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double max_solar_radiation (double latitude, time_t t, int press10, int sealevel10)
{
  struct tm *tp;
  int doy;                      // day of year
  double e0 = 1380.0;           // exo-atmos. solar irradiance
  double tau = 0.7;             // atmos transmittance
  int hour_angle = 0;           // solar hour angle
  double p_alt, p_sea;          // altitude pressure, sealevel pressure
  double deg2rad = (2 * 3.1415927) / 360.0;
  double declination, cos_zenith, air_mass;
  double i_direct, i_diffuse;

  tp = localtime (&t);
  doy = tp->tm_yday;
  p_alt = press10 / 10.0;
  p_sea = sealevel10 / 10.0;
  latitude *= deg2rad;
  hour_angle *= deg2rad;

  declination = -23.4 * deg2rad * cos (deg2rad * 360.0 * (doy + 10) / 365.0);
  cos_zenith =  (sin (latitude) * sin (declination)) +
                (cos (latitude) * cos (declination) * cos (hour_angle));
  air_mass = (p_alt/p_sea) / cos_zenith;
  i_direct = e0 * pow (tau, air_mass) * cos_zenith;
  i_diffuse = 0.3 * (1 - pow (tau, air_mass)) * e0 * cos_zenith;

  return i_direct + i_diffuse;
}