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 <simgear/constants.h>
  31 #include <simgear/debug/logstream.hxx>
  32 #include <simgear/math/interpolater.hxx>
  33 #include <simgear/props/props.hxx>
  34 #include <simgear/environment/visual_enviro.hxx>
  35
  36 #include <Main/fg_props.hxx>
  37
  38 #include "environment.hxx"
  39
  40
  41 \f
  42 ////////////////////////////////////////////////////////////////////////
  43 // Atmosphere model.
  44 ////////////////////////////////////////////////////////////////////////
  45
  46 // Calculated based on the ISA standard day, as found at e.g.
  47 // http://www.av8n.com/physics/altimetry.htm
  48
  49 // Each line of data has 3 elements:
  50 //   Elevation (ft),
  51 //   temperature factor (dimensionless ratio of absolute temp),
  52 //   pressure factor (dimensionless ratio)
  53 static double atmosphere_data[][3] = {
  54  {  -3000.00,   1.021,  1.1133 },
  55  {      0.00,   1.000,  1.0000 },
  56  {   2952.76,   0.980,  0.8978 },
  57  {   5905.51,   0.959,  0.8042 },
  58  {   8858.27,   0.939,  0.7187 },
  59  {  11811.02,   0.919,  0.6407 },
  60  {  14763.78,   0.898,  0.5697 },
  61  {  17716.54,   0.878,  0.5052 },
  62  {  20669.29,   0.858,  0.4468 },
  63  {  23622.05,   0.838,  0.3940 },
  64  {  26574.80,   0.817,  0.3463 },
  65  {  29527.56,   0.797,  0.3034 },
  66  {  32480.31,   0.777,  0.2649 },
  67  {  35433.07,   0.756,  0.2305 },
  68  {  38385.83,   0.752,  0.2000 },
  69  {  41338.58,   0.752,  0.1736 },
  70  {  44291.34,   0.752,  0.1506 },
  71  {  47244.09,   0.752,  0.1307 },
  72  {  50196.85,   0.752,  0.1134 },
  73  {  53149.61,   0.752,  0.0984 },
  74  {  56102.36,   0.752,  0.0854 },
  75  {  59055.12,   0.752,  0.0741 },
  76  {  62007.87,   0.752,  0.0643 },
  77  {  65000.00,   0.752,  0.0557 },
  78  {  68000.00,   0.754,  0.0482 },
  79  {  71000.00,   0.758,  0.0418 },
  80  {  74000.00,   0.761,  0.0362 },
  81  {  77000.00,   0.764,  0.0314 },
  82  {  80000.00,   0.767,  0.0273 },
  83  {  83000.00,   0.770,  0.0237 },
  84  {  86000.00,   0.773,  0.0206 },
  85  {  89000.00,   0.777,  0.0179 },
  86  {  92000.00,   0.780,  0.0156 },
  87  {  95000.00,   0.783,  0.0135 },
  88  {  98000.00,   0.786,  0.0118 },
  89  { 101000.00,   0.789,  0.0103 },
  90  { -1, -1, -1 }
  91 };
  92
  93 static SGInterpTable * _temperature_degc_table = 0;
  94 static SGInterpTable * _pressure_inhg_table = 0;
  95
  96 static void
  97 _setup_tables ()
  98 {
  99   if (_temperature_degc_table != 0)
 100       return;
 101
 102   _temperature_degc_table = new SGInterpTable;
 103   _pressure_inhg_table = new SGInterpTable;
 104
 105   for (int i = 0; atmosphere_data[i][0] != -1; i++) {
 106     _temperature_degc_table->addEntry(atmosphere_data[i][0],
 107                                       atmosphere_data[i][1]);
 108     _pressure_inhg_table->addEntry(atmosphere_data[i][0],
 109                                    atmosphere_data[i][2]);
 110   }
 111 }
 112
 113
 114 \f
 115 ////////////////////////////////////////////////////////////////////////
 116 // Implementation of FGEnvironment.
 117 ////////////////////////////////////////////////////////////////////////
 118
 119 void FGEnvironment::_init()
 120 {
 121     elevation_ft = 0;
 122     visibility_m = 32000;
 123     temperature_sea_level_degc = 15;
 124     temperature_degc = 15;
 125     dewpoint_sea_level_degc = 5; // guess
 126     dewpoint_degc = 5;
 127     pressure_sea_level_inhg = 29.92;
 128     pressure_inhg = 29.92;
 129     turbulence_magnitude_norm = 0;
 130     turbulence_rate_hz = 1;
 131     wind_from_heading_deg = 0;
 132     wind_speed_kt = 0;
 133     wind_from_north_fps = 0;
 134     wind_from_east_fps = 0;
 135     wind_from_down_fps = 0;
 136     thermal_lift_fps = 0;
 137     ridge_lift_fps= 0;
 138     altitude_half_to_sun_m = 1000;
 139     altitude_tropo_top_m = 10000;
 140     _setup_tables();
 141     _recalc_density();
 142     _recalc_relative_humidity();
 143     live_update = true;
 144 }
 145
 146 FGEnvironment::FGEnvironment()
 147 {
 148     _init();
 149 }
 150
 151 FGEnvironment::FGEnvironment (const FGEnvironment &env)
 152 {
 153     _init();
 154     copy(env);
 155 }
 156
 157 FGEnvironment::~FGEnvironment()
 158 {
 159 }
 160
 161 void
 162 FGEnvironment::copy (const FGEnvironment &env)
 163 {
 164     elevation_ft = env.elevation_ft;
 165     visibility_m = env.visibility_m;
 166     temperature_sea_level_degc = env.temperature_sea_level_degc;
 167     temperature_degc = env.temperature_degc;
 168     dewpoint_sea_level_degc = env.dewpoint_sea_level_degc;
 169     dewpoint_degc = env.dewpoint_degc;
 170     pressure_sea_level_inhg = env.pressure_sea_level_inhg;
 171     wind_from_heading_deg = env.wind_from_heading_deg;
 172     wind_speed_kt = env.wind_speed_kt;
 173     wind_from_north_fps = env.wind_from_north_fps;
 174     wind_from_east_fps = env.wind_from_east_fps;
 175     wind_from_down_fps = env.wind_from_down_fps;
 176     thermal_lift_fps = env.thermal_lift_fps;
 177     ridge_lift_fps= env.ridge_lift_fps;
 178     turbulence_magnitude_norm = env.turbulence_magnitude_norm;
 179     turbulence_rate_hz = env.turbulence_rate_hz;
 180 }
 181
 182 static inline bool
 183 maybe_copy_value (FGEnvironment * env, const SGPropertyNode * node,
 184                   const char * name, void (FGEnvironment::*setter)(double))
 185 {
 186     const SGPropertyNode * child = node->getNode(name);
 187                                 // fragile: depends on not being typed
 188                                 // as a number
 189     if (child != 0 && child->hasValue() &&
 190         child->getStringValue()[0] != '\0') {
 191         (env->*setter)(child->getDoubleValue());
 192         return true;
 193     } else {
 194         return false;
 195     }
 196 }
 197
 198 void
 199 FGEnvironment::read (const SGPropertyNode * node)
 200 {
 201     maybe_copy_value(this, node, "visibility-m",
 202                      &FGEnvironment::set_visibility_m);
 203
 204     if (!maybe_copy_value(this, node, "temperature-sea-level-degc",
 205                           &FGEnvironment::set_temperature_sea_level_degc))
 206         maybe_copy_value(this, node, "temperature-degc",
 207                          &FGEnvironment::set_temperature_degc);
 208
 209     if (!maybe_copy_value(this, node, "dewpoint-sea-level-degc",
 210                           &FGEnvironment::set_dewpoint_sea_level_degc))
 211         maybe_copy_value(this, node, "dewpoint-degc",
 212                          &FGEnvironment::set_dewpoint_degc);
 213
 214     if (!maybe_copy_value(this, node, "pressure-sea-level-inhg",
 215                           &FGEnvironment::set_pressure_sea_level_inhg))
 216         maybe_copy_value(this, node, "pressure-inhg",
 217                          &FGEnvironment::set_pressure_inhg);
 218
 219     maybe_copy_value(this, node, "wind-from-heading-deg",
 220                      &FGEnvironment::set_wind_from_heading_deg);
 221
 222     maybe_copy_value(this, node, "wind-speed-kt",
 223                      &FGEnvironment::set_wind_speed_kt);
 224
 225     maybe_copy_value(this, node, "elevation-ft",
 226                      &FGEnvironment::set_elevation_ft);
 227
 228     maybe_copy_value(this, node, "turbulence/magnitude-norm",
 229                      &FGEnvironment::set_turbulence_magnitude_norm);
 230
 231     maybe_copy_value(this, node, "turbulence/rate-hz",
 232                      &FGEnvironment::set_turbulence_rate_hz);
 233 }
 234
 235
 236 double
 237 FGEnvironment::get_visibility_m () const
 238 {
 239   return visibility_m;
 240 }
 241
 242 double
 243 FGEnvironment::get_temperature_sea_level_degc () const
 244 {
 245   return temperature_sea_level_degc;
 246 }
 247
 248 double
 249 FGEnvironment::get_temperature_degc () const
 250 {
 251   return temperature_degc;
 252 }
 253
 254 double
 255 FGEnvironment::get_temperature_degf () const
 256 {
 257   return (temperature_degc * 9.0 / 5.0) + 32.0;
 258 }
 259
 260 double
 261 FGEnvironment::get_dewpoint_sea_level_degc () const
 262 {
 263   return dewpoint_sea_level_degc;
 264 }
 265
 266 double
 267 FGEnvironment::get_dewpoint_degc () const
 268 {
 269   return dewpoint_degc;
 270 }
 271
 272 double
 273 FGEnvironment::get_pressure_sea_level_inhg () const
 274 {
 275   return pressure_sea_level_inhg;
 276 }
 277
 278 double
 279 FGEnvironment::get_pressure_inhg () const
 280 {
 281   return pressure_inhg;
 282 }
 283
 284 double
 285 FGEnvironment::get_density_slugft3 () const
 286 {
 287   return density_slugft3;
 288 }
 289
 290 double
 291 FGEnvironment::get_relative_humidity () const
 292 {
 293   return relative_humidity;
 294 }
 295
 296 double
 297 FGEnvironment::get_density_tropo_avg_kgm3 () const
 298 {
 299   return density_tropo_avg_kgm3;
 300 }
 301
 302 double
 303 FGEnvironment::get_altitude_half_to_sun_m () const
 304 {
 305   return altitude_half_to_sun_m;
 306 }
 307
 308 double
 309 FGEnvironment::get_altitude_tropo_top_m () const
 310 {
 311   return altitude_tropo_top_m;
 312 }
 313
 314 double
 315 FGEnvironment::get_wind_from_heading_deg () const
 316 {
 317   return wind_from_heading_deg;
 318 }
 319
 320 double
 321 FGEnvironment::get_wind_speed_kt () const
 322 {
 323   return wind_speed_kt;
 324 }
 325
 326 double
 327 FGEnvironment::get_wind_from_north_fps () const
 328 {
 329   return wind_from_north_fps;
 330 }
 331
 332 double
 333 FGEnvironment::get_wind_from_east_fps () const
 334 {
 335   return wind_from_east_fps;
 336 }
 337
 338 double
 339 FGEnvironment::get_wind_from_down_fps () const
 340 {
 341   return wind_from_down_fps;
 342 }
 343
 344 double
 345 FGEnvironment::get_thermal_lift_fps () const
 346 {
 347   return thermal_lift_fps;
 348 }
 349
 350 double
 351 FGEnvironment::get_ridge_lift_fps () const
 352 {
 353   return ridge_lift_fps;
 354 }
 355
 356 double
 357 FGEnvironment::get_turbulence_magnitude_norm () const
 358 {
 359   if( sgEnviro.get_turbulence_enable_state() )
 360     if (fgGetBool("/environment/params/real-world-weather-fetch") == true)
 361       return sgEnviro.get_cloud_turbulence();
 362   return turbulence_magnitude_norm;
 363 }
 364
 365 double
 366 FGEnvironment::get_turbulence_rate_hz () const
 367 {
 368   return turbulence_rate_hz;
 369 }
 370
 371 double
 372 FGEnvironment::get_elevation_ft () const
 373 {
 374   return elevation_ft;
 375 }
 376
 377 void
 378 FGEnvironment::set_visibility_m (double v)
 379 {
 380   visibility_m = v;
 381 }
 382
 383 void
 384 FGEnvironment::set_temperature_sea_level_degc (double t)
 385 {
 386   temperature_sea_level_degc = t;
 387   if (dewpoint_sea_level_degc > t)
 388       dewpoint_sea_level_degc = t;
 389   if( live_update ) {
 390     _recalc_alt_temperature();
 391     _recalc_density();
 392   }
 393 }
 394
 395 void
 396 FGEnvironment::set_temperature_degc (double t)
 397 {
 398   temperature_degc = t;
 399   if( live_update ) {
 400     _recalc_sl_temperature();
 401     _recalc_density();
 402     _recalc_relative_humidity();
 403   }
 404 }
 405
 406 void
 407 FGEnvironment::set_dewpoint_sea_level_degc (double t)
 408 {
 409   dewpoint_sea_level_degc = t;
 410   if (temperature_sea_level_degc < t)
 411       temperature_sea_level_degc = t;
 412   if( live_update ) {
 413     _recalc_alt_dewpoint();
 414     _recalc_density();
 415   }
 416 }
 417
 418 void
 419 FGEnvironment::set_dewpoint_degc (double t)
 420 {
 421   dewpoint_degc = t;
 422   if( live_update ) {
 423     _recalc_sl_dewpoint();
 424     _recalc_density();
 425     _recalc_relative_humidity();
 426   }
 427 }
 428
 429 void
 430 FGEnvironment::set_pressure_sea_level_inhg (double p)
 431 {
 432   pressure_sea_level_inhg = p;
 433   if( live_update ) {
 434     _recalc_alt_pressure();
 435     _recalc_density();
 436   }
 437 }
 438
 439 void
 440 FGEnvironment::set_pressure_inhg (double p)
 441 {
 442   pressure_inhg = p;
 443   if( live_update ) {
 444     _recalc_sl_pressure();
 445     _recalc_density();
 446   }
 447 }
 448
 449 void
 450 FGEnvironment::set_wind_from_heading_deg (double h)
 451 {
 452   wind_from_heading_deg = h;
 453   if( live_update ) {
 454     _recalc_ne();
 455   }
 456 }
 457
 458 void
 459 FGEnvironment::set_wind_speed_kt (double s)
 460 {
 461   wind_speed_kt = s;
 462   if( live_update ) {
 463     _recalc_ne();
 464   }
 465 }
 466
 467 void
 468 FGEnvironment::set_wind_from_north_fps (double n)
 469 {
 470   wind_from_north_fps = n;
 471   if( live_update ) {
 472     _recalc_hdgspd();
 473   }
 474 }
 475
 476 void
 477 FGEnvironment::set_wind_from_east_fps (double e)
 478 {
 479   wind_from_east_fps = e;
 480   if( live_update ) {
 481     _recalc_hdgspd();
 482   }
 483 }
 484
 485 void
 486 FGEnvironment::set_wind_from_down_fps (double d)
 487 {
 488   wind_from_down_fps = d;
 489   if( live_update ) {
 490     _recalc_hdgspd();
 491   }
 492 }
 493
 494 void
 495 FGEnvironment::set_thermal_lift_fps (double th)
 496 {
 497   thermal_lift_fps = th;
 498   if( live_update ) {
 499     _recalc_updraft();
 500   }
 501 }
 502
 503 void
 504 FGEnvironment::set_ridge_lift_fps (double ri)
 505 {
 506   ridge_lift_fps = ri;
 507   if( live_update ) {
 508     _recalc_updraft();
 509 }
 510 }
 511
 512 void
 513 FGEnvironment::set_turbulence_magnitude_norm (double t)
 514 {
 515   turbulence_magnitude_norm = t;
 516 }
 517
 518 void
 519 FGEnvironment::set_turbulence_rate_hz (double r)
 520 {
 521   turbulence_rate_hz = r;
 522 }
 523
 524 void
 525 FGEnvironment::set_elevation_ft (double e)
 526 {
 527   elevation_ft = e;
 528   if( live_update ) {
 529     _recalc_alt_temperature();
 530     _recalc_alt_dewpoint();
 531     _recalc_alt_pressure();
 532     _recalc_density();
 533     _recalc_relative_humidity();
 534   }
 535 }
 536
 537 void
 538 FGEnvironment::set_altitude_half_to_sun_m (double alt)
 539 {
 540   altitude_half_to_sun_m = alt;
 541   if( live_update ) {
 542     _recalc_density_tropo_avg_kgm3();
 543   }
 544 }
 545
 546 void
 547 FGEnvironment::set_altitude_tropo_top_m (double alt)
 548 {
 549   altitude_tropo_top_m = alt;
 550   if( live_update ) {
 551     _recalc_density_tropo_avg_kgm3();
 552   }
 553 }
 554
 555
 556 void
 557 FGEnvironment::_recalc_hdgspd ()
 558 {
 559   double angle_rad;
 560
 561   if (wind_from_east_fps == 0) {
 562     angle_rad = (wind_from_north_fps >= 0 ? SGD_PI_2 : -SGD_PI_2);
 563   } else {
 564     angle_rad = atan(wind_from_north_fps/wind_from_east_fps);
 565   }
 566   wind_from_heading_deg = angle_rad * SGD_RADIANS_TO_DEGREES;
 567   if (wind_from_east_fps >= 0)
 568     wind_from_heading_deg = 90 - wind_from_heading_deg;
 569   else
 570     wind_from_heading_deg = 270 - wind_from_heading_deg;
 571
 572 #if 0
 573   // FIXME: Windspeed can become negative with these formulas.
 574   //        which can cause problems for animations that rely
 575   //        on the windspeed property.
 576   if (angle_rad == 0)
 577     wind_speed_kt = fabs(wind_from_east_fps
 578                          * SG_METER_TO_NM * SG_FEET_TO_METER * 3600);
 579   else
 580     wind_speed_kt = (wind_from_north_fps / sin(angle_rad))
 581       * SG_METER_TO_NM * SG_FEET_TO_METER * 3600;
 582 #else
 583   wind_speed_kt = sqrt(wind_from_north_fps * wind_from_north_fps +
 584                        wind_from_east_fps * wind_from_east_fps)
 585                   * SG_METER_TO_NM * SG_FEET_TO_METER * 3600;
 586 #endif
 587 }
 588
 589 void
 590 FGEnvironment::_recalc_ne ()
 591 {
 592   double speed_fps =
 593     wind_speed_kt * SG_NM_TO_METER * SG_METER_TO_FEET * (1.0/3600);
 594
 595   wind_from_north_fps = speed_fps *
 596     cos(wind_from_heading_deg * SGD_DEGREES_TO_RADIANS);
 597   wind_from_east_fps = speed_fps *
 598     sin(wind_from_heading_deg * SGD_DEGREES_TO_RADIANS);
 599 }
 600
 601 void
 602 FGEnvironment::_recalc_updraft ()
 603 {
 604   wind_from_down_fps = thermal_lift_fps + ridge_lift_fps ;
 605 }
 606
 607 void
 608 FGEnvironment::_recalc_sl_temperature ()
 609 {
 610   // If we're in the stratosphere, leave sea-level temp alone
 611   if (elevation_ft < 38000) {
 612     temperature_sea_level_degc = (temperature_degc + 273.15)
 613         / _temperature_degc_table->interpolate(elevation_ft)
 614       - 273.15;
 615   }
 616 }
 617
 618 void
 619 FGEnvironment::_recalc_alt_temperature ()
 620 {
 621   if (elevation_ft < 38000) {
 622     temperature_degc = (temperature_sea_level_degc + 273.15) *
 623         _temperature_degc_table->interpolate(elevation_ft) - 273.15;
 624   } else {
 625     temperature_degc = -56.49;  // Stratosphere is constant
 626   }
 627 }
 628
 629 void
 630 FGEnvironment::_recalc_sl_dewpoint ()
 631 {
 632                                 // 0.2degC/1000ft
 633                                 // FIXME: this will work only for low
 634                                 // elevations
 635   dewpoint_sea_level_degc = dewpoint_degc + (elevation_ft * .0002);
 636   if (dewpoint_sea_level_degc > temperature_sea_level_degc)
 637     dewpoint_sea_level_degc = temperature_sea_level_degc;
 638 }
 639
 640 void
 641 FGEnvironment::_recalc_alt_dewpoint ()
 642 {
 643                                 // 0.2degC/1000ft
 644                                 // FIXME: this will work only for low
 645                                 // elevations
 646   dewpoint_degc = dewpoint_sea_level_degc + (elevation_ft * .0002);
 647   if (dewpoint_degc > temperature_degc)
 648     dewpoint_degc = temperature_degc;
 649 }
 650
 651 void
 652 FGEnvironment::_recalc_sl_pressure ()
 653 {
 654   pressure_sea_level_inhg =
 655     pressure_inhg / _pressure_inhg_table->interpolate(elevation_ft);
 656 }
 657
 658 void
 659 FGEnvironment::_recalc_alt_pressure ()
 660 {
 661   pressure_inhg =
 662     pressure_sea_level_inhg * _pressure_inhg_table->interpolate(elevation_ft);
 663 }
 664
 665 void
 666 FGEnvironment::_recalc_density ()
 667 {
 668   double pressure_psf = pressure_inhg * 70.7487;
 669
 670   // adjust for humidity
 671   // calculations taken from USA Today (oops!) at
 672   // http://www.usatoday.com/weather/basics/density-calculations.htm
 673   double temperature_degk = temperature_degc + 273.15;
 674   double pressure_mb = pressure_inhg * 33.86;
 675   double vapor_pressure_mb =
 676     6.11 * pow(10.0, 7.5 * dewpoint_degc / (237.7 + dewpoint_degc));
 677   double virtual_temperature_degk = temperature_degk / (1 - (vapor_pressure_mb / pressure_mb) * (1.0 - 0.622));
 678   double virtual_temperature_degr = virtual_temperature_degk * 1.8;
 679
 680   density_slugft3 = pressure_psf / (virtual_temperature_degr * 1718);
 681   _recalc_density_tropo_avg_kgm3();
 682 }
 683
 684 // This is used to calculate the average density on the path
 685 // of sunlight to the observer for calculating sun-color
 686 void
 687 FGEnvironment::_recalc_density_tropo_avg_kgm3 ()
 688 {
 689   double pressure_mb = pressure_inhg * 33.86;
 690   double vaporpressure = 6.11 * pow(10.0, ((7.5 * dewpoint_degc) / (237.7 + dewpoint_degc)));
 691
 692   double virtual_temp = (temperature_degc + 273.15) / (1 - 0.379 * (vaporpressure/pressure_mb));
 693
 694   double density_half = (100 * pressure_mb * exp(-altitude_half_to_sun_m / 8000))
 695       / (287.05 * virtual_temp);
 696   double density_tropo = (100 * pressure_mb * exp((-1 * altitude_tropo_top_m) / 8000))
 697       / ( 287.05 * virtual_temp);
 698
 699   density_tropo_avg_kgm3 = ((density_slugft3 * 515.379) + density_half + density_tropo) / 3;
 700 }
 701
 702 void
 703 FGEnvironment::_recalc_relative_humidity ()
 704 {
 705 /*
 706   double vaporpressure = 6.11 * pow(10.0, ((7.5 * dewpoint_degc) / ( 237.7 + dewpoint_degc)));
 707   double sat_vaporpressure = 6.11 * pow(10.0, ((7.5 * temperature_degc)
 708       / ( 237.7 + temperature_degc)) );
 709   relative_humidity = 100 * vaporpressure / sat_vaporpressure ;
 710
 711   with a little algebra, this gets the same result and spares two multiplications and one pow()
 712 */
 713   double a = (7.5 * dewpoint_degc)    / ( 237.7 + dewpoint_degc);
 714   double b = (7.5 * temperature_degc) / ( 237.7 + temperature_degc);
 715   relative_humidity = 100 * pow(10,a-b);
 716 }
 717
 718 bool
 719 FGEnvironment::set_live_update( bool _live_update )
 720 {
 721   bool b = live_update;
 722   live_update = _live_update;
 723   return b;
 724 }
 725
 726
 727 ////////////////////////////////////////////////////////////////////////
 728 // Functions.
 729 ////////////////////////////////////////////////////////////////////////
 730
 731 static inline double
 732 do_interp (double a, double b, double fraction)
 733 {
 734     double retval = (a + ((b - a) * fraction));
 735     return retval;
 736 }
 737
 738 static inline double
 739 do_interp_deg (double a, double b, double fraction)
 740 {
 741     a = fmod(a, 360);
 742     b = fmod(b, 360);
 743     if (fabs(b-a) > 180) {
 744         if (a < b)
 745             a += 360;
 746         else
 747             b += 360;
 748     }
 749     return fmod(do_interp(a, b, fraction), 360);
 750 }
 751
 752 void
 753 interpolate (const FGEnvironment * env1, const FGEnvironment * env2,
 754              double fraction, FGEnvironment * result)
 755 {
 756     // don't calculate each internal property every time we set a single value
 757     // we trigger that at the end of the interpolation process
 758     bool live_update = result->set_live_update( false );
 759
 760     result->set_visibility_m
 761         (do_interp(env1->get_visibility_m(),
 762                    env2->get_visibility_m(),
 763                    fraction));
 764
 765     result->set_temperature_sea_level_degc
 766         (do_interp(env1->get_temperature_sea_level_degc(),
 767                    env2->get_temperature_sea_level_degc(),
 768                    fraction));
 769
 770     result->set_dewpoint_degc
 771         (do_interp(env1->get_dewpoint_sea_level_degc(),
 772                    env2->get_dewpoint_sea_level_degc(),
 773                    fraction));
 774
 775     result->set_pressure_sea_level_inhg
 776         (do_interp(env1->get_pressure_sea_level_inhg(),
 777                    env2->get_pressure_sea_level_inhg(),
 778                    fraction));
 779
 780     result->set_wind_from_heading_deg
 781         (do_interp_deg(env1->get_wind_from_heading_deg(),
 782                        env2->get_wind_from_heading_deg(),
 783                        fraction));
 784
 785     result->set_wind_speed_kt
 786         (do_interp(env1->get_wind_speed_kt(),
 787                    env2->get_wind_speed_kt(),
 788                    fraction));
 789
 790     result->set_elevation_ft
 791         (do_interp(env1->get_elevation_ft(),
 792                    env2->get_elevation_ft(),
 793                    fraction));
 794
 795     result->set_turbulence_magnitude_norm
 796         (do_interp(env1->get_turbulence_magnitude_norm(),
 797                    env2->get_turbulence_magnitude_norm(),
 798                    fraction));
 799
 800     result->set_turbulence_rate_hz
 801         (do_interp(env1->get_turbulence_rate_hz(),
 802                    env2->get_turbulence_rate_hz(),
 803                    fraction));
 804
 805     // calculate derived properties here to avoid duplicate expensive computations
 806     result->_recalc_ne();
 807     result->_recalc_alt_temperature();
 808     result->_recalc_alt_dewpoint();
 809     result->_recalc_alt_pressure();
 810     result->_recalc_density();
 811     result->_recalc_relative_humidity();
 812
 813     result->set_live_update(live_update);
 814 }
 815
 816 // end of environment.cxx