What is the dew point?

The dew point is the temperature to which air must be cooled — at constant pressure and water-vapour content — before it becomes saturated and water starts to condense. When the air temperature falls to the dew point, you see dew, fog, or frost forming on surfaces.

What formula does this calculator use?

It uses the August–Roche–Magnus approximation with Alduchov and Eskridge (1996) constants: γ = ln(RH/100) + b·T/(c+T), Td = c·γ/(b−γ). Over water: b = 17.368, c = 238.88. Over ice (frost point): b = 17.966, c = 247.15. The formula is accurate to within 0.1 °C for temperatures between −40 °C and +60 °C.

What is a comfortable dew point?

Most people find dew points below 10 °C dry and comfortable. Between 10 °C and 16 °C feels pleasant. From 16 °C to 18 °C is slightly muggy. Above 21 °C is oppressive and above 24 °C is considered dangerous during heat waves, as sweat cannot evaporate efficiently.

What is the difference between dew point and relative humidity?

Relative humidity (RH) is the ratio of the actual water-vapour pressure to the maximum possible (saturation) pressure at the current temperature, expressed as a percentage. It changes as temperature changes even if the actual moisture content stays the same. Dew point is an absolute measure of moisture — it does not change when temperature changes, only when water vapour is added to or removed from the air.

What is the wet-bulb temperature?

The wet-bulb temperature is the lowest temperature achievable by evaporating water into the air. It is always between the dew point and the dry-bulb air temperature. The calculator uses the Stull (2011) polynomial approximation, accurate to within ±0.3 °C for relative humidity above 5 %. Wet-bulb temperature is critical for heat-stress assessments — a wet-bulb temperature above 35 °C is considered unsurvivable for healthy humans without cooling.

When should I use the frost-point option?

Enable the frost-point checkbox when the air temperature is below 0 °C. The standard Magnus constants are calibrated over liquid water. Below freezing, water vapour deposits directly as ice (frost), so slightly different constants (b = 17.966, c = 247.15) give a more accurate result for surface condensation and icing forecasts.

Dew Point Calculator

The dew point is one of the most informative single numbers in meteorology and everyday comfort assessment. Unlike relative humidity — which rises and falls as the temperature changes even if the actual moisture in the air stays constant — the dew point is an absolute measure of atmospheric water vapour. When the dew point is high, the air is genuinely laden with moisture; when it is low, the air is dry regardless of what the relative-humidity percentage says.

How the Magnus formula works

This calculator uses the August–Roche–Magnus approximation, one of the most widely cited empirical formulas in applied meteorology. Given air temperature T (in °C) and relative humidity RH (0–100 %), it first computes an intermediate variable:

γ = ln(RH / 100) + b·T / (c + T)

The dew point follows directly:

Td = c · γ / (b − γ)

The constants come from Alduchov and Eskridge (1996) and are optimised for accuracy over liquid water between −40 °C and +60 °C:

Surface	b	c
Water	17.368	238.88 °C
Ice	17.966	247.15 °C

For temperatures below 0 °C tick the frost-point checkbox and the ice constants are used instead, giving the temperature at which ice crystals (rather than liquid droplets) would form on surfaces — directly relevant for de-icing and cold-chain logistics.

The calculator also derives:

Wet-bulb temperature via the Stull (2011) approximation — the lowest temperature achievable by evaporating water into the air. Used in heat-stress and industrial cooling assessments.
Saturation and actual vapour pressure (hPa) from the Magnus exponential form e = 6.112 · exp(b·T / (c+T)).
Heat index (NOAA / Rothfusz 1990 regression) when temperature is at or above 27 °C and humidity is at or above 40 % — the “feels like” figure familiar from weather apps.

Worked example

Situation: A summer afternoon in Athens. Air temperature 34 °C, relative humidity 55 %.

Compute γ: ln(55/100) + 17.368×34 / (238.88 + 34) = −0.5978 + 2.1674 = 1.5696
Dew point: 238.88 × 1.5696 / (17.368 − 1.5696) = 23.7 °C
Comfort band: Extremely humid — dew points above 24 °C are tropical-oppressive.
Wet-bulb temperature (Stull): approximately 26.8 °C.
Heat index (Rothfusz): approximately 38 °C — noticeably hotter than the thermometer reading.

Compare with a dry desert afternoon: 38 °C, RH 15 %. The dew point drops to just 5 °C — classified as dry and comfortable — even though the air temperature is higher.

Location	T (°C)	RH (%)	Dew point (°C)	Comfort
Athens summer	34	55	23.7	Extremely humid
London average	18	75	13.6	Comfortable
Phoenix desert	38	15	5.0	Dry / comfortable
Singapore rainy season	30	85	27.2	Extremely humid

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