You can view the various "Pressure -vs-(whatever)" correlation plots,
and see in the small -multiples on the map, that the pressure in 
certain areas of the map gets divided into 2 "clumps".  See the following
map.

The "Pressure -vs- Cloudhigh" is a good example.

Notice that in the Western US, and Mexico, and southern South America,
many of the small-multiple correlation plots show the pressure values
clumping into 2 different ranges.  

This re-inforces the 'strange' values we were seeing in the time-series 
plots for individual long/lat areas, such as 
this one.