausius-Clapeyron equation, but the Magnus form is sufficient for all practical weather and engineering applications.

Dew point vs. relative humidity

Relative humidity (RH) expresses the current water vapor content as a percentage of the maximum possible at the current temperature. The problem: that maximum changes with temperature, so RH changes throughout the day even when no moisture is added or removed.

• At 6 AM with T = 60°F (15.6°C) and RH = 90%: dew point ≈ 57°F (13.9°C)
• At noon with T = 85°F (29.4°C) and RH = 45%: dew point ≈ 61°F (16.1°C)

The dew point barely changed; the air has essentially the same moisture content, but relative humidity swung from 90% to 45%. This is why a morning forecast of "90% humidity" does not mean the afternoon will feel just as oppressive: relative humidity naturally drops as temperatures rise, even though the actual moisture in the air is nearly constant.

Dew point and human comfort

The human body cools itself primarily through sweat evaporation. When the dew point is high, the air is already close to saturation and evaporation slows or stops. Your sweat cannot evaporate efficiently, and the body struggles to regulate its core temperature. This is what "muggy" or "oppressive" humidity actually means physiologically.

• Below 35°F (2°C): Very dry. Common in arid and polar climates. May cause dry skin, chapped lips, and static electricity. Air feels crisp and clean.
• 35–45°F (2–7°C): Dry and comfortable. Typical of Mediterranean climates and many spring days.
• 45–55°F (7–13°C): Comfortable. The ideal dew point range for most humans: fresh, pleasant air.
• 55–60°F (13–16°C): Mild. Slightly noticeable but still comfortable. Common in coastal and temperate regions in summer.
• 60–65°F (16–18°C): Sticky. Tropical feel. Sweating is less effective; some people begin to feel uncomfortable.
• 65–70°F (18–21°C): Uncomfortable. Classic muggy summer day. Outdoor exertion becomes difficult, particularly for the elderly and those with heart or respiratory conditions.
• 70–75°F (21–24°C): Oppressive. Common in the Gulf Coast, South Asia, and equatorial regions. Heat-related illness risk increases significantly with physical activity.
• Above 75°F (24°C): Dangerous. One of the highest dew points ever reliably recorded is 95°F (35°C) in Dhahran, Saudi Arabia (July 2003). At these levels, the human body cannot cool itself effectively even at rest.

Condensation in everyday life

Condensation occurs whenever a surface is cooled below the local dew point temperature. Common examples:

• Cold drinks: A glass of ice water has a surface temperature near 32°F (0°C). If the dew point is 55°F (13°C), the glass will be covered in water droplets within seconds of being set on a table.
• Morning dew: On clear nights, the ground and vegetation radiate heat away rapidly and cool below the dew point. Water vapor in the air condenses onto grass, leaves, and car hoods as dew.
• Window condensation: In winter, indoor air (warm and relatively humid) contacts a cold window pane. If the glass surface is below the dew point, condensation forms on the interior. Double-pane windows exist largely to keep the interior glass surface warmer.
• Foggy mirrors: A bathroom mirror after a hot shower condenses moisture because the mirror is cooler than the dew point of the humid air filling the room.
• Aircraft icing: When a plane flies through air at or near dew point in sub-freezing temperatures, supercooled water droplets freeze instantly on contact with wing surfaces, a critical aviation hazard.
• Pipe sweating: Uninsulated cold-water pipes run through warm, humid basements will "sweat" condensation, enough to drip onto floors and cause mold damage over time.

Dew point and fog formation

Fog is simply a cloud at ground level; it forms when the air temperature drops to the dew point and condensation occurs on microscopic particles (dust, sea salt, pollution) suspended in the air. There are several types:

• Radiation fog: The most common type. On clear, calm nights, the ground radiates heat and cools the air near the surface below its dew point. Often dissipates by mid-morning as the sun warms the surface.
• Advection fog: Warm, moist air moves over a cold surface (ocean current, cold land). San Francisco's famous summer fog is advection fog from warm Pacific air flowing over the cold California current.
• Upslope fog: Moist air forced upward along a hillside cools adiabatically to its dew point and condenses.

The "spread", the difference between air temperature and dew point — is a key forecasting tool. A spread of 5°F or less suggests fog is possible or likely overnight. A spread of 1–2°F means fog is almost certain.

Absolute humidity vs. relative humidity vs. dew point

• Absolute humidity (g/m³): The actual mass of water vapor in a cubic metre of air. Does not change as temperature changes (assuming no moisture is added or removed). Useful for engineering and HVAC but hard to measure directly.
• Relative humidity (%): Water vapor as a fraction of the maximum possible at the current temperature. Easy to measure with a hygrometer. Changes with temperature. Not an intuitive comfort indicator.
• Dew point (°F or °C): The temperature at which saturation occurs. Directly proportional to absolute moisture content. The most intuitive comfort indicator; a dew point of 70°F feels oppressive regardless of the actual temperature.

Dew point in weather forecasting

Meteorologists use dew point for a wide range of forecasting applications:

• Thunderstorm potential: High dew points (60°F+) indicate plentiful low-level moisture, a key ingredient for severe thunderstorms. Tornado outbreaks in the US Great Plains are often preceded by dew points above 65°F (18°C).
• Frost advisories: When overnight temperatures are forecast to fall near the frost point, agriculture agencies issue frost advisories to protect crops and gardens.
• Aviation: Pilots use the temperature/dew point spread to estimate cloud base height. The spread in °C × 400 feet gives an approximate cloud base altitude (e.g., spread of 10°C → cloud base ≈ 4,000 feet AGL).
• Lifting Condensation Level (LCL): The altitude at which a rising parcel of air cools to its dew point and clouds begin to form. Critical for convective forecasting.

Temperature and humidity conversion

• °F to °C: (°F − 32) × 5/9
• °C to °F: (°C × 9/5) + 32
• 1 hPa = 100 Pa (Pascal) = 0.02953 inHg
• Absolute humidity formula: AH ≈ 216.7 × e / (273.15 + T°C) g/m³