Dew Point Calculator
The dew point is the temperature at which air becomes saturated and dew forms. At 25 °C (77 °F) and 60% humidity the dew point is about 16.7 °C (62 °F), which feels slightly humid.
Dew point
16.7°C
Slightly humid
Estimated cloud base: about 1,038 m (3,405 ft)
The bigger the gap between temperature and dew point, the higher clouds form.
Starting to feel a little sticky during activity.
Show the math
- 1
Calculate the Magnus term γ
γ = ln(60 ÷ 100) + (17.625 × 25) ÷ (243.04 + 25) = 1.1331
This combines how humid the air is with how warm it is.
= 1.1331
Read the steps as text
- Calculate the Magnus term γ. γ = ln(60 ÷ 100) + (17.625 × 25) ÷ (243.04 + 25) = 1.1331 This combines how humid the air is with how warm it is.
- Solve for the dew point. Td = (243.04 × 1.1331) ÷ (17.625 − 1.1331) = 16.7 °C The dew point is the temperature the air must cool to before it can no longer hold its water vapour, so dew or fog forms.
- Read the comfort level. Dew point 16.7 °C → Slightly humid Starting to feel a little sticky during activity.
- Estimate the cloud base height. (25 − 16.7) × 125 m = 1,038 m Rising air cools about 8 °C per km while the dew point falls about 1.5 °C per km, so air reaches saturation about 125 m up for each degree of gap. This is a rough guide for convective clouds.
What the dew point tells you
Air can hold a limited amount of water vapour, and warm air holds more than cold air. The dew point is the temperature the air would have to cool to before it is full, at which point vapour condenses into dew, fog or cloud. A high dew point means a lot of moisture is in the air.
Relative humidity is less useful for comfort because it changes with the temperature: the same air can be 90% humid at dawn and 45% humid in the afternoon even though its moisture has not changed. The dew point stays put, which is why forecasters and pilots prefer it.
How the calculator works it out
It uses the Magnus formula. First it calculates γ = ln(RH ÷ 100) + (17.625 × T) ÷ (243.04 + T), with T in °C. Then the dew point is Td = (243.04 × γ) ÷ (17.625 − γ). This is accurate to a fraction of a degree between about −45 and 60 °C. If you use Fahrenheit, it converts first and back at the end, and every step is shown.
As a quick check, at 100% humidity the dew point equals the air temperature, and every 5% drop in humidity lowers the dew point a little under 1 °C at typical temperatures.
Dew point comfort scale
A dew point under 50 °F (10 °C) feels dry and comfortable. From 50 to 59 °F it is comfortable, 60 to 64 °F slightly humid, 65 to 69 °F humid and sticky, 70 to 74 °F oppressive, and 75 °F (24 °C) or higher is extremely humid, the kind of air that makes summer heat hard to bear.
The calculator also estimates the cloud base, roughly 125 metres per degree Celsius of gap between the temperature and the dew point, and warns when the gap is under 2.5 °C, when fog, mist and dew are likely. The cloud base figure is a guide for fair-weather clouds, not a forecast.
Frequently asked questions
- What is a good dew point?
- Dew points between about 10 and 15 °C (50 to 59 °F) are comfortable for most people. Above 18 °C (65 °F) the air feels humid, and above 21 °C (70 °F) it feels oppressive.
- What is the difference between dew point and humidity?
- Relative humidity says how full the air is compared with its capacity at the current temperature. The dew point says how much moisture is actually in the air, so it does not change through the day the way humidity does.
- Can the dew point be higher than the air temperature?
- No. At most it equals the air temperature, when humidity is 100% and fog or dew forms.
- How is cloud base estimated from the dew point?
- Rising air cools faster than its dew point falls, so they meet after about 125 metres of climb for each degree Celsius of gap. That level is where cloud starts to form.