The foggy, foggy dew.

This post does get to weather eventually.  Trust me!  (Whether what happens in that discussion is comprehensible is a separate question.)

I hate to disappoint any followers of archetypal Pom folk songs but I'm not a weaver and no young maidens will be saved in this blog post.  I will disappoint them further by not including a you-tube link to that song as everything on line seemed to either be:

• by Burl Ives or Marty Robbins - who have no connection with England - or 
• a version by Benjamin Britten which is about as folky as "Nessun dorma"  (Incidentally the answer to the question posed in that video is clearly Jussi Bjorling.).  or
• a much more modern Irish song (The Foggy Dew) about the Easter Uprising. 

To meteorological business.  In recent mornings I have noticed a heavy dew on the pavers out the front of our house.  The area concerned is approximated by the red/yellow rectangle in this image.

My weather station (WS), located at the aqua circle, produces data on the dew point as well as the actual temperature.  I was interested to see what the WS said about dew point, temperatures and humidity.  In principle I thought that to get a heavy dew the actual temperature and the dew point should be the same.  This first chart shows the hourly temperature and dew point for the first 20 days of May ( I haven't been able to work out how to get the dates in the axis ).

 Note the area boxed with dashed lines.  In that area of the chart, overnight the temperature and dew point are very close together.

The two measures never actually achieve equal values but the difference is less than 1^oC for some periods, which I suspect is equality within the accuracy of the equipment.

I also wanted to compare this with measures of relative Humidity (since dew point is a function of humidity and temperature).  When Humidity is high (ie close to 100) the difference between temperature and dew point is low which makes a messy chart, so I plotted (100 - rH ) which I am calling "inverted rH". As the two series are of different scale I have used the LH scale for difference in temperature and the RH one for inverted humidity.

With all that adjustment I am unsure how much reality is left in the information!  However it does show that the basic story of high humidity gives dew point close to the recorded temperature is supported by my data.  The two series plotted show a correlation coefficient f 0.996!

As a third way of looking at dew, Frances noted that on the morning of the 21st there was much less dew on the pavers and no condensation on the glass screens on the northern side of the house (red lines in the first image).  As with the paver area the screens are sheltered from the wind so are possibly more likely to get condensation than the more exposed WS position.