19991105: gempak metar ceiling

>From: address@hidden
>Organization: .
>Keywords: 199911051649.JAA28539

>We decode the metar data for gempak with the default hrly.pack file.
>The cloud height is put into the CHCx variable where x is 1, 2 or 3
>of the sky condition groups.  The entry from the help sfparm command
>has this entry
>
>       CHCx - Numeric combined cloud height and coverage
>
> For this variable I get numbers like 2502.00  or sometimes 1.00.  
>How is this interpreted?  Which is the cloud height and which is 
>the cloud amount?  Is the cloud height 25,000 ft, 2500 ft, or 
>2000 ft?  Does a value of 1.00 mean that this variable is missing?
>METAR uses CLR,FEW,SCT,BKN,OVC for cloud amount.  How does CLCx 
>interprete this?  Are FEW, 1/8, 1/10?
>If I am really interested in the cloud height should I
>use the variable CLHx in the packing file instead?
>
>Donna Tucker                      http://chinook.phsx.ukans.edu/tucker.html
>address@hidden      Department of Physics and Astronomy  
>(785) 864-4738 (new area code!)    University of Kansas                 
>(785) 864-5262 (fax)               Lawrence, KS  66045-2151                   
>  
>
>

Donna,

Your first question as to CHCx is explained in appendix A of the User Guide as:

CHCx = CLHx * 10 + CLCx

So, CHC3 = 2502 yeilds  CLC3 = 2 and CLHH = 250
The CLHx parameter is cloud height in hundreds of feet, so CLHH=250 means
25000 feet. You can use the CLHx parameters to obtain the cloud height in
hundreds of feet for output in sflist.

A value of CHCx=1 means CLCx = 1 --> clear from the table on A-12.
The fractional representation of the cloud representations are available from
xCLO and the cloud layer names from xCLD.

From the subject line "metar ceiling", I looked back in my archives
at a former question I answered for providing the ceiling height-
eg the height at which the cloud layer is at least BKN.
I provided a function at:
http://www.unidata.ucar.edu/glimpse/gempak/2626
which provides the ceiling level from CHC1,2,3.

Steve Chiswell