What are dew and frost points?

by | Mar 21, 2017

Maybe have you ever asked, while using Weather Station, what are the dew and frost points? To better understand what these two indicators are and what they show, here is a small explanation.

About temperature, humidity… and pressure

The dew point is the temperature below which the air has no longer enough energy to maintain the water it contains as vapor. Below the dew point, the energy associated with the molecular agitation becomes too low and the water, present in the form of gas (vapor), condenses and forms droplets: it’s dew! All this occurs at constant pressure, but in the case of atmospheric pressure and short observation times, a possible variation in pressure is relatively negligible.
Example of directly observable effect: if the outside temperature is 12 °C and the dew point is at 10 °C, it is sufficient that I put to air an object which have a temperature lower than 10 °C so that it is instantly covered by water droplets, the presence of this object having the effect of a local temperature fall of the air (near the object’s surface). This is what happens at night when our cars are covered with dew… Another example of this is when you blow air (which comes out of your lungs at a temperature of 33-34 °C and almost saturated of water vapor) on a colder surface than the dew point of the air you exhale – for example a window glass when it is colder outside than inside: the water vapor in this air that you exhale condenses, forming micro-droplets on this surface: the moisture! This condensation, again, occurred because the air (out of your lungs) is locally cooled in contact with the glass, to pass below its dew point.

This dew point is of relatively high importance for all processes that require to occur in dry conditions. This is the practical case, for example, when you want to know if you can repaint your window shutters: you must ensure that there is a good gap (at least 5 °C) between the air temperature and the dew point, otherwise if the surface you are about to paint is a bit cold… You now have an idea of what will happen!

Frost point

What the preceding explanation does not say is that this dew point, its calculation and its implications are valid only for a positive air temperature…

The frost point is, in fact, the temperature below which the air no longer has enough energy to keep the water it contains in the form of vapor. But this time, we no longer speak of condensation but of solidification: water always present in the form of gas in the air is transformed into crystals! This phenomenon is often seen in winter, it is the white frost…
Where it becomes astonishing is that for negative values of atmospheric temperature, the frost point is higher than the dew point. That is why under 0 °C, the frost point is used, because it will be reached before the dew point. Surprising, right?

As you will have understood, the scope of use of this value is very broad. The value of the frost point is used, for example, in air sports and transports, to calculate “in advance” the altitude at which frost is likely to form. But it can also serve you to know in which state you will find the windscreen of your car…

 

Rather than using Heinrich Gustav Magnus‘s formula which is relatively complex to implement, Weather Station uses an approximation calculation.

Dew point

\({T}_{d}=\sqrt[8]{H}[112+(0,9{T})]+(0,1{T})-112\)

with \({T}_{d}\) the dew point in °C, \({T}\) the atmospheric temperature in °C and \({H}\) the relative humidity (from 0 to 1).

Frost point

\({T}_{f}={T}_{d}+\frac{2671,02}{\frac{2954,61}{T}+2,193665\ln{(T)}-13,3448}-T\)

with \({T}_{f}\) the frost point in K, \({T}_{d}\) the dew point in K and \({T}\) the atmospheric temperature in K.

 

In Weather Station, dew and frost point values are available in all controls and widgets.

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