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Re: Frost?/Dewpoint?

Square Foot Gardening List - http://www.flinet.com/~gallus/sqft.html


>Is "dewpoint" related to frost at certain temps??  What is
>"dewpoint"???
>
OK, at the risk of oversimplifying:  Air contains various amounts
of moisture.  When the temperature falls, air can hold _less_ 
moisture so water comes out.  The temperature at which air becomes
100% saturated is called the "dew point."  Now certain substances are
not able to hold heat as well as others, so they cool faster, and so does
the air right around them. If there is very little wind, the water that comes
out of the air clings to stuff like the metal on car hoods or the leaves of 
plants, which do not hold heat well.
  If the temperature is close to the freezing point, frost will form; 
it's frozen dew.  Now, frost may form even when the air temperature generally
is just above freezing.  Let's say the relative humidity drops a bit so that
the dew can start evaporating.  When water evaporates--for example from
wet clothes--it cools.  So dew at, say, 33 deg F. that starts evaporating
drops a degree and freezes.
Dew is like the condensation on the outside of your glass of ice water.
The air right next to the glass will reach the dew point.
If you put that glass in the freezer, frost will form on it.
If you take a fan and blow warmer and drier air around your
glass of ice water, no condensation is likely to occur because
the air cannot reach the dew point.


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