RE: [ATM] asymptote to ambient

["Donald Good" <donald.good@comcast.net>]

Air temperature cannot fall below the dew point.  When it is falling to the
dew point, relative humidity rises.  At the dew point, fog and clouds form.
Go home, because the visibility is shot.   If the temperature continues to
fall, the dew point falls with it and it is raining.  See
http://ww2010.atmos.uiuc.edu/(Gh)/guides/maps/sfcobs/dwp.rxml 

Clouds exist when the air temperature at the cloud altitude is at the dew
point.

Our problem exists when ambient temperature is slightly above the dew point
(high relative humidity).  Optics dew up when the optics glass temperature
falls to or below the dew point (that radiative cooling thing again),
cooling the air in contact with the glass to the dew point causing
condensation.  If the absolute humidity is low, condensation is minor (small
drops) and if the absolute humidity is high, those big drops form quickly.
So the fan can help by blowing the (warmer than dew point) ambient air
across the optics, warming it and keeping the radiative cooling at bay.
This is less effective as the ambient temperature is closer to the dew point
and particularly when the absolute humidity is high.  Then active heating
becomes necessary.

Clear skies (low relative humidity)
Don

> -----Original Message-----
> From: atm-bounces@atmlist.net 
> [mailto:atm-bounces@atmlist.net] On Behalf Of Mark Holm
> Sent: Tuesday, February 08, 2005 3:01 PM
> To: atm@atmlist.net
> Subject: RE: [ATM] asymptote to ambient
> 
> Donald Good wrote:
> 
> 
> >I read in a recent review of dew heaters that a mirror can 
> drop below 
> >ambient by radiative cooling.
> >
> Yes, and with today's much more open designs, the mirror is 
> more open to the sky, thus will cool more by radiative cooling.
> 
> This is also another argument for fans.  When radiative 
> cooling is significant, a fan will actually help keep the 
> mirror warmer.  If you happen to be in a locale where air 
> temperature does not often drop below dew point, but a 
> radiatively cooled surface might, then a fan might be all the 
> extra edge you need to keep away dew.
> 
> -- 
> Mark Holm
> mdholm@telerama.com
> 

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