  We perform calculations to obtain the $H_{c2}$ curve of high temperature
superconductors (HTSC). We consider explicitly the fact that the HTSC possess
intrinsic inhomogeneities by taking into account a non uniform charge density
$\rho(r)$. The transition to a coherent superconducting phase at a critical
temperature $T_c$ corresponds to a percolation threshold among different
superconducting regions, each one characterized by a given $T_c(\rho(r))$.
Within this model we calculate the upper critical field $H_{c2}$ by means of an
average linearized Ginzburg-Landau (GL) equation to take into account the
distribution of local superconducting temperatures $T_c(\rho(r))$. This
approach explains some of the anomalies associated with $H_{c2}$ and why
several properties like the Meissner and Nernst effects are detected at
temperatures much higher than $T_c$.
