What is Dew Point Temperature?

Definition

Dew point is the temperature at which air must be cooled to become saturated with water vapor.

When the temparature is below freezing point of water, the dew point is called the frost point.

Relationship to human comfort

When the air temperature is high, human body will perspire to cool down. If the air is already saturated with moisture (high RH), perspiration will not evaporate. In which case, human body can become coated with sweat on humid days.

Discomfort also exists when the dew point is very low. The drier air can cause skin to crack and beocme irritated more easily.

People living in tropical zone (e.g. Singapore) acclimatize somewhat to higher dew points.

Formula

Simple formula

From How do I calculate dew point? (columbia.edu)

$T_{dp} \approx T - \frac{100 - \varphi }{5}$

$T_{dp}$ is dewpoint temperature, resulted in °C,
$T$ is Temperature (dry-bulb), measured in °C,
$\varphi$ is Relative Humidity, measured in %. This RH must be above 50% for this formula to be accurate.

Magnus formula

From Dew point – Wikipedia, with coefficient recommended by Improved Magnus Form Approximation of Saturation Vapor Pressure in: Journal of Applied Meteorology and Climatology Volume 35 Issue 4 (1996) (ametsoc.org)

$T_{dp} = \frac{243.04 ln(\frac{\varphi }{100})+\frac{17.625 T}{243.04 + T}}{17.625 - ln(\frac{\varphi }{100})+\frac{17.625 T}{243.04 + T}}$

$T_{dp}$ is dewpoint temperature, resulted in °C,
$T$ is Temperature (dry-bulb), measured in °C,
$\varphi$ is Relative Humidity, measured in %.

Dew Point	Comfort Level
< 10°C	A bit dry for some
10 – 16 °C	Dry and comfortable
16 – 18 °C	Getting sticky
18 – 21 °C	Unpleasant, lots of moisture in the air
> 21°C	Uncomfortable, oppressive

Dew Point