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