RE: [APML] dew formation. Was: Film Creep

["Gene Horr" <genehorr@houston.rr.com>]

seds.org
[mailto:astro-photo-bounces@seds.org]On Behalf Of Steve Walters

> This is an excellent exposition but I have a question. You said:

>> The atmosphere will not get cooler than the dew point. Once the
temperature
>> of the atmosphere reaches the dew point, fog will form (a relative
humidity
>> of 100%, of course) and the temperature remains pretty stable until other
>> factors cause changes.

> Can you explain why this is true? And what are the other factors that will
> cause changes?

I'm going to jump in.  The statement is somewhat incorrect but still
pretty much applies to most cases for a period of time.  Keep in mind
that I am not a meteorologist nor do a portray one on television.
However I do have some practical and professional experience with
this.

Fog formation is an endothermic reaction.  And quite an energy-hungry
one.  It takes far more calories for a given volume of water vapour
to condense into fog then it does to lower the temperature than to
lower the temperature one degree (C or F).  So if you have the air
radiating a fixed amount of calories per minute if you track the
air temp. it will steadily decrease in a linear manner until it
reaches the dewpoint, at which time it level off and maintain the
same temp. as the water forms.

At the same time the fog is sheilding the lower air from space,
which lowers the radiation rate.  So you get a temporary condition
where the temperature stays the same.  But since in most locals
you reach this point in the early morning, this temporary conditions
lasts until sunrise, which means that in most cases the ground level
air never does drop below the dewpoint.

Now if you start the evening with close to 100% humidity and fog
forms early I have seen it drop below that point by the early
morning.  I have a hypothesis that this is caused by convection
from the upper air but have no data to verify this.

The other factors that he mentioned is most likely wind.  If
you have airflow from a dryer region that would push the fog
out of the way and allow the radiation cooling to continue.

Gene Horr
genehorr@houston.rr.com


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