Last night I had an awful nightmare about dew formation, in which I was assailed by PWM monsters, complicated circuit diagrams and MOSFET misiles.
Although I’ve experienced ‘time to pack up’ dewing in real life, one of the PWM creatures insisted it just couldn’t happen … there was no way a passive object could drop below the temperature of its surroundings. Nightmarish or not, this PWM knew his physics!
The whole unsettling episode is of course down to recent dew-prevention posts on IIS and my inability to cope with the technical details. Perhaps this and similar items should be quarantined in some sort of ‘sealed section’ to keep it away from members prone to night terrors. In fact, because of its addictive properties, perhaps IIS should just be banned.
Anyway, as I couldn’t get back to sleep, I felt the best way to exorcise the demons was to tackle them on the Internet, their home ground (ref:
http://en.wikipedia.org/wiki/Internet_monsters 
).
Thankfully, within a few minutes I came across this:
“Temperature of a Glass Plate Exposed to the Night Sky … or … Why Does Dew Form so Readily on My Corrector Plate?
The author, Howard C. Anderson, seems to be a mathematical wizard and may be familiar to IIS regulars, addicted or not. His sensible approach brought me gently down to earth and banished the physics monster forever.
Here's a few extracts:
“I had often wondered about whether it was possible to predict when dew would form on a corrector plate …
… [There is] lots of misinformation out there on the internet. Some said the night sky temperature is 3 deg. K (which is the background temperature of the universe.) However, the night sky is full of air that radiates back at us. So the generally accepted value is approximately -70 C or 203 K.
… I used Mathcad to solve the system of equations …
… if the air temperature is 40 F, the ground temperature is 45 F, and the sky is -70 C, [calculations show that] the top surface of a piece of 1/4 inch thick glass suspended above the ground will achieve equilibrium when its top surface reaches 25.2 F! That's about 7 degrees below freezing [32 F] ... The radiative effect is much stronger than intuition had led me to believe...”
The full article is here:
http://www.astroshow.com/dewtell/dewtell.htm
You can also view Howard Anderson’s remarkable whole-Mars-day animated image here:
http://antwrp.gsfc.nasa.gov/apod/ap030826.html
Goodnight everyone … and sweet dreams!
