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Array multiplication: implicit loop query

Hi there

I was just calculating the following equation:

DEN_H = MMR_H * Pres * RMT / ( atomic_mass_H * Gas_constant * TN )

These  numbers are 3D arrays, 1D arrays and constants, i.e.,

MMR_H = fltarr(30,91,40)
Pres = fltarr(30)
RMT = fltarr(30,91,40)
atomic_mass_H = constant
Gas_constant  = constant
TN =fltarr(30,91,40)

The result of this is DEN_H (previously undefined) which ends up being
fltarr(30) - i.e., 1 dimensional.
To my mind DEN_H should be 3D (30,91,40) - shouldn't it ?  Doesn't IDL
understand that I am implicity doing a full 3D calculation here ?  It
would seem that, to get this to work I need to make
Pres=fltarr(30,91,40).  However, this seems uneccessary because that
means that the 2 new dimensions for pressure are redundant.
Any advice ?? Cheers,
George Millward