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     bool live_update = set_live_update( false );
 202     maybe_copy_value(this, node, "visibility-m",
 203                      &FGEnvironment::set_visibility_m);
 204
 205     if (!maybe_copy_value(this, node, "temperature-sea-level-degc",
 206                           &FGEnvironment::set_temperature_sea_level_degc))
 207         maybe_copy_value(this, node, "temperature-degc",
 208                          &FGEnvironment::set_temperature_degc);
 209
 210     if (!maybe_copy_value(this, node, "dewpoint-sea-level-degc",
 211                           &FGEnvironment::set_dewpoint_sea_level_degc))
 212         maybe_copy_value(this, node, "dewpoint-degc",
 213                          &FGEnvironment::set_dewpoint_degc);
 214
 215     if (!maybe_copy_value(this, node, "pressure-sea-level-inhg",
 216                           &FGEnvironment::set_pressure_sea_level_inhg))
 217         maybe_copy_value(this, node, "pressure-inhg",
 218                          &FGEnvironment::set_pressure_inhg);
 219
 220     maybe_copy_value(this, node, "wind-from-heading-deg",
 221                      &FGEnvironment::set_wind_from_heading_deg);
 222
 223     maybe_copy_value(this, node, "wind-speed-kt",
 224                      &FGEnvironment::set_wind_speed_kt);
 225
 226     maybe_copy_value(this, node, "elevation-ft",
 227                      &FGEnvironment::set_elevation_ft);
 228
 229     maybe_copy_value(this, node, "turbulence/magnitude-norm",
 230                      &FGEnvironment::set_turbulence_magnitude_norm);
 231
 232     maybe_copy_value(this, node, "turbulence/rate-hz",
 233                      &FGEnvironment::set_turbulence_rate_hz);
 234     // calculate derived properties here to avoid duplicate expensive computations
 235     _recalc_ne();
 236     _recalc_alt_temperature();
 237     _recalc_alt_dewpoint();
 238     _recalc_alt_pressure();
 239     _recalc_density();
 240     _recalc_relative_humidity();
 241
 242     set_live_update(live_update);
 243 }
 244
 245
 246 double
 247 FGEnvironment::get_visibility_m () const
 248 {
 249   return visibility_m;
 250 }
 251
 252 double
 253 FGEnvironment::get_temperature_sea_level_degc () const
 254 {
 255   return temperature_sea_level_degc;
 256 }
 257
 258 double
 259 FGEnvironment::get_temperature_degc () const
 260 {
 261   return temperature_degc;
 262 }
 263
 264 double
 265 FGEnvironment::get_temperature_degf () const
 266 {
 267   return (temperature_degc * 9.0 / 5.0) + 32.0;
 268 }
 269
 270 double
 271 FGEnvironment::get_dewpoint_sea_level_degc () const
 272 {
 273   return dewpoint_sea_level_degc;
 274 }
 275
 276 double
 277 FGEnvironment::get_dewpoint_degc () const
 278 {
 279   return dewpoint_degc;
 280 }
 281
 282 double
 283 FGEnvironment::get_pressure_sea_level_inhg () const
 284 {
 285   return pressure_sea_level_inhg;
 286 }
 287
 288 double
 289 FGEnvironment::get_pressure_inhg () const
 290 {
 291   return pressure_inhg;
 292 }
 293
 294 double
 295 FGEnvironment::get_density_slugft3 () const
 296 {
 297   return density_slugft3;
 298 }
 299
 300 double
 301 FGEnvironment::get_relative_humidity () const
 302 {
 303   return relative_humidity;
 304 }
 305
 306 double
 307 FGEnvironment::get_density_tropo_avg_kgm3 () const
 308 {
 309   return density_tropo_avg_kgm3;
 310 }
 311
 312 double
 313 FGEnvironment::get_altitude_half_to_sun_m () const
 314 {
 315   return altitude_half_to_sun_m;
 316 }
 317
 318 double
 319 FGEnvironment::get_altitude_tropo_top_m () const
 320 {
 321   return altitude_tropo_top_m;
 322 }
 323
 324 double
 325 FGEnvironment::get_wind_from_heading_deg () const
 326 {
 327   return wind_from_heading_deg;
 328 }
 329
 330 double
 331 FGEnvironment::get_wind_speed_kt () const
 332 {
 333   return wind_speed_kt;
 334 }
 335
 336 double
 337 FGEnvironment::get_wind_from_north_fps () const
 338 {
 339   return wind_from_north_fps;
 340 }
 341
 342 double
 343 FGEnvironment::get_wind_from_east_fps () const
 344 {
 345   return wind_from_east_fps;
 346 }
 347
 348 double
 349 FGEnvironment::get_wind_from_down_fps () const
 350 {
 351   return wind_from_down_fps;
 352 }
 353
 354 double
 355 FGEnvironment::get_thermal_lift_fps () const
 356 {
 357   return thermal_lift_fps;
 358 }
 359
 360 double
 361 FGEnvironment::get_ridge_lift_fps () const
 362 {
 363   return ridge_lift_fps;
 364 }
 365
 366 double
 367 FGEnvironment::get_turbulence_magnitude_norm () const
 368 {
 369   if( sgEnviro.get_turbulence_enable_state() )
 370     if (fgGetBool("/environment/params/real-world-weather-fetch") == true)
 371       return sgEnviro.get_cloud_turbulence();
 372   return turbulence_magnitude_norm;
 373 }
 374
 375 double
 376 FGEnvironment::get_turbulence_rate_hz () const
 377 {
 378   return turbulence_rate_hz;
 379 }
 380
 381 double
 382 FGEnvironment::get_elevation_ft () const
 383 {
 384   return elevation_ft;
 385 }
 386
 387 void
 388 FGEnvironment::set_visibility_m (double v)
 389 {
 390   visibility_m = v;
 391 }
 392
 393 void
 394 FGEnvironment::set_temperature_sea_level_degc (double t)
 395 {
 396   temperature_sea_level_degc = t;
 397   if (dewpoint_sea_level_degc > t)
 398       dewpoint_sea_level_degc = t;
 399   if( live_update ) {
 400     _recalc_alt_temperature();
 401     _recalc_density();
 402   }
 403 }
 404
 405 void
 406 FGEnvironment::set_temperature_degc (double t)
 407 {
 408   temperature_degc = t;
 409   if( live_update ) {
 410     _recalc_sl_temperature();
 411     _recalc_density();
 412     _recalc_relative_humidity();
 413   }
 414 }
 415
 416 void
 417 FGEnvironment::set_dewpoint_sea_level_degc (double t)
 418 {
 419   dewpoint_sea_level_degc = t;
 420   if (temperature_sea_level_degc < t)
 421       temperature_sea_level_degc = t;
 422   if( live_update ) {
 423     _recalc_alt_dewpoint();
 424     _recalc_density();
 425   }
 426 }
 427
 428 void
 429 FGEnvironment::set_dewpoint_degc (double t)
 430 {
 431   dewpoint_degc = t;
 432   if( live_update ) {
 433     _recalc_sl_dewpoint();
 434     _recalc_density();
 435     _recalc_relative_humidity();
 436   }
 437 }
 438
 439 void
 440 FGEnvironment::set_pressure_sea_level_inhg (double p)
 441 {
 442   pressure_sea_level_inhg = p;
 443   if( live_update ) {
 444     _recalc_alt_pressure();
 445     _recalc_density();
 446   }
 447 }
 448
 449 void
 450 FGEnvironment::set_pressure_inhg (double p)
 451 {
 452   pressure_inhg = p;
 453   if( live_update ) {
 454     _recalc_sl_pressure();
 455     _recalc_density();
 456   }
 457 }
 458
 459 void
 460 FGEnvironment::set_wind_from_heading_deg (double h)
 461 {
 462   wind_from_heading_deg = h;
 463   if( live_update ) {
 464     _recalc_ne();
 465   }
 466 }
 467
 468 void
 469 FGEnvironment::set_wind_speed_kt (double s)
 470 {
 471   wind_speed_kt = s;
 472   if( live_update ) {
 473     _recalc_ne();
 474   }
 475 }
 476
 477 void
 478 FGEnvironment::set_wind_from_north_fps (double n)
 479 {
 480   wind_from_north_fps = n;
 481   if( live_update ) {
 482     _recalc_hdgspd();
 483   }
 484 }
 485
 486 void
 487 FGEnvironment::set_wind_from_east_fps (double e)
 488 {
 489   wind_from_east_fps = e;
 490   if( live_update ) {
 491     _recalc_hdgspd();
 492   }
 493 }
 494
 495 void
 496 FGEnvironment::set_wind_from_down_fps (double d)
 497 {
 498   wind_from_down_fps = d;
 499   if( live_update ) {
 500     _recalc_hdgspd();
 501   }
 502 }
 503
 504 void
 505 FGEnvironment::set_thermal_lift_fps (double th)
 506 {
 507   thermal_lift_fps = th;
 508   if( live_update ) {
 509     _recalc_updraft();
 510   }
 511 }
 512
 513 void
 514 FGEnvironment::set_ridge_lift_fps (double ri)
 515 {
 516   ridge_lift_fps = ri;
 517   if( live_update ) {
 518     _recalc_updraft();
 519 }
 520 }
 521
 522 void
 523 FGEnvironment::set_turbulence_magnitude_norm (double t)
 524 {
 525   turbulence_magnitude_norm = t;
 526 }
 527
 528 void
 529 FGEnvironment::set_turbulence_rate_hz (double r)
 530 {
 531   turbulence_rate_hz = r;
 532 }
 533
 534 void
 535 FGEnvironment::set_elevation_ft (double e)
 536 {
 537   elevation_ft = e;
 538   if( live_update ) {
 539     _recalc_alt_temperature();
 540     _recalc_alt_dewpoint();
 541     _recalc_alt_pressure();
 542     _recalc_density();
 543     _recalc_relative_humidity();
 544   }
 545 }
 546
 547 void
 548 FGEnvironment::set_altitude_half_to_sun_m (double alt)
 549 {
 550   altitude_half_to_sun_m = alt;
 551   if( live_update ) {
 552     _recalc_density_tropo_avg_kgm3();
 553   }
 554 }
 555
 556 void
 557 FGEnvironment::set_altitude_tropo_top_m (double alt)
 558 {
 559   altitude_tropo_top_m = alt;
 560   if( live_update ) {
 561     _recalc_density_tropo_avg_kgm3();
 562   }
 563 }
 564
 565
 566 void
 567 FGEnvironment::_recalc_hdgspd ()
 568 {
 569   double angle_rad;
 570
 571   if (wind_from_east_fps == 0) {
 572     angle_rad = (wind_from_north_fps >= 0 ? SGD_PI_2 : -SGD_PI_2);
 573   } else {
 574     angle_rad = atan(wind_from_north_fps/wind_from_east_fps);
 575   }
 576   wind_from_heading_deg = angle_rad * SGD_RADIANS_TO_DEGREES;
 577   if (wind_from_east_fps >= 0)
 578     wind_from_heading_deg = 90 - wind_from_heading_deg;
 579   else
 580     wind_from_heading_deg = 270 - wind_from_heading_deg;
 581
 582 #if 0
 583   // FIXME: Windspeed can become negative with these formulas.
 584   //        which can cause problems for animations that rely
 585   //        on the windspeed property.
 586   if (angle_rad == 0)
 587     wind_speed_kt = fabs(wind_from_east_fps
 588                          * SG_METER_TO_NM * SG_FEET_TO_METER * 3600);
 589   else
 590     wind_speed_kt = (wind_from_north_fps / sin(angle_rad))
 591       * SG_METER_TO_NM * SG_FEET_TO_METER * 3600;
 592 #else
 593   wind_speed_kt = sqrt(wind_from_north_fps * wind_from_north_fps +
 594                        wind_from_east_fps * wind_from_east_fps)
 595                   * SG_METER_TO_NM * SG_FEET_TO_METER * 3600;
 596 #endif
 597 }
 598
 599 void
 600 FGEnvironment::_recalc_ne ()
 601 {
 602   double speed_fps =
 603     wind_speed_kt * SG_NM_TO_METER * SG_METER_TO_FEET * (1.0/3600);
 604
 605   wind_from_north_fps = speed_fps *
 606     cos(wind_from_heading_deg * SGD_DEGREES_TO_RADIANS);
 607   wind_from_east_fps = speed_fps *
 608     sin(wind_from_heading_deg * SGD_DEGREES_TO_RADIANS);
 609 }
 610
 611 void
 612 FGEnvironment::_recalc_updraft ()
 613 {
 614   wind_from_down_fps = thermal_lift_fps + ridge_lift_fps ;
 615 }
 616
 617 void
 618 FGEnvironment::_recalc_sl_temperature ()
 619 {
 620   // If we're in the stratosphere, leave sea-level temp alone
 621   if (elevation_ft < 38000) {
 622     temperature_sea_level_degc = (temperature_degc + 273.15)
 623         / _temperature_degc_table->interpolate(elevation_ft)
 624       - 273.15;
 625   }
 626 }
 627
 628 void
 629 FGEnvironment::_recalc_alt_temperature ()
 630 {
 631   if (elevation_ft < 38000) {
 632     temperature_degc = (temperature_sea_level_degc + 273.15) *
 633         _temperature_degc_table->interpolate(elevation_ft) - 273.15;
 634   } else {
 635     temperature_degc = -56.49;  // Stratosphere is constant
 636   }
 637 }
 638
 639 void
 640 FGEnvironment::_recalc_sl_dewpoint ()
 641 {
 642                                 // 0.2degC/1000ft
 643                                 // FIXME: this will work only for low
 644                                 // elevations
 645   dewpoint_sea_level_degc = dewpoint_degc + (elevation_ft * .0002);
 646   if (dewpoint_sea_level_degc > temperature_sea_level_degc)
 647     dewpoint_sea_level_degc = temperature_sea_level_degc;
 648 }
 649
 650 void
 651 FGEnvironment::_recalc_alt_dewpoint ()
 652 {
 653                                 // 0.2degC/1000ft
 654                                 // FIXME: this will work only for low
 655                                 // elevations
 656   dewpoint_degc = dewpoint_sea_level_degc + (elevation_ft * .0002);
 657   if (dewpoint_degc > temperature_degc)
 658     dewpoint_degc = temperature_degc;
 659 }
 660
 661 void
 662 FGEnvironment::_recalc_sl_pressure ()
 663 {
 664   pressure_sea_level_inhg =
 665     pressure_inhg / _pressure_inhg_table->interpolate(elevation_ft);
 666 }
 667
 668 void
 669 FGEnvironment::_recalc_alt_pressure ()
 670 {
 671   pressure_inhg =
 672     pressure_sea_level_inhg * _pressure_inhg_table->interpolate(elevation_ft);
 673 }
 674
 675 void
 676 FGEnvironment::_recalc_density ()
 677 {
 678   double pressure_psf = pressure_inhg * 70.7487;
 679
 680   // adjust for humidity
 681   // calculations taken from USA Today (oops!) at
 682   // http://www.usatoday.com/weather/basics/density-calculations.htm
 683   double temperature_degk = temperature_degc + 273.15;
 684   double pressure_mb = pressure_inhg * 33.86;
 685   double vapor_pressure_mb =
 686     6.11 * pow(10.0, 7.5 * dewpoint_degc / (237.7 + dewpoint_degc));
 687   double virtual_temperature_degk = temperature_degk / (1 - (vapor_pressure_mb / pressure_mb) * (1.0 - 0.622));
 688   double virtual_temperature_degr = virtual_temperature_degk * 1.8;
 689
 690   density_slugft3 = pressure_psf / (virtual_temperature_degr * 1718);
 691   _recalc_density_tropo_avg_kgm3();
 692 }
 693
 694 // This is used to calculate the average density on the path
 695 // of sunlight to the observer for calculating sun-color
 696 void
 697 FGEnvironment::_recalc_density_tropo_avg_kgm3 ()
 698 {
 699   double pressure_mb = pressure_inhg * 33.86;
 700   double vaporpressure = 6.11 * pow(10.0, ((7.5 * dewpoint_degc) / (237.7 + dewpoint_degc)));
 701
 702   double virtual_temp = (temperature_degc + 273.15) / (1 - 0.379 * (vaporpressure/pressure_mb));
 703
 704   double density_half = (100 * pressure_mb * exp(-altitude_half_to_sun_m / 8000))
 705       / (287.05 * virtual_temp);
 706   double density_tropo = (100 * pressure_mb * exp((-1 * altitude_tropo_top_m) / 8000))
 707       / ( 287.05 * virtual_temp);
 708
 709   density_tropo_avg_kgm3 = ((density_slugft3 * 515.379) + density_half + density_tropo) / 3;
 710 }
 711
 712 void
 713 FGEnvironment::_recalc_relative_humidity ()
 714 {
 715 /*
 716   double vaporpressure = 6.11 * pow(10.0, ((7.5 * dewpoint_degc) / ( 237.7 + dewpoint_degc)));
 717   double sat_vaporpressure = 6.11 * pow(10.0, ((7.5 * temperature_degc)
 718       / ( 237.7 + temperature_degc)) );
 719   relative_humidity = 100 * vaporpressure / sat_vaporpressure ;
 720
 721   with a little algebra, this gets the same result and spares two multiplications and one pow()
 722 */
 723   double a = (7.5 * dewpoint_degc)    / ( 237.7 + dewpoint_degc);
 724   double b = (7.5 * temperature_degc) / ( 237.7 + temperature_degc);
 725   relative_humidity = 100 * pow(10,a-b);
 726 }
 727
 728 bool
 729 FGEnvironment::set_live_update( bool _live_update )
 730 {
 731   bool b = live_update;
 732   live_update = _live_update;
 733   return b;
 734 }
 735
 736
 737 ////////////////////////////////////////////////////////////////////////
 738 // Functions.
 739 ////////////////////////////////////////////////////////////////////////
 740
 741 static inline double
 742 do_interp (double a, double b, double fraction)
 743 {
 744     double retval = (a + ((b - a) * fraction));
 745     return retval;
 746 }
 747
 748 static inline double
 749 do_interp_deg (double a, double b, double fraction)
 750 {
 751     a = fmod(a, 360);
 752     b = fmod(b, 360);
 753     if (fabs(b-a) > 180) {
 754         if (a < b)
 755             a += 360;
 756         else
 757             b += 360;
 758     }
 759     return fmod(do_interp(a, b, fraction), 360);
 760 }
 761
 762 void
 763 interpolate (const FGEnvironment * env1, const FGEnvironment * env2,
 764              double fraction, FGEnvironment * result)
 765 {
 766     // don't calculate each internal property every time we set a single value
 767     // we trigger that at the end of the interpolation process
 768     bool live_update = result->set_live_update( false );
 769
 770     result->set_visibility_m
 771         (do_interp(env1->get_visibility_m(),
 772                    env2->get_visibility_m(),
 773                    fraction));
 774
 775     result->set_temperature_sea_level_degc
 776         (do_interp(env1->get_temperature_sea_level_degc(),
 777                    env2->get_temperature_sea_level_degc(),
 778                    fraction));
 779
 780     result->set_dewpoint_degc
 781         (do_interp(env1->get_dewpoint_sea_level_degc(),
 782                    env2->get_dewpoint_sea_level_degc(),
 783                    fraction));
 784
 785     result->set_pressure_sea_level_inhg
 786         (do_interp(env1->get_pressure_sea_level_inhg(),
 787                    env2->get_pressure_sea_level_inhg(),
 788                    fraction));
 789
 790     result->set_wind_from_heading_deg
 791         (do_interp_deg(env1->get_wind_from_heading_deg(),
 792                        env2->get_wind_from_heading_deg(),
 793                        fraction));
 794
 795     result->set_wind_speed_kt
 796         (do_interp(env1->get_wind_speed_kt(),
 797                    env2->get_wind_speed_kt(),
 798                    fraction));
 799
 800     result->set_elevation_ft
 801         (do_interp(env1->get_elevation_ft(),
 802                    env2->get_elevation_ft(),
 803                    fraction));
 804
 805     result->set_turbulence_magnitude_norm
 806         (do_interp(env1->get_turbulence_magnitude_norm(),
 807                    env2->get_turbulence_magnitude_norm(),
 808                    fraction));
 809
 810     result->set_turbulence_rate_hz
 811         (do_interp(env1->get_turbulence_rate_hz(),
 812                    env2->get_turbulence_rate_hz(),
 813                    fraction));
 814
 815     // calculate derived properties here to avoid duplicate expensive computations
 816     result->_recalc_ne();
 817     result->_recalc_alt_temperature();
 818     result->_recalc_alt_dewpoint();
 819     result->_recalc_alt_pressure();
 820     result->_recalc_density();
 821     result->_recalc_relative_humidity();
 822
 823     result->set_live_update(live_update);
 824 }
 825
 826 // end of environment.cxx