Creating a monthly average from a matrix

My data, which has 30 years of monthly data, over a 2 degree grid of the globe:

>> whos KoppenA_prate
  Name                Size                    Bytes  Class     Attributes

    KoppenA_prate      90x180x360            23328000  single

Not every pixel has data, just the pixels that fit into Koppen classification A

>> sum(A(:))

ans =

     559

My question: Any ideas on how I could write a loop to create an average monthly value over the 30 years?

Thanks a bunch!

[Azzi Abdelmalek]

This is not clear for me. What are your data? and what do you want?

[Image Analyst]

What does each of the 3 dimensions represent? Is any one of the dimensions the time point? If so, are the time points all one month apart? Or, are they days of the year, so that one month might have 30 data points sometimes, and 31 points at other times?

[Kate]

90x180 is a 2 degree grid of the globe, 360 is 12 months x 30 years

[Kate]

I would like to create a vector of precip data, for each month averaged over 30 years.

[Kate]

This is as far as I've gotten, but my for loop isn't working:

%Create monthly averages over 30 years
Jan=[1:12:349];
Feb=[2:12:350];
Mar=[3:12:351];
Apr=[4:12:352];
May=[5:12:353];
Jun=[6:12:354];
Jul=[7:12:355];
Aug=[8:12:356];
Sep=[9:12:357];
Oct=[10:12:358];
Nov=[11:12:359];
Dec=[12:12:360];

for ii=length(num_mons)
    if num_mons==Jan;
        Jan_dat=[Jan,KoppenA_prate(3)];
    elseif num_mons==Feb;
        Feb_dat=[Feb,KoppenA_prate(3)];
    end
end

Answer 1

[Cedric]

Just

 avgAllYears = mean(KoppenA_prate, 3) ;

where 3 is the dimension along which the mean must be computed. With that you get a 90x180 array of means over all times.

EDIT:

 siz = size(KoppenA_prate) ;
 avg = zeros(siz(1), siz(2), 12) ;
 for mId = 1 : 12
    avg(:,:,mId) = mean(KoppenA_prate(:,:,mId:12:end), 3) ;
 end

EDIT2: or, more efficient but more complicated

 siz = size(KoppenA_prate) ;
 buf = mean(reshape(KoppenA_prate, siz(1), 12*siz(2), []), 3) ;
 avg = reshape(buf, siz(1), siz(2), []) ;

Note: both produce a 90x180x12 array of monthly averages, were e.g. avg(:,:,5) is the average of 5th months over all years.

[Kate]

Thanks Cedric, but that would create a 30 year average, not a monthly average

[Cedric]

Ok, I misunderstood "monthly average" for "average over monthly values". See my edits.

[Kate]

Thanks Cedric, that's great. How could I get a single, say Jan, value over the whole grid? I need to plot average yearly precip.

Thanks for all of your help!

[Cedric]

You're welcome; see my last note.