<VV> Car Covers and Dew

[RoboMan91324 at aol.com]

Alan,

While there is moisture in the ground that can rise up, it is more difficult 
for that moisture to rise through concrete which is what most of us park on.  
If you park on dirt, that would be the worst situation in regard to ground 
moisture.  During a rain, water will soak into the concrete and slowing evaporate 
out which could condense on the underside of your car if it is cool.  A 
sealed or painted concrete surface is best unless there is a depression where you 
park and a puddle forms.

Dew generally comes from the air.  Hot air holds more moisture.  (This is why 
politicians are all wet.)  As the air cools during the night, it reaches what 
is called the "dew point" and water vapor will condense on any surface.  This 
generally happens on the cooler surfaces first and most.  Since most car 
covers are porous to let moisture out, they will also let moisture in.  During the 
night, the moisture will soak through to the car's surfaces which are 
typically cool.  It probably seems to you that the moisture originates between the 
car and the cover because the moisture on the surface of the car cover 
evaporates first in the morning sun and the moisture below the surface evaporates last. 
 Of course, the cover will lengthen the time it takes for any inside moisture 
to evaporate.

The worst case would be covering your car with a cover that reaches the 
ground.  Any moisture from the ground, rain flowing under the car or dew, will be 
trapped there longer because the cover prevents air circulation and thus slows 
evaporation.

Doc
1960 Vette; 1961 Rampside; 1962 Rampside; 1964 Spyder coupe; 1965 Greenbrier; 
1966 Corsa Turbo Coupe; 1967 Nova SS; 1968 Camaro ragtop
~~~~~~~~~~~~~~~~~~~~~~~~
In a message dated 3/10/2009 12:21:20 PM Pacific Daylight Time, 
virtualvairs-request at corvair.org writes:

> Message: 2
> Date: Tue, 10 Mar 2009 18:29:11 -0000
> From: "Alan and Clare Wesson" <alan.wesson at atlas.co.uk>
> Subject: Re: <VV> Car Covers
> To: "Ron" <ronh at owt.com>, "mike hicks" <mhicks130 at cox.net>,
>     <virtualvairs at corvair.org>
> Message-ID: <C303FCFA02A6435EBC9AB5E5B23528E9 at ALAN>
> Content-Type: text/plain; format=flowed; charset="iso-8859-1";
>     reply-type=response
> 
> I am prepared to take your word for it, but personal experience indicates 
> otherwise! I have had several very unfortunate experiences with 
> micro-blistering, on cars that were left outside in *dry* but dewy 
> conditions, and because extremely micro-blistered where the cover was in 
> contact with the paint. When I removed the cover the contact surface where 
> the cover touched the paint was wet, as were various places under the car 
> where the moisture could only (apparently) have risen.
> 
> And I have just done a quick web search on the subject, and the best 
> explanation seems to be that dew is formed from moisture that condenses from 
> 
> air close to the ground, as the ground has harboured heat in the daytime, so 
> 
> when night falls the vapour nearer the ground condenses more.
> 
> That in itself strikes me as a good reason to avoid car covers, because the 
> dew is going to be condensing under your car where it is near the ground, 
> and wetting it (this is certainly what happens). So whatever the cause of 
> the dew (and I'm prepared to believe what you say about dew, and I stand 
> corrected) I still wouldn't touch a car cover with a ten foot pole, except a 
> 
> dust cover on a car kept in a very dry atmosphere.
> 
> Cheers
> 
> Alan


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