Dew Point

on . Posted in Fluid Dynamics

Dew point, abbreviated as \(T_d\) is the temperature at which air must be cooled to become saturated with water vapor.  It is porportional to the amount of water vapor in a given amount of air and when the dew point is raised the more water vapor present, also the opposite.  The dew point is determined by the amount of water vapor in the air and the air temperature.  When the air temperature drops below the dew point, the excess water vapor in the air begins to condense onto surfaces as liquid water, forming dew, fog, or clouds.

Here's how the dew point works

  • Moisture in the Air  -  The air around us can hold a certain amount of water vapor, which is the gaseous form of water.  The maximum amount of water vapor that air can hold depends on its temperature.  Warmer air can hold more moisture, while colder air can hold less.
  • Saturation  -  When air reaches its saturation point, it can no longer hold all the moisture it contains, and some of the water vapor begins to condense into tiny water droplets.  This process is what creates dew on surfaces like grass, leaves, or car windows.
  • Dew Point Temperature  -  The temperature at which this saturation and condensation occur is called the dew point temperature.  It's the temperature at which the air becomes saturated with moisture, and the relative humidity reaches 100%.

The dew point is an important parameter in meteorology, as it can affect weather conditions such as the formation of clouds, fog, and precipitation.  It is also important in many industrial processes, such as refrigeration, where controlling the dew point can prevent condensation from forming and causing damage to equipment or products.  The dew point can be calculated using various equations based on the temperature and humidity of the air.  One common equation is the Magnus-Tetens formula, which relates the dew point to the saturation vapor pressure of water at a given temperature.  Other methods for measuring the dew point include using a hygrometer or a dew point sensor.


Dew Point formula

\(\large{ T_d =  T -  \frac{ 100 \;-\; RH  }{ 5 }  }\) 
Symbol English Metric
\(\large{ T_d }\) = dew point \(\large{F}\) \(\large{C}\)
\(\large{ T }\) = temperature \(\large{F}\) \(\large{C}\)
\(\large{ RH }\) = relative humidity \(\large{dimensionless}\)


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Tags: Temperature Equations Water Equations HVAC Equations