formula from curves for two inputs

dear all,

Hello,

I need your help in this matter,

the discription of the subject is:

we have two input vectors ( included in one vector):

T ( temperature ) and H ( humidity ), there is a corresponding one output

P ( power demand ), and all these data are normalized

I have thousands of data for these values done for many years.

for sample example: ( l assume that we have the data for 5 hours )

T_H=[0.3333 0.3333 0.3542 0.3750 0.3750 ; 0.6970 0.8384 0.6869 0.7980 0.7677]

and the corresponding P is:

P=[0.6561 0.6618 0.6568 0.6694 0.6622]

I need to draw a curves relating all data,

and THE MOST IMPORTANT THING WHICH IS THE AIM OF MY PROBLEM IS :

finding ( inventing ) a formula depending on the curves drawn that relates
the power as an output with the temperature & humidity as inputs,

meaning:

P = f (T,H)

for example: the formula model may be:

P = a0 + a1 * T + a2*H*sin(T) + a3*T^3*H + a4*T*H

then finding a0,a1,a2 and a3 and this is easy and I
know how to find them.

BUT, my problem is how can I find this above example model function P ?
is it by dividing the curve to parts and then finding a function
for each part ? but how is this done by MATLAB ?
Any idea ?

Can anybody help me? I am appreciating any help.
and remember, any service will be valuable for me

Best regards