Re: [ferret_users] mm/day to mm/month

[mick spillane <mick.spillane@xxxxxxxx>]

Hi Peter & Ansley,

   I actually prefer Russell's DAYS_IN_MONTH approach which beats by 
miles the clunky things I've used at various times in the past in 
similar situations. There is a slight omission in not having a YEAR0 in 
the definition of YEAR and maybe its worth generalizing to situations 
where a monthly series did not begin in January.  With these thoughts in 
mind, I'm putting the attached DaysPerMonth.jnl script in my toolkit. 
Apologies to anyone whose reaction is "duh" -- though there's probably 
no one like that in the ferret-users group.

Mick

PS As an example of how to use it in a situation like yours Peter (but 
with series starting with  Feb 1960 say)

! demo of russell's days-in-month approach to mm/day --> mm/month conversion
def axis/t="15-Feb-1960":"15-JAN-1990"/npoints=360 tax ; def grid/t=tax grd
let/title="rainfall"/units="mm/day" drain=randu(t[g=grd])  ! a fake 
rainfall series
! generate the days/month variable for the daily rainfall variable "drain"
go dayspermonth 1960 2 drain
! and apply it to generate monthly rate
let/title="rainfall"/units="mm/month" mrain=dpm*drain
! and list some results to check dpm in leap and non-leap years
list/nohead/L=1:16 yrser,mnser,d1900,dpm,drain,mrain
15-FEB-1960 /  1:  1960.   2.00  21945.  29.00  0.6862  19.90    <== 29 
days in Feb'60
16-MAR-1960 /  2:  1960.   3.00  21974.  31.00  0.6201  19.22
15-APR-1960 /  3:  1960.   4.00  22005.  30.00  0.5975  17.92
16-MAY-1960 /  4:  1960.   5.00  22035.  31.00  0.4654  14.43
15-JUN-1960 /  5:  1960.   6.00  22066.  30.00  0.6546  19.64
16-JUL-1960 /  6:  1960.   7.00  22096.  31.00  0.8547  26.50
15-AUG-1960 /  7:  1960.   8.00  22127.  31.00  0.2217   6.87
15-SEP-1960 /  8:  1960.   9.00  22158.  30.00  0.4683  14.05
15-OCT-1960 /  9:  1960.  10.00  22188.  31.00  0.8639  26.78
14-NOV-1960 / 10:  1960.  11.00  22219.  30.00  0.3296   9.89
15-DEC-1960 / 11:  1960.  12.00  22249.  31.00  0.9992  30.97
14-JAN-1961 / 12:  1961.   1.00  22280.  31.00  0.4089  12.67
14-FEB-1961 / 13:  1961.   2.00  22311.  28.00  0.7811  21.87  <== & 28 
in Feb'61
16-MAR-1961 / 14:  1961.   3.00  22339.  31.00  0.4778  14.81
16-APR-1961 / 15:  1961.   4.00  22370.  30.00  0.4869  14.61
16-MAY-1961 / 16:  1961.   5.00  22400.  31.00  0.5987  18.56


! DaysPerMonth : Creates a variable "dpm" for days per month to associate with a
!                particular monthly time series. Other variables introduced may
!                be useful, eg mnser in generating monthly climatologies, d1900
!                which is days1900 for the first of the month
 
! Usage Example: If series "myvar" starts May 1959 then before using "dpm"
!   go DaysPerMonth 1959 5 myvar

let LVAL=L[g=$3]
let/title="Year Series"     yrser=`$1`+int((LVAL+`$2`-2)/12)
let mser0=mod(LVAL+`$2`-1,12)
let/title="Month Series"    mnser=if(mser0 eq 0)then 12 else mser0
let/title="Days1900 Series" d1900=days1900(yrser,mnser,1)
let/title="Days/Month"      dpm=d1900[L=@shf]-d1900