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     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
 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     turbulence_magnitude_norm = env.turbulence_magnitude_norm;
 184     turbulence_rate_hz = env.turbulence_rate_hz;
 185 }
 186
 187 static inline bool
 188 maybe_copy_value (FGEnvironment * env, const SGPropertyNode * node,
 189                   const char * name, void (FGEnvironment::*setter)(double))
 190 {
 191     const SGPropertyNode * child = node->getNode(name);
 192                                 // fragile: depends on not being typed
 193                                 // as a number
 194     if (child != 0 && child->hasValue() &&
 195         child->getStringValue()[0] != '\0') {
 196         (env->*setter)(child->getDoubleValue());
 197         return true;
 198     } else {
 199         return false;
 200     }
 201 }
 202
 203 void
 204 FGEnvironment::read (const SGPropertyNode * node)
 205 {
 206     maybe_copy_value(this, node, "visibility-m",
 207                      &FGEnvironment::set_visibility_m);
 208
 209     if (!maybe_copy_value(this, node, "temperature-sea-level-degc",
 210                           &FGEnvironment::set_temperature_sea_level_degc))
 211         maybe_copy_value(this, node, "temperature-degc",
 212                          &FGEnvironment::set_temperature_degc);
 213
 214     if (!maybe_copy_value(this, node, "dewpoint-sea-level-degc",
 215                           &FGEnvironment::set_dewpoint_sea_level_degc))
 216         maybe_copy_value(this, node, "dewpoint-degc",
 217                          &FGEnvironment::set_dewpoint_degc);
 218
 219     if (!maybe_copy_value(this, node, "pressure-sea-level-inhg",
 220                           &FGEnvironment::set_pressure_sea_level_inhg))
 221         maybe_copy_value(this, node, "pressure-inhg",
 222                          &FGEnvironment::set_pressure_inhg);
 223
 224     maybe_copy_value(this, node, "wind-from-heading-deg",
 225                      &FGEnvironment::set_wind_from_heading_deg);
 226
 227     maybe_copy_value(this, node, "wind-speed-kt",
 228                      &FGEnvironment::set_wind_speed_kt);
 229
 230     maybe_copy_value(this, node, "elevation-ft",
 231                      &FGEnvironment::set_elevation_ft);
 232
 233     maybe_copy_value(this, node, "turbulence/magnitude-norm",
 234                      &FGEnvironment::set_turbulence_magnitude_norm);
 235
 236     maybe_copy_value(this, node, "turbulence/rate-hz",
 237                      &FGEnvironment::set_turbulence_rate_hz);
 238 }
 239
 240
 241 double
 242 FGEnvironment::get_visibility_m () const
 243 {
 244   return visibility_m;
 245 }
 246
 247 double
 248 FGEnvironment::get_temperature_sea_level_degc () const
 249 {
 250   return temperature_sea_level_degc;
 251 }
 252
 253 double
 254 FGEnvironment::get_temperature_degc () const
 255 {
 256   return temperature_degc;
 257 }
 258
 259 double
 260 FGEnvironment::get_temperature_degf () const
 261 {
 262   return (temperature_degc * 9.0 / 5.0) + 32.0;
 263 }
 264
 265 double
 266 FGEnvironment::get_dewpoint_sea_level_degc () const
 267 {
 268   return dewpoint_sea_level_degc;
 269 }
 270
 271 double
 272 FGEnvironment::get_dewpoint_degc () const
 273 {
 274   return dewpoint_degc;
 275 }
 276
 277 double
 278 FGEnvironment::get_pressure_sea_level_inhg () const
 279 {
 280   return pressure_sea_level_inhg;
 281 }
 282
 283 double
 284 FGEnvironment::get_pressure_inhg () const
 285 {
 286   return pressure_inhg;
 287 }
 288
 289 double
 290 FGEnvironment::get_density_slugft3 () const
 291 {
 292   return density_slugft3;
 293 }
 294
 295 double
 296 FGEnvironment::get_relative_humidity () const
 297 {
 298   return relative_humidity;
 299 }
 300
 301 double
 302 FGEnvironment::get_density_tropo_avg_kgm3 () const
 303 {
 304   return density_tropo_avg_kgm3;
 305 }
 306
 307 double
 308 FGEnvironment::get_altitude_half_to_sun_m () const
 309 {
 310   return altitude_half_to_sun_m;
 311 }
 312
 313 double
 314 FGEnvironment::get_altitude_tropo_top_m () const
 315 {
 316   return altitude_tropo_top_m;
 317 }
 318
 319 double
 320 FGEnvironment::get_wind_from_heading_deg () const
 321 {
 322   return wind_from_heading_deg;
 323 }
 324
 325 double
 326 FGEnvironment::get_wind_speed_kt () const
 327 {
 328   return wind_speed_kt;
 329 }
 330
 331 double
 332 FGEnvironment::get_wind_from_north_fps () const
 333 {
 334   return wind_from_north_fps;
 335 }
 336
 337 double
 338 FGEnvironment::get_wind_from_east_fps () const
 339 {
 340   return wind_from_east_fps;
 341 }
 342
 343 double
 344 FGEnvironment::get_wind_from_down_fps () const
 345 {
 346   return wind_from_down_fps;
 347 }
 348
 349 double
 350 FGEnvironment::get_turbulence_magnitude_norm () const
 351 {
 352   if( sgEnviro.get_turbulence_enable_state() )
 353     if (fgGetBool("/environment/params/real-world-weather-fetch") == true)
 354       return sgEnviro.get_cloud_turbulence();
 355   return turbulence_magnitude_norm;
 356 }
 357
 358 double
 359 FGEnvironment::get_turbulence_rate_hz () const
 360 {
 361   return turbulence_rate_hz;
 362 }
 363
 364 double
 365 FGEnvironment::get_elevation_ft () const
 366 {
 367   return elevation_ft;
 368 }
 369
 370 void
 371 FGEnvironment::set_visibility_m (double v)
 372 {
 373   visibility_m = v;
 374 }
 375
 376 void
 377 FGEnvironment::set_temperature_sea_level_degc (double t)
 378 {
 379   temperature_sea_level_degc = t;
 380   if (dewpoint_sea_level_degc > t)
 381       dewpoint_sea_level_degc = t;
 382   _recalc_alt_pt();
 383   _recalc_density();
 384 }
 385
 386 void
 387 FGEnvironment::set_temperature_degc (double t)
 388 {
 389   temperature_degc = t;
 390   _recalc_sl_temperature();
 391   _recalc_sl_pressure();
 392   _recalc_alt_pt();
 393   _recalc_density();
 394   _recalc_relative_humidity();
 395 }
 396
 397 void
 398 FGEnvironment::set_dewpoint_sea_level_degc (double t)
 399 {
 400   dewpoint_sea_level_degc = t;
 401   if (temperature_sea_level_degc < t)
 402       temperature_sea_level_degc = t;
 403   _recalc_alt_dewpoint();
 404   _recalc_density();
 405 }
 406
 407 void
 408 FGEnvironment::set_dewpoint_degc (double t)
 409 {
 410   dewpoint_degc = t;
 411   _recalc_sl_dewpoint();
 412   _recalc_density();
 413   _recalc_relative_humidity();
 414 }
 415
 416 void
 417 FGEnvironment::set_pressure_sea_level_inhg (double p)
 418 {
 419   pressure_sea_level_inhg = p;
 420   _recalc_alt_pt();
 421   _recalc_density();
 422 }
 423
 424 void
 425 FGEnvironment::set_pressure_inhg (double p)
 426 {
 427   pressure_inhg = p;
 428   _recalc_sl_pressure();
 429   _recalc_density();
 430 }
 431
 432 void
 433 FGEnvironment::set_wind_from_heading_deg (double h)
 434 {
 435   wind_from_heading_deg = h;
 436   _recalc_ne();
 437 }
 438
 439 void
 440 FGEnvironment::set_wind_speed_kt (double s)
 441 {
 442   wind_speed_kt = s;
 443   _recalc_ne();
 444 }
 445
 446 void
 447 FGEnvironment::set_wind_from_north_fps (double n)
 448 {
 449   wind_from_north_fps = n;
 450   _recalc_hdgspd();
 451 }
 452
 453 void
 454 FGEnvironment::set_wind_from_east_fps (double e)
 455 {
 456   wind_from_east_fps = e;
 457   _recalc_hdgspd();
 458 }
 459
 460 void
 461 FGEnvironment::set_wind_from_down_fps (double d)
 462 {
 463   wind_from_down_fps = d;
 464   _recalc_hdgspd();
 465 }
 466
 467 void
 468 FGEnvironment::set_turbulence_magnitude_norm (double t)
 469 {
 470   turbulence_magnitude_norm = t;
 471 }
 472
 473 void
 474 FGEnvironment::set_turbulence_rate_hz (double r)
 475 {
 476   turbulence_rate_hz = r;
 477 }
 478
 479 void
 480 FGEnvironment::set_elevation_ft (double e)
 481 {
 482   elevation_ft = e;
 483   _recalc_alt_dewpoint();
 484   _recalc_alt_pt();
 485   _recalc_density();
 486   _recalc_relative_humidity();
 487 }
 488
 489 void
 490 FGEnvironment::set_altitude_half_to_sun_m (double alt)
 491 {
 492  altitude_half_to_sun_m = alt;
 493  _recalc_density_tropo_avg_kgm3();
 494 }
 495
 496 void
 497 FGEnvironment::set_altitude_tropo_top_m (double alt)
 498 {
 499  altitude_tropo_top_m = alt;
 500  _recalc_density_tropo_avg_kgm3();
 501 }
 502
 503
 504 void
 505 FGEnvironment::_recalc_hdgspd ()
 506 {
 507   double angle_rad;
 508
 509   if (wind_from_east_fps == 0) {
 510     angle_rad = (wind_from_north_fps >= 0 ? SGD_PI_2 : -SGD_PI_2);
 511   } else {
 512     angle_rad = atan(wind_from_north_fps/wind_from_east_fps);
 513   }
 514   wind_from_heading_deg = angle_rad * SGD_RADIANS_TO_DEGREES;
 515   if (wind_from_east_fps >= 0)
 516     wind_from_heading_deg = 90 - wind_from_heading_deg;
 517   else
 518     wind_from_heading_deg = 270 - wind_from_heading_deg;
 519
 520 #if 0
 521   // FIXME: Windspeed can become negative with these formulas.
 522   //        which can cause problems for animations that rely
 523   //        on the windspeed property.
 524   if (angle_rad == 0)
 525     wind_speed_kt = fabs(wind_from_east_fps
 526                          * SG_METER_TO_NM * SG_FEET_TO_METER * 3600);
 527   else
 528     wind_speed_kt = (wind_from_north_fps / sin(angle_rad))
 529       * SG_METER_TO_NM * SG_FEET_TO_METER * 3600;
 530 #else
 531   wind_speed_kt = sqrt(wind_from_north_fps * wind_from_north_fps +
 532                        wind_from_east_fps * wind_from_east_fps)
 533                   * SG_METER_TO_NM * SG_FEET_TO_METER * 3600;
 534 #endif
 535 }
 536
 537 void
 538 FGEnvironment::_recalc_ne ()
 539 {
 540   double speed_fps =
 541     wind_speed_kt * SG_NM_TO_METER * SG_METER_TO_FEET * (1.0/3600);
 542
 543   wind_from_north_fps = speed_fps *
 544     cos(wind_from_heading_deg * SGD_DEGREES_TO_RADIANS);
 545   wind_from_east_fps = speed_fps *
 546     sin(wind_from_heading_deg * SGD_DEGREES_TO_RADIANS);
 547 }
 548
 549 // Intended to help with the interpretation of METAR data,
 550 // not for random in-flight outside-air temperatures.
 551 void
 552 FGEnvironment::_recalc_sl_temperature ()
 553 {
 554
 555 #if 0
 556   {
 557     SG_LOG(SG_GENERAL, SG_DEBUG, "recalc_sl_temperature: using "
 558       << temperature_degc << " @ " << elevation_ft << " :: " << this);
 559   }
 560 #endif
 561
 562   if (elevation_ft >= ISA_def[1].height) {
 563     SG_LOG(SG_GENERAL, SG_ALERT, "recalc_sl_temperature: "
 564         << "valid only in troposphere, not " << elevation_ft);
 565     return;
 566   }
 567
 568 // Clamp: temperature of the stratosphere, in degrees C:
 569   double t_strato = ISA_def[1].temp - atmodel::freezing;
 570   if (temperature_degc < t_strato) temperature_sea_level_degc = t_strato;
 571   else temperature_sea_level_degc =
 572       temperature_degc - elevation_ft * ISA_def[0].lapse;
 573
 574 // Alternative implemenation:
 575 //  else temperature_sea_level_inhg = T_layer(0., elevation_ft * foot,
 576 //      pressure_inhg * inHg, temperature_degc + freezing, ISA_def[0].lapse) - freezing;
 577 }
 578
 579 void
 580 FGEnvironment::_recalc_sl_dewpoint ()
 581 {
 582                                 // 0.2degC/1000ft
 583                                 // FIXME: this will work only for low
 584                                 // elevations
 585   dewpoint_sea_level_degc = dewpoint_degc + (elevation_ft * .0002);
 586   if (dewpoint_sea_level_degc > temperature_sea_level_degc)
 587     dewpoint_sea_level_degc = temperature_sea_level_degc;
 588 }
 589
 590 void
 591 FGEnvironment::_recalc_alt_dewpoint ()
 592 {
 593                                 // 0.2degC/1000ft
 594                                 // FIXME: this will work only for low
 595                                 // elevations
 596   dewpoint_degc = dewpoint_sea_level_degc + (elevation_ft * .0002);
 597   if (dewpoint_degc > temperature_degc)
 598     dewpoint_degc = temperature_degc;
 599 }
 600
 601 void
 602 FGEnvironment::_recalc_sl_pressure ()
 603 {
 604   using namespace atmodel;
 605 #if 0
 606   {
 607     SG_LOG(SG_GENERAL, SG_ALERT, "recalc_sl_pressure: using "
 608       << pressure_inhg << " and "
 609       << temperature_degc << " @ " << elevation_ft << " :: " << this);
 610   }
 611 #endif
 612   pressure_sea_level_inhg = P_layer(0., elevation_ft * foot,
 613       pressure_inhg * inHg, temperature_degc + freezing, ISA_def[0].lapse) / inHg;
 614 }
 615
 616 // This gets called at frame rate, to account for the aircraft's
 617 // changing altitude.
 618 // Called by set_elevation_ft() which is called by FGEnvironmentMgr::update
 619
 620 void
 621 FGEnvironment::_recalc_alt_pt ()
 622 {
 623   using namespace atmodel;
 624 #if 0
 625   {
 626     static int count(0);
 627     if (++count % 1000 == 0) {
 628       SG_LOG(SG_GENERAL, SG_ALERT,
 629            "recalc_alt_pt for: " << elevation_ft
 630         << "  using "  << pressure_sea_level_inhg
 631         << "  and "  << temperature_sea_level_degc
 632         << " :: " << this
 633         << "  # " << count);
 634         ///////////////////////////////////raise(SIGUSR1);
 635     }
 636   }
 637 #endif
 638   double press, temp;
 639   boost::tie(press, temp) = PT_vs_hpt(elevation_ft * foot,
 640         pressure_sea_level_inhg * inHg, temperature_sea_level_degc + freezing);
 641   temperature_degc = temp - freezing;
 642   pressure_inhg = press / inHg;
 643 }
 644
 645 void
 646 FGEnvironment::_recalc_density ()
 647 {
 648   double pressure_psf = pressure_inhg * 70.7487;
 649
 650   // adjust for humidity
 651   // calculations taken from USA Today (oops!) at
 652   // http://www.usatoday.com/weather/basics/density-calculations.htm
 653   double temperature_degk = temperature_degc + 273.15;
 654   double pressure_mb = pressure_inhg * 33.86;
 655   double vapor_pressure_mb =
 656     6.11 * pow(10.0, 7.5 * dewpoint_degc / (237.7 + dewpoint_degc));
 657   double virtual_temperature_degk = temperature_degk / (1 - (vapor_pressure_mb / pressure_mb) * (1.0 - 0.622));
 658   double virtual_temperature_degr = virtual_temperature_degk * 1.8;
 659
 660   density_slugft3 = pressure_psf / (virtual_temperature_degr * 1718);
 661   _recalc_density_tropo_avg_kgm3();
 662 }
 663
 664 // This is used to calculate the average density on the path
 665 // of sunlight to the observer for calculating sun-color
 666 void
 667 FGEnvironment::_recalc_density_tropo_avg_kgm3 ()
 668 {
 669   double pressure_mb = pressure_inhg * 33.86;
 670   double vaporpressure = 6.11 * pow(10.0, ((7.5 * dewpoint_degc) / (237.7 + dewpoint_degc)));
 671
 672   double virtual_temp = (temperature_degc + 273.15) / (1 - 0.379 * (vaporpressure/pressure_mb));
 673
 674   double density_half = (100 * pressure_mb * exp(-altitude_half_to_sun_m / 8000))
 675       / (287.05 * virtual_temp);
 676   double density_tropo = (100 * pressure_mb * exp((-1 * altitude_tropo_top_m) / 8000))
 677       / ( 287.05 * virtual_temp);
 678
 679   density_tropo_avg_kgm3 = ((density_slugft3 * 515.379) + density_half + density_tropo) / 3;
 680 }
 681
 682 void
 683 FGEnvironment::_recalc_relative_humidity ()
 684 {
 685   double vaporpressure = 6.11 * pow(10.0, ((7.5 * dewpoint_degc) / ( 237.7 + dewpoint_degc)));
 686   double sat_vaporpressure = 6.11 * pow(10.0, ((7.5 * temperature_degc)
 687       / ( 237.7 + temperature_degc)) );
 688   relative_humidity = 100 * vaporpressure / sat_vaporpressure ;
 689 }
 690
 691
 692
 693 ////////////////////////////////////////////////////////////////////////
 694 // Functions.
 695 ////////////////////////////////////////////////////////////////////////
 696
 697 static inline double
 698 do_interp (double a, double b, double fraction)
 699 {
 700     double retval = (a + ((b - a) * fraction));
 701     return retval;
 702 }
 703
 704 static inline double
 705 do_interp_deg (double a, double b, double fraction)
 706 {
 707     a = fmod(a, 360);
 708     b = fmod(b, 360);
 709     if (fabs(b-a) > 180) {
 710         if (a < b)
 711             a += 360;
 712         else
 713             b += 360;
 714     }
 715     return fmod(do_interp(a, b, fraction), 360);
 716 }
 717
 718 void
 719 interpolate (const FGEnvironment * env1, const FGEnvironment * env2,
 720              double fraction, FGEnvironment * result)
 721 {
 722     result->set_visibility_m
 723         (do_interp(env1->get_visibility_m(),
 724                    env2->get_visibility_m(),
 725                    fraction));
 726
 727     result->set_temperature_sea_level_degc
 728         (do_interp(env1->get_temperature_sea_level_degc(),
 729                    env2->get_temperature_sea_level_degc(),
 730                    fraction));
 731
 732     result->set_dewpoint_degc
 733         (do_interp(env1->get_dewpoint_sea_level_degc(),
 734                    env2->get_dewpoint_sea_level_degc(),
 735                    fraction));
 736
 737     result->set_pressure_sea_level_inhg
 738         (do_interp(env1->get_pressure_sea_level_inhg(),
 739                    env2->get_pressure_sea_level_inhg(),
 740                    fraction));
 741
 742     result->set_wind_from_heading_deg
 743         (do_interp_deg(env1->get_wind_from_heading_deg(),
 744                        env2->get_wind_from_heading_deg(),
 745                        fraction));
 746
 747     result->set_wind_speed_kt
 748         (do_interp(env1->get_wind_speed_kt(),
 749                    env2->get_wind_speed_kt(),
 750                    fraction));
 751
 752     result->set_elevation_ft
 753         (do_interp(env1->get_elevation_ft(),
 754                    env2->get_elevation_ft(),
 755                    fraction));
 756
 757     result->set_turbulence_magnitude_norm
 758         (do_interp(env1->get_turbulence_magnitude_norm(),
 759                    env2->get_turbulence_magnitude_norm(),
 760                    fraction));
 761
 762     result->set_turbulence_rate_hz
 763         (do_interp(env1->get_turbulence_rate_hz(),
 764                    env2->get_turbulence_rate_hz(),
 765                    fraction));
 766 }
 767
 768 // end of environment.cxx