Data comments for the CASES 1997 HSI operations.

Dr. Charles N. Long
e-mail:  chuck.long@noaa.gov
Phone: (303) 497-6056


General:

The first Hemispheric Sky Imager (HSI) prototype was operated 
as part of the Cases 1997 experiment. This was the very first 
campaign field deployment of any early prototype HSI version. 
The basic design includes a digital camera looking down on a 
hemispheric mirror, with a curved black metal strip to block 
the sun. It should be noted that this early strip design was 
abandoned in favor of a rotating mirror in large part because 
the strip curvature would need to be changed in relation to 
the actual sun path across the sky, with the exact shape and 
evolution dependent on location and date. Note that toward the 
end of the Cases97 campaign the sun blocking strip failed to adequately block the sun at lower solar elevations. This early 
prototype also used an early model Connectix "golf ball" digital 
camera, which as can be seen had rather more image "bleed" issues 
than desired. The data processing also used an early version of 
the fractional sky cover retrieval code, thus the output file 
format is a bit different than later campaigns.
   
Note that the sky cover retrievals are generally only valid for solar 
elevation angles greater than 10 degrees (zenith angles less than
80 degrees), and images are processed for a 160 degree field-of-
view (ignoring the 10 degrees of sky near the horizon).



Data Formats --

Sky images:
are in JPEG image format, 320 X 240 pixel resolution. Image
file names are in YMDDhhmm.jpg format, where:

Y = last digit of 4-digit yearin 199#
M = month of year 
DD = day of month
hh = hour of day (0-23)
mm = minute of hour



Fractional Sky Cover:

date	Date in  YYMMDD format
Ltim	Local standard time in hhmm format  
Ztim	Corresponding UTC time in hhmm format 
adfl	Processing code flags, can be ignored by user    
CF	Total fractional sky cover   
TCF	Thin fractional sky cover   
ZCF	Fractional sky cover in 20 degree zenith circle   
SCF	Fractional sky cover in 20 degree circle around sun position in image   
E2A	Ratio of average cloud edge to cloud area in sky view, experimental    
CosZ	Cosine of the solar zenith angle   
Azim	Solar azimuth angle referenced to east (degrees)
Decl	Solar declination angle (degrees)