src/Environment/environment.cxx

   1 // environment.cxx -- routines to model the natural environment
   2 //
   3 // Written by David Megginson, started February 2002.
   4 //
   5 // Copyright (C) 2002  David Megginson - david@megginson.com
   6 //
   7 // This program is free software; you can redistribute it and/or
   8 // modify it under the terms of the GNU General Public License as
   9 // published by the Free Software Foundation; either version 2 of the
  10 // License, or (at your option) any later version.
  11 //
  12 // This program is distributed in the hope that it will be useful, but
  13 // WITHOUT ANY WARRANTY; without even the implied warranty of
  14 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  15 // General Public License for more details.
  16 //
  17 // You should have received a copy of the GNU General Public License
  18 // along with this program; if not, write to the Free Software
  19 // Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
  20 //
  21 // $Id$
  22
  23
  24 #ifdef HAVE_CONFIG_H
  25 #  include <config.h>
  26 #endif
  27
  28 #include <math.h>
  29
  30 #include <boost/tuple/tuple.hpp>
  31
  32 #include <plib/sg.h>
  33
  34 #include <simgear/constants.h>
  35 #include <simgear/debug/logstream.hxx>
  36 #include <simgear/math/interpolater.hxx>
  37 #include <simgear/props/props.hxx>
  38 #include <simgear/environment/visual_enviro.hxx>
  39
  40 #include <Main/fg_props.hxx>
  41 #include <signal.h>
  42
  43 #include "environment.hxx"
  44 #include "atmosphere.hxx"
  45
  46 \f
  47 ////////////////////////////////////////////////////////////////////////
  48 // Atmosphere model.
  49 ////////////////////////////////////////////////////////////////////////
  50
  51 #ifdef USING_TABLES
  52
  53 // Calculated based on the ISA standard day, as found at e.g.
  54 // http://www.av8n.com/physics/altimetry.htm
  55
  56 // Each line of data has 3 elements:
  57 //   Elevation (ft),
  58 //   temperature factor (dimensionless ratio of absolute temp),
  59 //   pressure factor (dimensionless ratio)
  60 static double atmosphere_data[][3] = {
  61  {  -3000.00,   1.021,  1.1133 },
  62  {      0.00,   1.000,  1.0000 },
  63  {   2952.76,   0.980,  0.8978 },
  64  {   5905.51,   0.959,  0.8042 },
  65  {   8858.27,   0.939,  0.7187 },
  66  {  11811.02,   0.919,  0.6407 },
  67  {  14763.78,   0.898,  0.5697 },
  68  {  17716.54,   0.878,  0.5052 },
  69  {  20669.29,   0.858,  0.4468 },
  70  {  23622.05,   0.838,  0.3940 },
  71  {  26574.80,   0.817,  0.3463 },
  72  {  29527.56,   0.797,  0.3034 },
  73  {  32480.31,   0.777,  0.2649 },
  74  {  35433.07,   0.756,  0.2305 },
  75  {  38385.83,   0.752,  0.2000 },
  76  {  41338.58,   0.752,  0.1736 },
  77  {  44291.34,   0.752,  0.1506 },
  78  {  47244.09,   0.752,  0.1307 },
  79  {  50196.85,   0.752,  0.1134 },
  80  {  53149.61,   0.752,  0.0984 },
  81  {  56102.36,   0.752,  0.0854 },
  82  {  59055.12,   0.752,  0.0741 },
  83  {  62007.87,   0.752,  0.0643 },
  84  {  65000.00,   0.752,  0.0557 },
  85  {  68000.00,   0.754,  0.0482 },
  86  {  71000.00,   0.758,  0.0418 },
  87  {  74000.00,   0.761,  0.0362 },
  88  {  77000.00,   0.764,  0.0314 },
  89  {  80000.00,   0.767,  0.0273 },
  90  {  83000.00,   0.770,  0.0237 },
  91  {  86000.00,   0.773,  0.0206 },
  92  {  89000.00,   0.777,  0.0179 },
  93  {  92000.00,   0.780,  0.0156 },
  94  {  95000.00,   0.783,  0.0135 },
  95  {  98000.00,   0.786,  0.0118 },
  96  { 101000.00,   0.789,  0.0103 },
  97  { -1, -1, -1 }
  98 };
  99
 100 static SGInterpTable * _temperature_degc_table = 0;
 101 static SGInterpTable * _pressure_inhg_table = 0;
 102
 103 static void
 104 _setup_tables ()
 105 {
 106   if (_temperature_degc_table != 0)
 107       return;
 108
 109   _temperature_degc_table = new SGInterpTable;
 110   _pressure_inhg_table = new SGInterpTable;
 111
 112   for (int i = 0; atmosphere_data[i][0] != -1; i++) {
 113     _temperature_degc_table->addEntry(atmosphere_data[i][0],
 114                                       atmosphere_data[i][1]);
 115     _pressure_inhg_table->addEntry(atmosphere_data[i][0],
 116                                    atmosphere_data[i][2]);
 117   }
 118 }
 119 #endif
 120
 121 \f
 122 ////////////////////////////////////////////////////////////////////////
 123 // Implementation of FGEnvironment.
 124 ////////////////////////////////////////////////////////////////////////
 125
 126 void FGEnvironment::_init()
 127 {
 128     elevation_ft = 0;
 129     visibility_m = 32000;
 130     temperature_sea_level_degc = 15;
 131     temperature_degc = 15;
 132     dewpoint_sea_level_degc = 5; // guess
 133     dewpoint_degc = 5;
 134     pressure_sea_level_inhg = 29.92;
 135     pressure_inhg = 29.92;
 136     turbulence_magnitude_norm = 0;
 137     turbulence_rate_hz = 1;
 138     wind_from_heading_deg = 0;
 139     wind_speed_kt = 0;
 140     wind_from_north_fps = 0;
 141     wind_from_east_fps = 0;
 142     wind_from_down_fps = 0;
 143     thermal_lift_fps = 0;
 144     ridge_lift_fps= 0;
 145     altitude_half_to_sun_m = 1000;
 146     altitude_tropo_top_m = 10000;
 147 #ifdef USING_TABLES
 148     _setup_tables();
 149 #endif
 150     _recalc_density();
 151     _recalc_relative_humidity();
 152     live_update = true;
 153 }
 154
 155 FGEnvironment::FGEnvironment()
 156 {
 157     _init();
 158 }
 159
 160 FGEnvironment::FGEnvironment (const FGEnvironment &env)
 161 {
 162     _init();
 163     copy(env);
 164 }
 165
 166 FGEnvironment::~FGEnvironment()
 167 {
 168 }
 169
 170 void
 171 FGEnvironment::copy (const FGEnvironment &env)
 172 {
 173     elevation_ft = env.elevation_ft;
 174     visibility_m = env.visibility_m;
 175     temperature_sea_level_degc = env.temperature_sea_level_degc;
 176     temperature_degc = env.temperature_degc;
 177     dewpoint_sea_level_degc = env.dewpoint_sea_level_degc;
 178     dewpoint_degc = env.dewpoint_degc;
 179     pressure_sea_level_inhg = env.pressure_sea_level_inhg;
 180     wind_from_heading_deg = env.wind_from_heading_deg;
 181     wind_speed_kt = env.wind_speed_kt;
 182     wind_from_north_fps = env.wind_from_north_fps;
 183     wind_from_east_fps = env.wind_from_east_fps;
 184     wind_from_down_fps = env.wind_from_down_fps;
 185     thermal_lift_fps = env.thermal_lift_fps;
 186     ridge_lift_fps= env.ridge_lift_fps;
 187     turbulence_magnitude_norm = env.turbulence_magnitude_norm;
 188     turbulence_rate_hz = env.turbulence_rate_hz;
 189 }
 190
 191 static inline bool
 192 maybe_copy_value (FGEnvironment * env, const SGPropertyNode * node,
 193                   const char * name, void (FGEnvironment::*setter)(double))
 194 {
 195     const SGPropertyNode * child = node->getNode(name);
 196                                 // fragile: depends on not being typed
 197                                 // as a number
 198     if (child != 0 && child->hasValue() &&
 199         child->getStringValue()[0] != '\0') {
 200         (env->*setter)(child->getDoubleValue());
 201         return true;
 202     } else {
 203         return false;
 204     }
 205 }
 206
 207 void
 208 FGEnvironment::read (const SGPropertyNode * node)
 209 {
 210     bool live_update = set_live_update( false );
 211     maybe_copy_value(this, node, "visibility-m",
 212                      &FGEnvironment::set_visibility_m);
 213
 214     if (!maybe_copy_value(this, node, "temperature-sea-level-degc",
 215                           &FGEnvironment::set_temperature_sea_level_degc))
 216         maybe_copy_value(this, node, "temperature-degc",
 217                          &FGEnvironment::set_temperature_degc);
 218
 219     if (!maybe_copy_value(this, node, "dewpoint-sea-level-degc",
 220                           &FGEnvironment::set_dewpoint_sea_level_degc))
 221         maybe_copy_value(this, node, "dewpoint-degc",
 222                          &FGEnvironment::set_dewpoint_degc);
 223
 224     if (!maybe_copy_value(this, node, "pressure-sea-level-inhg",
 225                           &FGEnvironment::set_pressure_sea_level_inhg))
 226         maybe_copy_value(this, node, "pressure-inhg",
 227                          &FGEnvironment::set_pressure_inhg);
 228
 229     maybe_copy_value(this, node, "wind-from-heading-deg",
 230                      &FGEnvironment::set_wind_from_heading_deg);
 231
 232     maybe_copy_value(this, node, "wind-speed-kt",
 233                      &FGEnvironment::set_wind_speed_kt);
 234
 235     maybe_copy_value(this, node, "elevation-ft",
 236                      &FGEnvironment::set_elevation_ft);
 237
 238     maybe_copy_value(this, node, "turbulence/magnitude-norm",
 239                      &FGEnvironment::set_turbulence_magnitude_norm);
 240
 241     maybe_copy_value(this, node, "turbulence/rate-hz",
 242                      &FGEnvironment::set_turbulence_rate_hz);
 243     // calculate derived properties here to avoid duplicate expensive computations
 244     _recalc_ne();
 245     _recalc_alt_pt();
 246     _recalc_alt_dewpoint();
 247     _recalc_density();
 248     _recalc_relative_humidity();
 249
 250     set_live_update(live_update);
 251 }
 252
 253
 254 double
 255 FGEnvironment::get_visibility_m () const
 256 {
 257   return visibility_m;
 258 }
 259
 260 double
 261 FGEnvironment::get_temperature_sea_level_degc () const
 262 {
 263   return temperature_sea_level_degc;
 264 }
 265
 266 double
 267 FGEnvironment::get_temperature_degc () const
 268 {
 269   return temperature_degc;
 270 }
 271
 272 double
 273 FGEnvironment::get_temperature_degf () const
 274 {
 275   return (temperature_degc * 9.0 / 5.0) + 32.0;
 276 }
 277
 278 double
 279 FGEnvironment::get_dewpoint_sea_level_degc () const
 280 {
 281   return dewpoint_sea_level_degc;
 282 }
 283
 284 double
 285 FGEnvironment::get_dewpoint_degc () const
 286 {
 287   return dewpoint_degc;
 288 }
 289
 290 double
 291 FGEnvironment::get_pressure_sea_level_inhg () const
 292 {
 293   return pressure_sea_level_inhg;
 294 }
 295
 296 double
 297 FGEnvironment::get_pressure_inhg () const
 298 {
 299   return pressure_inhg;
 300 }
 301
 302 double
 303 FGEnvironment::get_density_slugft3 () const
 304 {
 305   return density_slugft3;
 306 }
 307
 308 double
 309 FGEnvironment::get_relative_humidity () const
 310 {
 311   return relative_humidity;
 312 }
 313
 314 double
 315 FGEnvironment::get_density_tropo_avg_kgm3 () const
 316 {
 317   return density_tropo_avg_kgm3;
 318 }
 319
 320 double
 321 FGEnvironment::get_altitude_half_to_sun_m () const
 322 {
 323   return altitude_half_to_sun_m;
 324 }
 325
 326 double
 327 FGEnvironment::get_altitude_tropo_top_m () const
 328 {
 329   return altitude_tropo_top_m;
 330 }
 331
 332 double
 333 FGEnvironment::get_wind_from_heading_deg () const
 334 {
 335   return wind_from_heading_deg;
 336 }
 337
 338 double
 339 FGEnvironment::get_wind_speed_kt () const
 340 {
 341   return wind_speed_kt;
 342 }
 343
 344 double
 345 FGEnvironment::get_wind_from_north_fps () const
 346 {
 347   return wind_from_north_fps;
 348 }
 349
 350 double
 351 FGEnvironment::get_wind_from_east_fps () const
 352 {
 353   return wind_from_east_fps;
 354 }
 355
 356 double
 357 FGEnvironment::get_wind_from_down_fps () const
 358 {
 359   return wind_from_down_fps;
 360 }
 361
 362 double
 363 FGEnvironment::get_thermal_lift_fps () const
 364 {
 365   return thermal_lift_fps;
 366 }
 367
 368 double
 369 FGEnvironment::get_ridge_lift_fps () const
 370 {
 371   return ridge_lift_fps;
 372 }
 373
 374 double
 375 FGEnvironment::get_turbulence_magnitude_norm () const
 376 {
 377   if( sgEnviro.get_turbulence_enable_state() )
 378     if (fgGetBool("/environment/params/real-world-weather-fetch") == true)
 379       return sgEnviro.get_cloud_turbulence();
 380   return turbulence_magnitude_norm;
 381 }
 382
 383 double
 384 FGEnvironment::get_turbulence_rate_hz () const
 385 {
 386   return turbulence_rate_hz;
 387 }
 388
 389 double
 390 FGEnvironment::get_elevation_ft () const
 391 {
 392   return elevation_ft;
 393 }
 394
 395 void
 396 FGEnvironment::set_visibility_m (double v)
 397 {
 398   visibility_m = v;
 399 }
 400
 401 void
 402 FGEnvironment::set_temperature_sea_level_degc (double t)
 403 {
 404   temperature_sea_level_degc = t;
 405   if (dewpoint_sea_level_degc > t)
 406       dewpoint_sea_level_degc = t;
 407   if( live_update ) {
 408     _recalc_alt_pt();
 409     _recalc_density();
 410   }
 411 }
 412
 413 void
 414 FGEnvironment::set_temperature_degc (double t)
 415 {
 416   temperature_degc = t;
 417   if( live_update ) {
 418     _recalc_sl_temperature();
 419     _recalc_sl_pressure();
 420     _recalc_alt_pt();
 421     _recalc_density();
 422     _recalc_relative_humidity();
 423   }
 424 }
 425
 426 void
 427 FGEnvironment::set_dewpoint_sea_level_degc (double t)
 428 {
 429   dewpoint_sea_level_degc = t;
 430   if (temperature_sea_level_degc < t)
 431       temperature_sea_level_degc = t;
 432   if( live_update ) {
 433     _recalc_alt_dewpoint();
 434     _recalc_density();
 435   }
 436 }
 437
 438 void
 439 FGEnvironment::set_dewpoint_degc (double t)
 440 {
 441   dewpoint_degc = t;
 442   if( live_update ) {
 443     _recalc_sl_dewpoint();
 444     _recalc_density();
 445     _recalc_relative_humidity();
 446   }
 447 }
 448
 449 void
 450 FGEnvironment::set_pressure_sea_level_inhg (double p)
 451 {
 452   pressure_sea_level_inhg = p;
 453   if( live_update ) {
 454     _recalc_alt_pt();
 455     _recalc_density();
 456   }
 457 }
 458
 459 void
 460 FGEnvironment::set_pressure_inhg (double p)
 461 {
 462   pressure_inhg = p;
 463   if( live_update ) {
 464     _recalc_sl_pressure();
 465     _recalc_density();
 466   }
 467 }
 468
 469 void
 470 FGEnvironment::set_wind_from_heading_deg (double h)
 471 {
 472   wind_from_heading_deg = h;
 473   if( live_update ) {
 474     _recalc_ne();
 475   }
 476 }
 477
 478 void
 479 FGEnvironment::set_wind_speed_kt (double s)
 480 {
 481   wind_speed_kt = s;
 482   if( live_update ) {
 483     _recalc_ne();
 484   }
 485 }
 486
 487 void
 488 FGEnvironment::set_wind_from_north_fps (double n)
 489 {
 490   wind_from_north_fps = n;
 491   if( live_update ) {
 492     _recalc_hdgspd();
 493   }
 494 }
 495
 496 void
 497 FGEnvironment::set_wind_from_east_fps (double e)
 498 {
 499   wind_from_east_fps = e;
 500   if( live_update ) {
 501     _recalc_hdgspd();
 502   }
 503 }
 504
 505 void
 506 FGEnvironment::set_wind_from_down_fps (double d)
 507 {
 508   wind_from_down_fps = d;
 509   if( live_update ) {
 510     _recalc_hdgspd();
 511   }
 512 }
 513
 514 void
 515 FGEnvironment::set_thermal_lift_fps (double th)
 516 {
 517   thermal_lift_fps = th;
 518   if( live_update ) {
 519     _recalc_updraft();
 520   }
 521 }
 522
 523 void
 524 FGEnvironment::set_ridge_lift_fps (double ri)
 525 {
 526   ridge_lift_fps = ri;
 527   if( live_update ) {
 528     _recalc_updraft();
 529 }
 530 }
 531
 532 void
 533 FGEnvironment::set_turbulence_magnitude_norm (double t)
 534 {
 535   turbulence_magnitude_norm = t;
 536 }
 537
 538 void
 539 FGEnvironment::set_turbulence_rate_hz (double r)
 540 {
 541   turbulence_rate_hz = r;
 542 }
 543
 544 void
 545 FGEnvironment::set_elevation_ft (double e)
 546 {
 547   elevation_ft = e;
 548   if( live_update ) {
 549     _recalc_alt_dewpoint();
 550     _recalc_alt_pt();
 551     _recalc_density();
 552     _recalc_relative_humidity();
 553   }
 554 }
 555
 556 void
 557 FGEnvironment::set_altitude_half_to_sun_m (double alt)
 558 {
 559   altitude_half_to_sun_m = alt;
 560   if( live_update ) {
 561     _recalc_density_tropo_avg_kgm3();
 562   }
 563 }
 564
 565 void
 566 FGEnvironment::set_altitude_tropo_top_m (double alt)
 567 {
 568   altitude_tropo_top_m = alt;
 569   if( live_update ) {
 570     _recalc_density_tropo_avg_kgm3();
 571   }
 572 }
 573
 574
 575 void
 576 FGEnvironment::_recalc_hdgspd ()
 577 {
 578   double angle_rad;
 579
 580   if (wind_from_east_fps == 0) {
 581     angle_rad = (wind_from_north_fps >= 0 ? SGD_PI_2 : -SGD_PI_2);
 582   } else {
 583     angle_rad = atan(wind_from_north_fps/wind_from_east_fps);
 584   }
 585   wind_from_heading_deg = angle_rad * SGD_RADIANS_TO_DEGREES;
 586   if (wind_from_east_fps >= 0)
 587     wind_from_heading_deg = 90 - wind_from_heading_deg;
 588   else
 589     wind_from_heading_deg = 270 - wind_from_heading_deg;
 590
 591 #if 0
 592   // FIXME: Windspeed can become negative with these formulas.
 593   //        which can cause problems for animations that rely
 594   //        on the windspeed property.
 595   if (angle_rad == 0)
 596     wind_speed_kt = fabs(wind_from_east_fps
 597                          * SG_METER_TO_NM * SG_FEET_TO_METER * 3600);
 598   else
 599     wind_speed_kt = (wind_from_north_fps / sin(angle_rad))
 600       * SG_METER_TO_NM * SG_FEET_TO_METER * 3600;
 601 #else
 602   wind_speed_kt = sqrt(wind_from_north_fps * wind_from_north_fps +
 603                        wind_from_east_fps * wind_from_east_fps)
 604                   * SG_METER_TO_NM * SG_FEET_TO_METER * 3600;
 605 #endif
 606 }
 607
 608 void
 609 FGEnvironment::_recalc_ne ()
 610 {
 611   double speed_fps =
 612     wind_speed_kt * SG_NM_TO_METER * SG_METER_TO_FEET * (1.0/3600);
 613
 614   wind_from_north_fps = speed_fps *
 615     cos(wind_from_heading_deg * SGD_DEGREES_TO_RADIANS);
 616   wind_from_east_fps = speed_fps *
 617     sin(wind_from_heading_deg * SGD_DEGREES_TO_RADIANS);
 618 }
 619
 620 void
 621 FGEnvironment::_recalc_updraft ()
 622 {
 623   wind_from_down_fps = thermal_lift_fps + ridge_lift_fps ;
 624 }
 625
 626 // Intended to help with the interpretation of METAR data,
 627 // not for random in-flight outside-air temperatures.
 628 void
 629 FGEnvironment::_recalc_sl_temperature ()
 630 {
 631
 632 #if 0
 633   {
 634     SG_LOG(SG_GENERAL, SG_DEBUG, "recalc_sl_temperature: using "
 635       << temperature_degc << " @ " << elevation_ft << " :: " << this);
 636   }
 637 #endif
 638
 639   if (elevation_ft >= ISA_def[1].height) {
 640     SG_LOG(SG_GENERAL, SG_ALERT, "recalc_sl_temperature: "
 641         << "valid only in troposphere, not " << elevation_ft);
 642     return;
 643   }
 644
 645 // Clamp: temperature of the stratosphere, in degrees C:
 646   double t_strato = ISA_def[1].temp - atmodel::freezing;
 647   if (temperature_degc < t_strato) temperature_sea_level_degc = t_strato;
 648   else temperature_sea_level_degc =
 649       temperature_degc + elevation_ft * atmodel::foot * ISA_def[0].lapse;
 650
 651 // Alternative implemenation:
 652 //  else temperature_sea_level_inhg = T_layer(0., elevation_ft * foot,
 653 //      pressure_inhg * inHg, temperature_degc + freezing, ISA_def[0].lapse) - freezing;
 654 }
 655
 656 void
 657 FGEnvironment::_recalc_sl_dewpoint ()
 658 {
 659                                 // 0.2degC/1000ft
 660                                 // FIXME: this will work only for low
 661                                 // elevations
 662   dewpoint_sea_level_degc = dewpoint_degc + (elevation_ft * .0002);
 663   if (dewpoint_sea_level_degc > temperature_sea_level_degc)
 664     dewpoint_sea_level_degc = temperature_sea_level_degc;
 665 }
 666
 667 void
 668 FGEnvironment::_recalc_alt_dewpoint ()
 669 {
 670                                 // 0.2degC/1000ft
 671                                 // FIXME: this will work only for low
 672                                 // elevations
 673   dewpoint_degc = dewpoint_sea_level_degc + (elevation_ft * .0002);
 674   if (dewpoint_degc > temperature_degc)
 675     dewpoint_degc = temperature_degc;
 676 }
 677
 678 void
 679 FGEnvironment::_recalc_sl_pressure ()
 680 {
 681   using namespace atmodel;
 682 #if 0
 683   {
 684     SG_LOG(SG_GENERAL, SG_ALERT, "recalc_sl_pressure: using "
 685       << pressure_inhg << " and "
 686       << temperature_degc << " @ " << elevation_ft << " :: " << this);
 687   }
 688 #endif
 689   pressure_sea_level_inhg = P_layer(0., elevation_ft * foot,
 690       pressure_inhg * inHg, temperature_degc + freezing, ISA_def[0].lapse) / inHg;
 691 }
 692
 693 // This gets called at frame rate, to account for the aircraft's
 694 // changing altitude.
 695 // Called by set_elevation_ft() which is called by FGEnvironmentMgr::update
 696
 697 void
 698 FGEnvironment::_recalc_alt_pt ()
 699 {
 700   using namespace atmodel;
 701 #if 0
 702   {
 703     static int count(0);
 704     if (++count % 1000 == 0) {
 705       SG_LOG(SG_GENERAL, SG_ALERT,
 706            "recalc_alt_pt for: " << elevation_ft
 707         << "  using "  << pressure_sea_level_inhg
 708         << "  and "  << temperature_sea_level_degc
 709         << " :: " << this
 710         << "  # " << count);
 711         ///////////////////////////////////raise(SIGUSR1);
 712     }
 713   }
 714 #endif
 715   double press, temp;
 716   boost::tie(press, temp) = PT_vs_hpt(elevation_ft * foot,
 717         pressure_sea_level_inhg * inHg, temperature_sea_level_degc + freezing);
 718   temperature_degc = temp - freezing;
 719   pressure_inhg = press / inHg;
 720 }
 721
 722 void
 723 FGEnvironment::_recalc_density ()
 724 {
 725   double pressure_psf = pressure_inhg * 70.7487;
 726
 727   // adjust for humidity
 728   // calculations taken from USA Today (oops!) at
 729   // http://www.usatoday.com/weather/basics/density-calculations.htm
 730   double temperature_degk = temperature_degc + 273.15;
 731   double pressure_mb = pressure_inhg * 33.86;
 732   double vapor_pressure_mb =
 733     6.11 * pow(10.0, 7.5 * dewpoint_degc / (237.7 + dewpoint_degc));
 734   double virtual_temperature_degk = temperature_degk / (1 - (vapor_pressure_mb / pressure_mb) * (1.0 - 0.622));
 735   double virtual_temperature_degr = virtual_temperature_degk * 1.8;
 736
 737   density_slugft3 = pressure_psf / (virtual_temperature_degr * 1718);
 738   _recalc_density_tropo_avg_kgm3();
 739 }
 740
 741 // This is used to calculate the average density on the path
 742 // of sunlight to the observer for calculating sun-color
 743 void
 744 FGEnvironment::_recalc_density_tropo_avg_kgm3 ()
 745 {
 746   double pressure_mb = pressure_inhg * 33.86;
 747   double vaporpressure = 6.11 * pow(10.0, ((7.5 * dewpoint_degc) / (237.7 + dewpoint_degc)));
 748
 749   double virtual_temp = (temperature_degc + 273.15) / (1 - 0.379 * (vaporpressure/pressure_mb));
 750
 751   double density_half = (100 * pressure_mb * exp(-altitude_half_to_sun_m / 8000))
 752       / (287.05 * virtual_temp);
 753   double density_tropo = (100 * pressure_mb * exp((-1 * altitude_tropo_top_m) / 8000))
 754       / ( 287.05 * virtual_temp);
 755
 756   density_tropo_avg_kgm3 = ((density_slugft3 * 515.379) + density_half + density_tropo) / 3;
 757 }
 758
 759 void
 760 FGEnvironment::_recalc_relative_humidity ()
 761 {
 762 /*
 763   double vaporpressure = 6.11 * pow(10.0, ((7.5 * dewpoint_degc) / ( 237.7 + dewpoint_degc)));
 764   double sat_vaporpressure = 6.11 * pow(10.0, ((7.5 * temperature_degc)
 765       / ( 237.7 + temperature_degc)) );
 766   relative_humidity = 100 * vaporpressure / sat_vaporpressure ;
 767
 768   with a little algebra, this gets the same result and spares two multiplications and one pow()
 769 */
 770   double a = (7.5 * dewpoint_degc)    / ( 237.7 + dewpoint_degc);
 771   double b = (7.5 * temperature_degc) / ( 237.7 + temperature_degc);
 772   relative_humidity = 100 * pow(10.0,a-b);
 773 }
 774
 775 bool
 776 FGEnvironment::set_live_update( bool _live_update )
 777 {
 778   bool b = live_update;
 779   live_update = _live_update;
 780   return b;
 781 }
 782
 783
 784 ////////////////////////////////////////////////////////////////////////
 785 // Functions.
 786 ////////////////////////////////////////////////////////////////////////
 787
 788 static inline double
 789 do_interp (double a, double b, double fraction)
 790 {
 791     double retval = (a + ((b - a) * fraction));
 792     return retval;
 793 }
 794
 795 static inline double
 796 do_interp_deg (double a, double b, double fraction)
 797 {
 798     a = fmod(a, 360);
 799     b = fmod(b, 360);
 800     if (fabs(b-a) > 180) {
 801         if (a < b)
 802             a += 360;
 803         else
 804             b += 360;
 805     }
 806     return fmod(do_interp(a, b, fraction), 360);
 807 }
 808
 809 void
 810 interpolate (const FGEnvironment * env1, const FGEnvironment * env2,
 811              double fraction, FGEnvironment * result)
 812 {
 813     // don't calculate each internal property every time we set a single value
 814     // we trigger that at the end of the interpolation process
 815     bool live_update = result->set_live_update( false );
 816
 817     result->set_visibility_m
 818         (do_interp(env1->get_visibility_m(),
 819                    env2->get_visibility_m(),
 820                    fraction));
 821
 822     result->set_temperature_sea_level_degc
 823         (do_interp(env1->get_temperature_sea_level_degc(),
 824                    env2->get_temperature_sea_level_degc(),
 825                    fraction));
 826
 827     result->set_dewpoint_degc
 828         (do_interp(env1->get_dewpoint_sea_level_degc(),
 829                    env2->get_dewpoint_sea_level_degc(),
 830                    fraction));
 831
 832     result->set_pressure_sea_level_inhg
 833         (do_interp(env1->get_pressure_sea_level_inhg(),
 834                    env2->get_pressure_sea_level_inhg(),
 835                    fraction));
 836
 837     result->set_wind_from_heading_deg
 838         (do_interp_deg(env1->get_wind_from_heading_deg(),
 839                        env2->get_wind_from_heading_deg(),
 840                        fraction));
 841
 842     result->set_wind_speed_kt
 843         (do_interp(env1->get_wind_speed_kt(),
 844                    env2->get_wind_speed_kt(),
 845                    fraction));
 846
 847     result->set_elevation_ft
 848         (do_interp(env1->get_elevation_ft(),
 849                    env2->get_elevation_ft(),
 850                    fraction));
 851
 852     result->set_turbulence_magnitude_norm
 853         (do_interp(env1->get_turbulence_magnitude_norm(),
 854                    env2->get_turbulence_magnitude_norm(),
 855                    fraction));
 856
 857     result->set_turbulence_rate_hz
 858         (do_interp(env1->get_turbulence_rate_hz(),
 859                    env2->get_turbulence_rate_hz(),
 860                    fraction));
 861
 862     // calculate derived properties here to avoid duplicate expensive computations
 863     result->_recalc_ne();
 864     result->_recalc_alt_pt();
 865     result->_recalc_alt_dewpoint();
 866     result->_recalc_density();
 867     result->_recalc_relative_humidity();
 868
 869     result->set_live_update(live_update);
 870 }
 871
 872 // end of environment.cxx