[ferret_users] precision issue ?

[Alexa Griesel <agriesel@xxxxxxxx>]

Dear ferret users,
I have variables defined on a lon,lat,depth grid.  I am calculating 
cross (uteddyc, vteddyc) and along (uteddya, vteddya) temperature 
contours components of eddy heat fluxes. As a test,  
uteddyc*uteddya+vteddyc*vteddya  should be zero.
but what I am getting is 2^n in a lot of places (i.e. 2, 8, 16, 32, 
64,128..etc.), e.g. see below:

yes? list UTEDDYC[k=1,j=500,i=205]*UTEDDYA[k=1,j=500,i=205]
           
        -1.458E+09
yes? list VTEDDYC[k=1,j=500,i=205]*VTEDDYA[k=1,j=500,i=205]
           
         1.458E+09
yes? list 
UTEDDYC[k=1,j=500,i=205]*UTEDDYA[k=1,j=500,i=205]+VTEDDYC[k=1,j=500,i=205]*VTEDDYA[k=1,j=500,i=205]
           
        -128.0

so listing the terms individually, they should add up to zero. Is this a 
consequence of ferret operating in single precision ? This makes
me not trust my ferret calculations any more..
thanks for any hints! Alexa