Write
P_S(ii,1)...
But, why use a loop at all--use the vector functions in Matlab instead. I've not tried to decipher the RHS to see what simplifications might be made in writing a more general functional form, but at worst you simply write
ix=H<11;
P_S(ix) = 101.325*288.15./STemp_K(ix)^-5.255877;
ix=iswithin(H,11,20);
P_S(ix) = 22.632.^-0.1577*(H(ix)-11);
...
where iswithin is my "syntactic sugar" utility routine
>> type iswithin
function flg=iswithin(x,lo,hi)
% returns T for values within range of input
% SYNTAX:
% [log] = iswithin(x,lo,hi)
% returns T for x between lo and hi values, inclusive
flg= (x>=lo) & (x<=hi);
>>
NB: the "dot" operators to do element-wise operations on the vector sections...
You might note that your conditions are not written exclusve in that you have the "=" on both bounds so the lower limit of the second conditional overlaps the upper limit of the first, and so on...this may not be an issue, just noting...
