OBSERVED PRECIPITATION AND CLOUD COVER
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The product shown here uses current surface observations from the 
United States and Canada to provide a plot of precipitation type and 
cloud cover.  The plot is created by dividing these areas into 1/4 
degree latitude by 1/4 degree longitude grid boxes.  Using the centroid 
of each box, the box is matched to the closest observing station and 
uses the reported precipitation type (Freezing Drizzle (ZL), Freezing 
Rain (ZR), Ice Pellets (IP), Snow (S), Rain (R), Drizzle (L)).  
If no precipitation is reported, then the closest observation will 
be checked for mention of thunder (TH) or fog (F).  If neither of 
those is found, then the closest observation will be checked for 
the greatest amount of cloud cover reported at any level (Obscured 
(XOB), Overcast (OVC), Broken (BKN), Scattered (SCT) or Clear (CLR)).

Each 1/4 degree by 1/4 degree box is filled using the color bar
at the bottom of the plot, depending upon what was indicated by
the nearest weather station.  The color is chosen using a hierarchy
of precipitation and cloud types, going from left to right along the
color bar.  If two types of precipitation are observed at a station
(e.g. ZR and S), the color chosen will be the one that is futher
left on the color bar.  If no weather data was available for any 
station within 100 km (150 km for pixels west of 100 degrees
longitude due to larger station seperation in the west), then
the pixel is filled with black to indicate that no data was available
for that pixel.

NOTE that these plots do not take differences in weather observing 
platforms into account.  This is important for automated weather 
stations, such as ASOS, which have trouble distinguishing between
some categories of precipitation and do not report cloud cover above 
12000 feet.  ASOS stations are often augmented to correct for 
errors in precipitation type reports.  Such corrections are used 
here to improve the quality of the plots.