Re: Dew Formation

[John Denker <jsd@AV8N.COM>]

Quoting Bob LaMontagne <rlamont@POSTOFFICE.PROVIDENCE.EDU>:

> When dew forms on clear nights, it will heavily wet lawns
> but will hardly ever leave visible moisture on concrete
> walkways or asphalt roads. There are many factors that
> regulate condensation: temperature, available condensation
> nucleii, curvature, hydrophilic surfaces, etc. - but I am
> having difficulty identifying the  primary one in this
> case. Any ideas out there?

(I assume the observation applies to clear skies with
_calm winds_.)

I'm not sure there is a "primary" factor but there are a
couple of overwhelming factors that distinguish grass from
concrete.

The first is that grass has roots!  The grass has a vascular
system that is in the business of sucking moisture out of
the ground.  So you would expect that early in the night,
the *local* relative humidity in the air near the grass
would go to 100%, and that any subsequent cooling would
re-preciptate some of this moisture.  Nothing similar happens
in/near the concrete.

Another overwhelming difference has to do with thermal inertia,
which involves heat capacity and thermal conductivity.
The thermal conductivity of concrete isn't super-super good,
but it is relativity higher than the air trapped between blades
of grass.  So the grass will cool quicker and get colder.