Averaging data for 4 months

[Amy Sundquist <afsundquist@xxxxxxxxxxx>]

Hello,

I am working with gridded monthly 700 mb heights.  The data for all 12
months is for approximately 49 years (588 months).  I would like to find
the average 700 mb height for Dec.-Mar. for each year and an average for
the entire 49 years.  The problem I am having is with incrementing the
time.

Since I was having difficulty with this, I decided to increment each month
individually by 12 months to see where my errors are.  This is what my
code looks like for the height data:

Let Dec = hgt[x=-160,y=60,z=700,L=12:582:12,d=2]
Let Jan = hgt[x=-160,y=60,z=700,L=13:582:12,d=2]
Let Feb = hgt[x=-160,y=60,z=700,L=14:582:12,d=2]
Let Mar = hgt[x=-160,y=60,z=700,L=15:582:12,d=2]
Let Htime = (Dec + Jan + Feb + Mar)/4

Dec works fine.  It starts in December 1949 and goes through December
1995.  When I list Jan, it has the exact same values as Dec. However, when
I change the time increment in Jan to "L=1:582:12" instead of
"L=13:582:12" I get the correct heights for Jan starting with January
1948, but I don't want to start with month 1, I want to start with month
13 (January 1949).

A similar thing happens for Feb.  I get December values.  But when I
change the time from "L=14:582:12" to "L=2:582:12", I get January Values
as I did when I used "L=1:582:12".

Finally, when I list Mar, I get Dec values for "L=15:582:12" and Jan
values for "L=3:582:12".  

I can't seem to figure out what is going on here.  There doesn't seem to
be a pattern on how Ferret is displaying the data.  Is there a simpler way
to go about averaging 4 months of data for a period of 50 years or so?  If
anyone has any suggestions, I would greatly appreciate it!

Thanks,
Amy Sundquist

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UC Davis Atmospheric Science Graduate Group