+void
+FGEnvironment::_recalc_sl_dewpoint ()
+{
+ // 0.2degC/1000ft
+ // FIXME: this will work only for low
+ // elevations
+ dewpoint_sea_level_degc = dewpoint_degc + (elevation_ft * .0002);
+ if (dewpoint_sea_level_degc > temperature_sea_level_degc)
+ dewpoint_sea_level_degc = temperature_sea_level_degc;
+}
+
+void
+FGEnvironment::_recalc_alt_dewpoint ()
+{
+ // 0.2degC/1000ft
+ // FIXME: this will work only for low
+ // elevations
+ dewpoint_degc = dewpoint_sea_level_degc + (elevation_ft * .0002);
+ if (dewpoint_degc > temperature_degc)
+ dewpoint_degc = temperature_degc;
+}
+
+void
+FGEnvironment::_recalc_sl_pressure ()
+{
+ using namespace atmodel;
+#if 0
+ {
+ SG_LOG(SG_GENERAL, SG_ALERT, "recalc_sl_pressure: using "
+ << pressure_inhg << " and "
+ << temperature_degc << " @ " << elevation_ft << " :: " << this);
+ }
+#endif
+ pressure_sea_level_inhg = P_layer(0., elevation_ft * foot,
+ pressure_inhg * inHg, temperature_degc + freezing, ISA_def[0].lapse) / inHg;
+}
+
+// This gets called at frame rate, to account for the aircraft's
+// changing altitude.
+// Called by set_elevation_ft() which is called by FGEnvironmentMgr::update
+
+void
+FGEnvironment::_recalc_alt_pt ()
+{
+ using namespace atmodel;
+#if 0
+ {
+ static int count(0);
+ if (++count % 1000 == 0) {
+ SG_LOG(SG_GENERAL, SG_ALERT,
+ "recalc_alt_pt for: " << elevation_ft
+ << " using " << pressure_sea_level_inhg
+ << " and " << temperature_sea_level_degc
+ << " :: " << this
+ << " # " << count);
+ }
+ }
+#endif
+ double press, temp;
+ boost::tie(press, temp) = PT_vs_hpt(elevation_ft * foot,
+ pressure_sea_level_inhg * inHg, temperature_sea_level_degc + freezing);
+ temperature_degc = temp - freezing;
+ pressure_inhg = press / inHg;
+}
+
+void
+FGEnvironment::_recalc_density ()
+{
+ double pressure_psf = pressure_inhg * 70.7487;
+
+ // adjust for humidity
+ // calculations taken from USA Today (oops!) at
+ // http://www.usatoday.com/weather/basics/density-calculations.htm
+ double temperature_degk = temperature_degc + 273.15;
+ double pressure_mb = pressure_inhg * 33.86;
+ double vapor_pressure_mb =
+ 6.11 * pow(10.0, 7.5 * dewpoint_degc / (237.7 + dewpoint_degc));
+ double virtual_temperature_degk = temperature_degk / (1 - (vapor_pressure_mb / pressure_mb) * (1.0 - 0.622));
+ double virtual_temperature_degr = virtual_temperature_degk * 1.8;
+
+ density_slugft3 = pressure_psf / (virtual_temperature_degr * 1718);
+ _recalc_density_tropo_avg_kgm3();
+}
+
+// This is used to calculate the average density on the path
+// of sunlight to the observer for calculating sun-color
+void
+FGEnvironment::_recalc_density_tropo_avg_kgm3 ()
+{
+ double pressure_mb = pressure_inhg * 33.86;
+ double vaporpressure = 6.11 * pow(10.0, ((7.5 * dewpoint_degc) / (237.7 + dewpoint_degc)));
+
+ double virtual_temp = (temperature_degc + 273.15) / (1 - 0.379 * (vaporpressure/pressure_mb));
+
+ double density_half = (100 * pressure_mb * exp(-altitude_half_to_sun_m / 8000))
+ / (287.05 * virtual_temp);
+ double density_tropo = (100 * pressure_mb * exp((-1 * altitude_tropo_top_m) / 8000))
+ / ( 287.05 * virtual_temp);
+
+ density_tropo_avg_kgm3 = ((density_slugft3 * 515.379) + density_half + density_tropo) / 3;
+}
+
+void
+FGEnvironment::_recalc_relative_humidity ()
+{
+/*
+ double vaporpressure = 6.11 * pow(10.0, ((7.5 * dewpoint_degc) / ( 237.7 + dewpoint_degc)));
+ double sat_vaporpressure = 6.11 * pow(10.0, ((7.5 * temperature_degc)
+ / ( 237.7 + temperature_degc)) );
+ relative_humidity = 100 * vaporpressure / sat_vaporpressure ;
+
+ with a little algebra, this gets the same result and spares two multiplications and one pow()
+*/
+ double a = (7.5 * dewpoint_degc) / ( 237.7 + dewpoint_degc);
+ double b = (7.5 * temperature_degc) / ( 237.7 + temperature_degc);
+ relative_humidity = 100 * pow(10.0,a-b);
+}
+
+bool
+FGEnvironment::set_live_update( bool _live_update )
+{
+ bool b = live_update;
+ live_update = _live_update;
+ return b;
+}
+
+
+////////////////////////////////////////////////////////////////////////
+// Functions.
+////////////////////////////////////////////////////////////////////////
+
+static inline double
+do_interp (double a, double b, double fraction)
+{
+ double retval = (a + ((b - a) * fraction));
+ return retval;
+}
+
+static inline double
+do_interp_deg (double a, double b, double fraction)
+{
+ a = fmod(a, 360);
+ b = fmod(b, 360);
+ if (fabs(b-a) > 180) {
+ if (a < b)
+ a += 360;
+ else
+ b += 360;
+ }
+ return fmod(do_interp(a, b, fraction), 360);
+}
+
+void
+interpolate (const FGEnvironment * env1, const FGEnvironment * env2,
+ double fraction, FGEnvironment * result)
+{
+ // don't calculate each internal property every time we set a single value
+ // we trigger that at the end of the interpolation process
+ bool live_update = result->set_live_update( false );
+
+ result->set_visibility_m
+ (do_interp(env1->get_visibility_m(),
+ env2->get_visibility_m(),
+ fraction));
+
+ result->set_temperature_sea_level_degc
+ (do_interp(env1->get_temperature_sea_level_degc(),
+ env2->get_temperature_sea_level_degc(),
+ fraction));
+
+ result->set_dewpoint_sea_level_degc
+ (do_interp(env1->get_dewpoint_sea_level_degc(),
+ env2->get_dewpoint_sea_level_degc(),
+ fraction));
+
+ result->set_pressure_sea_level_inhg
+ (do_interp(env1->get_pressure_sea_level_inhg(),
+ env2->get_pressure_sea_level_inhg(),
+ fraction));
+
+ result->set_wind_from_heading_deg
+ (do_interp_deg(env1->get_wind_from_heading_deg(),
+ env2->get_wind_from_heading_deg(),
+ fraction));
+
+ result->set_wind_speed_kt
+ (do_interp(env1->get_wind_speed_kt(),
+ env2->get_wind_speed_kt(),
+ fraction));
+
+ result->set_elevation_ft
+ (do_interp(env1->get_elevation_ft(),
+ env2->get_elevation_ft(),
+ fraction));
+
+ result->set_turbulence_magnitude_norm
+ (do_interp(env1->get_turbulence_magnitude_norm(),
+ env2->get_turbulence_magnitude_norm(),
+ fraction));
+
+ result->set_turbulence_rate_hz
+ (do_interp(env1->get_turbulence_rate_hz(),
+ env2->get_turbulence_rate_hz(),
+ fraction));
+
+ // calculate derived properties here to avoid duplicate expensive computations
+ result->_recalc_ne();
+ result->_recalc_alt_pt();
+ result->_recalc_alt_dewpoint();
+ result->_recalc_density();
+ result->_recalc_relative_humidity();
+
+ result->set_live_update(live_update);
+}
+
+// end of environment.cxx