SURFRAD monthly average files

Filenames:

Filenames of the monthly average SURFRAD files are of the form [STA]yy.spr, 
where STA is the station three-letter identifier (e.g., bon for Bondville, sxf 
for Sioux Falls), yy is the two-digit year (e.g., 00 for 2000, and 99 for 
1999), and the extension (.spr) represents "spreadsheet," because these files 
spreadsheet-like and thus are easily loaded into programs such as Excel.  For 
example, the file "sxf03.spr" contains monthly averages for the Sioux Falls 
station for 2003.  

File structure:

The file structure of the SURFRAD monthly average files is a header line 
containing column labels, followed by 12 lines of monthly average data.  The 
first line is for January, and the last line is for December.  The columns 
represent the following:

Column	Label		Quantity		Units

1	month		number of month		unitless
2	dpsp		downwelling short wave	W/m2
3	upsp		upwelling short wave	W/m2
4	nip		direct solar		W/m2
5	par		photosynthetically active radiation	W/m2
6	uvb		erythemal ultraviolet irradiance	milliW/m2
7	diffuse		diffuse short wave	W/m2
8	dpir		downwelling long wave	W/m2
9	upir		upwelling long wave	W/m2
10	netsolar	dpsp-upsp		W/m2
11	netir		dpir-upir		W/m2
12	totalnet	netsolar+netir		W/m2
13	convfac		conversion factor (par/dpsp)	unitless
14	trans		transmission (dpsp/solar constant)	unitless
15	tc		air temperature (C)	deg. C
16	rh		relative humidity	%
17	speed		wind speed		m/s
18	albedo		upsp/dpsp		unitless
19	q		specific humidity	g/Kg
20	pres		station pressure	hPa
21	virtual_t	virtual temperature	deg. C


Average calculation:

The averages in the SURFRAD monthly average files represent the average of all 
instantaneous measurements during one month.  This type of average is easily 
interpreted for meteorological measurements such as temperature and wind 
speed, however the irradiance averages need explanation.  The irradiance 
averages in the SURFRAD average files are reported as an average instantaneous 
flux in Wm-2, and include all data from the month, including the nighttime 
values.  Thus for the solar parameters, the average includes all nighttime 
zero values.  Another popular way of presenting average radiation data is to 
integrate over the period of interest to compute a dose, which would have 
units of J/m2.  Doses are not reported in the SURFRAD monthly average files.  
Also note that the average UVB irradiance is reported in milliW/m2.  

The average for downwelling shortwave (dpsp) is computed with the best data 
possible.  That is, if the direct and diffuse solar measurements are available, 
they are combined to represent the most accurate downwelling total short wave 
measurement.  If either the direct or diffuse short wave is unavailable for a 
particular time, the global short wave measurement is used.  

The values used for the calculation of average transmission, albedo, and 
conversion are limited to measurements when the solar zenith angle is less 
than or equal to 75 degrees.  In other words, no data collected when the sun 
is within 15 degrees of the horizon is used in those calculations.  

If less than 70% of the possible data for a particular parameter are available 
for a month, then an average is not computed, and a missing value is reported. 
Missing values are represented by -9999.9000