Surface Diver uses the hydrophobicity scale introduced by Rose et al.:

residue hydrophobicity
ALA 0.74
GLY 0.72
VAL 0.86
LEU 0.85
ILE 0.88
PRO 0.64
PHE 0.88
TYR 0.76
TRP 0.85
SER 0.66
THR 0.70
ASN 0.63
GLN 0.62
CYS 0.91
MET 0.85
ASP 0.62
GLU 0.62
HIS 0.78
LYS 0.52
ARG 0.64

To map hydrophobicity onto the surface of a molecule, using the above table, each atom of a certain residue is assigned a similar hydrophobicity index, which is equal to the hydrophobicity of the residue that contains this atom.
Hydrophobicity at a particular point of the surface of the molecule is computed as:


where HBa,aa is the hydrophobicity index of a given atom (residue) and ga is a Gaussian function centered on atom a