Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Dew Formation

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

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.