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Dew Point

Dew Point is the the temperature to which air must be cooled to become saturated.  In other words, at that temperature water vapour in the air will condense to liquid water.

If you were to visit Homershams (and you'd be very welcome!) on certain days one might see our still in action.  See video here

However don't be concerned.  We've not augmented our business by manufacturing moonshine!  We make our own distilled water to create ice slurry for ice point thermometer checks.

The process of distillation has always fascinated me.  Ordinary tap water is boiled till it begins to produce steam/water-vapour.  A cold water jacket is then used to reduce the steam to its Dew Point, where it condenses back to water, now free of most impurities.

We have many instruments to measure Dew Point and relative Humidity and being the nerd I am, I revel in measuring this.


Here's 45-days of temperature, humidity and Dew Point measurement in my lounge at my home. The red trace is temperature, the blue is humidity and the green Dew Point.   

Annotation 2019-07-30 082133
And here's a few hours, zoomed in for clarity.  See how the Dew Point is 8.1 ℃ so it's very unlikely I'll get condensation forming.  The exception would be if it's very cold outside.  If the temperature of my window falls to 8.1 ℃, I could have water forming on the windows.

Humidity Temp Dew Point edited-1
But as it happens, it was 9.3 ℃ and so ipso-facto, no wet windows!

Now Dew-Point can be calculated if one knows the RH, Temperature, Vapour Pressure, Barometric Pressure etc but it's not easy!  The charmingly named Bögel modification, also known as the Arden Buck equation is needed to calculate it

Thank goodness all of this is taken care of by software in some instruments!  here's a screen shot from a Delta Ohm data logger.  Note that it shows many more humidity measurements than just RH, Dew Point and temperature

IMG 2226 edited-1 

Some definitions  (I've paraphrased for simplicity.  Feel free to e-mail me corrections 😊)

Vapour Pressure.  The tendency of a liquid to become a gas/vapour.  This increases with temperature, so for example at 100 ℃ the vapour pressure of water is equal to atmospheric pressure and it boils

Relative Humidity. The amount of water in the air compared to the theoretical maximum  the air could hold e.g. 100% RH means the air is fully saturated and can not hold any more water.  

Absolute Humidity.  How much actual water is 1 kg of air e.g. 6.6 grams of water in the example above

Mixing Ratio.   A measure of how much water could be held by 1 kg of Dry Air.   

Wet Bulb temperature.  A measure of the temperature that a wet thermometer would read in current conditions

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