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Re: (meteorobs) Cloud

Hi Kim,

Obstructions to a field of view can be caused by trees, buildings, or
clouds. While observing, the percentage of sky that is blocked by these 
obstructions is noted at the beginning of the session and at any time a
change has taken place. This percentage is noted on the Visual Summary
Report.

Clouds present a difficult challenge to observers since they are constantly
changing. It would be almost impossible to list every change in cloud cover
that takes place. Therefore, it is recommended that if clouds are present,
the average cloud percentage over the last 10 to 15 minutes be reported.

After the observation is over, the percentages you obtained from the above
can be converted to a decimal value by using an equation provided by the
IMO. The equation that is used is:

      1
F = -----
    1 - k

           percent blockage x minutes
where k = ----------------------------
             total observing period


As an example, on the night of August 11/12, assume an observing period of
0500 to 0630. Teff for the period was 1.50 (90 minutes), and the sky was 
covered with 15% clouds for 15 minutes.

Therefore,

     15% x 15 minutes*       2.25
k = -----------------  =  ----------  =  0.025
       90 minutes             90


Then,

      1            1              1
F = -----  =  -----------  =  ---------  =  1.03
    1 - k      1 - 0.025        0.975

Note that when calculating k, be sure to include all individual cloud cover
estimates in the percent x minutes value.

The correction factor F, equals 1.03 for the above mentioned observing
period, and should be listed under the OBSERVING PERIODS section of the
NAMN report. Note that when there are no obstructions to a field of 
view, F would then equal 1.00 and should also be listed for each 
observing period on the report.

If any of the above isn't clear, just let me know.

-Mark


-----Original Message-----
From: Kim Hay <kimhay@adan.kingstondot net>
To: meteorobs@latrade.com <meteorobs@latrade.com>
Date: Monday, August 03, 1998 8:48 AM
Subject: (meteorobs) Cloud


Hi Mark!!

To calculate the f factor, I am wondering how to do this. Last night/
this morning when I was out, this large mothership of a cloud sits
itself on the milkyway. Ok, so it is starting to obstruct my view.
Then it decides to slowly spread out.

How can I account for that percentage of the sky being obscured??

Kim
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