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: environment.cxx,v 1.1 2009/01/30 15:07:04 jsd Exp $
  22
  23
  24 #ifdef HAVE_CONFIG_H
  25 #  include <config.h>
  26 #endif
  27
  28 #include <math.h>
  29
  30 #include <plib/sg.h>
  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     altitude_half_to_sun_m = 1000;
 142     altitude_tropo_top_m = 10000;
 143 #ifdef USING_TABLES
 144     _setup_tables();
 145 #endif
 146     _recalc_density();
 147     _recalc_relative_humidity();
 148
 149 }
 150
 151 FGEnvironment::FGEnvironment()
 152 {
 153     _init();
 154 }
 155
 156 FGEnvironment::FGEnvironment (const FGEnvironment &env)
 157 {
 158     _init();
 159     copy(env);
 160 }
 161
 162 FGEnvironment::~FGEnvironment()
 163 {
 164 }
 165
 166 void
 167 FGEnvironment::copy (const FGEnvironment &env)
 168 {
 169     elevation_ft = env.elevation_ft;
 170     visibility_m = env.visibility_m;
 171     temperature_sea_level_degc = env.temperature_sea_level_degc;
 172     temperature_degc = env.temperature_degc;
 173     dewpoint_sea_level_degc = env.dewpoint_sea_level_degc;
 174     dewpoint_degc = env.dewpoint_degc;
 175     pressure_sea_level_inhg = env.pressure_sea_level_inhg;
 176     wind_from_heading_deg = env.wind_from_heading_deg;
 177     wind_speed_kt = env.wind_speed_kt;
 178     wind_from_north_fps = env.wind_from_north_fps;
 179     wind_from_east_fps = env.wind_from_east_fps;
 180     wind_from_down_fps = env.wind_from_down_fps;
 181     turbulence_magnitude_norm = env.turbulence_magnitude_norm;
 182     turbulence_rate_hz = env.turbulence_rate_hz;
 183 }
 184
 185 static inline bool
 186 maybe_copy_value (FGEnvironment * env, const SGPropertyNode * node,
 187                   const char * name, void (FGEnvironment::*setter)(double))
 188 {
 189     const SGPropertyNode * child = node->getNode(name);
 190                                 // fragile: depends on not being typed
 191                                 // as a number
 192     if (child != 0 && child->hasValue() &&
 193         child->getStringValue()[0] != '\0') {
 194         (env->*setter)(child->getDoubleValue());
 195         return true;
 196     } else {
 197         return false;
 198     }
 199 }
 200
 201 void
 202 FGEnvironment::read (const SGPropertyNode * node)
 203 {
 204     maybe_copy_value(this, node, "visibility-m",
 205                      &FGEnvironment::set_visibility_m);
 206
 207     if (!maybe_copy_value(this, node, "temperature-sea-level-degc",
 208              &FGEnvironment::set_temperature_sea_level_degc))
 209         maybe_copy_value(this, node, "temperature-degc",
 210                          &FGEnvironment::set_temperature_degc);
 211
 212     if (!maybe_copy_value(this, node, "dewpoint-sea-level-degc",
 213              &FGEnvironment::set_dewpoint_sea_level_degc))
 214         maybe_copy_value(this, node, "dewpoint-degc",
 215                          &FGEnvironment::set_dewpoint_degc);
 216
 217     if (!maybe_copy_value(this, node, "pressure-sea-level-inhg",
 218                           &FGEnvironment::set_pressure_sea_level_inhg))
 219         maybe_copy_value(this, node, "pressure-inhg",
 220                          &FGEnvironment::set_pressure_inhg);
 221
 222     maybe_copy_value(this, node, "wind-from-heading-deg",
 223                      &FGEnvironment::set_wind_from_heading_deg);
 224
 225     maybe_copy_value(this, node, "wind-speed-kt",
 226                      &FGEnvironment::set_wind_speed_kt);
 227
 228     maybe_copy_value(this, node, "elevation-ft",
 229                      &FGEnvironment::set_elevation_ft);
 230
 231     maybe_copy_value(this, node, "turbulence/magnitude-norm",
 232                      &FGEnvironment::set_turbulence_magnitude_norm);
 233
 234     maybe_copy_value(this, node, "turbulence/rate-hz",
 235                      &FGEnvironment::set_turbulence_rate_hz);
 236 }
 237
 238
 239 double
 240 FGEnvironment::get_visibility_m () const
 241 {
 242   return visibility_m;
 243 }
 244
 245 double
 246 FGEnvironment::get_temperature_sea_level_degc () const
 247 {
 248   return temperature_sea_level_degc;
 249 }
 250
 251 double
 252 FGEnvironment::get_temperature_degc () const
 253 {
 254   return temperature_degc;
 255 }
 256
 257 double
 258 FGEnvironment::get_temperature_degf () const
 259 {
 260   return (temperature_degc * 9.0 / 5.0) + 32.0;
 261 }
 262
 263 double
 264 FGEnvironment::get_dewpoint_sea_level_degc () const
 265 {
 266   return dewpoint_sea_level_degc;
 267 }
 268
 269 double
 270 FGEnvironment::get_dewpoint_degc () const
 271 {
 272   return dewpoint_degc;
 273 }
 274
 275 double
 276 FGEnvironment::get_pressure_sea_level_inhg () const
 277 {
 278   return pressure_sea_level_inhg;
 279 }
 280
 281 double
 282 FGEnvironment::get_pressure_inhg () const
 283 {
 284   return pressure_inhg;
 285 }
 286
 287 double
 288 FGEnvironment::get_density_slugft3 () const
 289 {
 290   return density_slugft3;
 291 }
 292
 293 double
 294 FGEnvironment::get_relative_humidity () const
 295 {
 296   return relative_humidity;
 297 }
 298
 299 double
 300 FGEnvironment::get_density_tropo_avg_kgm3 () const
 301 {
 302   return density_tropo_avg_kgm3;
 303 }
 304
 305 double
 306 FGEnvironment::get_altitude_half_to_sun_m () const
 307 {
 308   return altitude_half_to_sun_m;
 309 }
 310
 311 double
 312 FGEnvironment::get_altitude_tropo_top_m () const
 313 {
 314   return altitude_tropo_top_m;
 315 }
 316
 317 double
 318 FGEnvironment::get_wind_from_heading_deg () const
 319 {
 320   return wind_from_heading_deg;
 321 }
 322
 323 double
 324 FGEnvironment::get_wind_speed_kt () const
 325 {
 326   return wind_speed_kt;
 327 }
 328
 329 double
 330 FGEnvironment::get_wind_from_north_fps () const
 331 {
 332   return wind_from_north_fps;
 333 }
 334
 335 double
 336 FGEnvironment::get_wind_from_east_fps () const
 337 {
 338   return wind_from_east_fps;
 339 }
 340
 341 double
 342 FGEnvironment::get_wind_from_down_fps () const
 343 {
 344   return wind_from_down_fps;
 345 }
 346
 347 double
 348 FGEnvironment::get_turbulence_magnitude_norm () const
 349 {
 350   if( sgEnviro.get_turbulence_enable_state() )
 351     if (fgGetBool("/environment/params/real-world-weather-fetch") == true)
 352       return sgEnviro.get_cloud_turbulence();
 353   return turbulence_magnitude_norm;
 354 }
 355
 356 double
 357 FGEnvironment::get_turbulence_rate_hz () const
 358 {
 359   return turbulence_rate_hz;
 360 }
 361
 362 double
 363 FGEnvironment::get_elevation_ft () const
 364 {
 365   return elevation_ft;
 366 }
 367
 368 void
 369 FGEnvironment::set_visibility_m (double v)
 370 {
 371   visibility_m = v;
 372 }
 373
 374 void
 375 FGEnvironment::set_temperature_sea_level_degc (double t)
 376 {
 377   temperature_sea_level_degc = t;
 378   if (dewpoint_sea_level_degc > t)
 379       dewpoint_sea_level_degc = t;
 380   _recalc_alt_pt();
 381   _recalc_density();
 382 }
 383
 384 void
 385 FGEnvironment::set_temperature_degc (double t)
 386 {
 387   temperature_degc = t;
 388   _recalc_sl_temperature();
 389   _recalc_sl_pressure();
 390   _recalc_alt_pt();
 391   _recalc_density();
 392   _recalc_relative_humidity();
 393 }
 394
 395 void
 396 FGEnvironment::set_dewpoint_sea_level_degc (double t)
 397 {
 398   dewpoint_sea_level_degc = t;
 399   if (temperature_sea_level_degc < t)
 400       temperature_sea_level_degc = t;
 401   _recalc_alt_dewpoint();
 402   _recalc_density();
 403 }
 404
 405 void
 406 FGEnvironment::set_dewpoint_degc (double t)
 407 {
 408   dewpoint_degc = t;
 409   _recalc_sl_dewpoint();
 410   _recalc_density();
 411   _recalc_relative_humidity();
 412 }
 413
 414 void
 415 FGEnvironment::set_pressure_sea_level_inhg (double p)
 416 {
 417   pressure_sea_level_inhg = p;
 418   _recalc_alt_pt();
 419   _recalc_density();
 420 }
 421
 422 void
 423 FGEnvironment::set_pressure_inhg (double p)
 424 {
 425   pressure_inhg = p;
 426   _recalc_sl_pressure();
 427   _recalc_density();
 428 }
 429
 430 void
 431 FGEnvironment::set_wind_from_heading_deg (double h)
 432 {
 433   wind_from_heading_deg = h;
 434   _recalc_ne();
 435 }
 436
 437 void
 438 FGEnvironment::set_wind_speed_kt (double s)
 439 {
 440   wind_speed_kt = s;
 441   _recalc_ne();
 442 }
 443
 444 void
 445 FGEnvironment::set_wind_from_north_fps (double n)
 446 {
 447   wind_from_north_fps = n;
 448   _recalc_hdgspd();
 449 }
 450
 451 void
 452 FGEnvironment::set_wind_from_east_fps (double e)
 453 {
 454   wind_from_east_fps = e;
 455   _recalc_hdgspd();
 456 }
 457
 458 void
 459 FGEnvironment::set_wind_from_down_fps (double d)
 460 {
 461   wind_from_down_fps = d;
 462   _recalc_hdgspd();
 463 }
 464
 465 void
 466 FGEnvironment::set_turbulence_magnitude_norm (double t)
 467 {
 468   turbulence_magnitude_norm = t;
 469 }
 470
 471 void
 472 FGEnvironment::set_turbulence_rate_hz (double r)
 473 {
 474   turbulence_rate_hz = r;
 475 }
 476
 477 void
 478 FGEnvironment::set_elevation_ft (double e)
 479 {
 480   elevation_ft = e;
 481   _recalc_alt_dewpoint();
 482   _recalc_alt_pt();
 483   _recalc_density();
 484   _recalc_relative_humidity();
 485 }
 486
 487 void
 488 FGEnvironment::set_altitude_half_to_sun_m (double alt)
 489 {
 490  altitude_half_to_sun_m = alt;
 491  _recalc_density_tropo_avg_kgm3();
 492 }
 493
 494 void
 495 FGEnvironment::set_altitude_tropo_top_m (double alt)
 496 {
 497  altitude_tropo_top_m = alt;
 498  _recalc_density_tropo_avg_kgm3();
 499 }
 500
 501
 502 void
 503 FGEnvironment::_recalc_hdgspd ()
 504 {
 505   double angle_rad;
 506
 507   if (wind_from_east_fps == 0) {
 508     angle_rad = (wind_from_north_fps >= 0 ? SGD_PI_2 : -SGD_PI_2);
 509   } else {
 510     angle_rad = atan(wind_from_north_fps/wind_from_east_fps);
 511   }
 512   wind_from_heading_deg = angle_rad * SGD_RADIANS_TO_DEGREES;
 513   if (wind_from_east_fps >= 0)
 514     wind_from_heading_deg = 90 - wind_from_heading_deg;
 515   else
 516     wind_from_heading_deg = 270 - wind_from_heading_deg;
 517
 518 #if 0
 519   // FIXME: Windspeed can become negative with these formulas.
 520   //        which can cause problems for animations that rely
 521   //        on the windspeed property.
 522   if (angle_rad == 0)
 523     wind_speed_kt = fabs(wind_from_east_fps
 524                          * SG_METER_TO_NM * SG_FEET_TO_METER * 3600);
 525   else
 526     wind_speed_kt = (wind_from_north_fps / sin(angle_rad))
 527       * SG_METER_TO_NM * SG_FEET_TO_METER * 3600;
 528 #else
 529   wind_speed_kt = sqrt(wind_from_north_fps * wind_from_north_fps +
 530                        wind_from_east_fps * wind_from_east_fps)
 531                   * SG_METER_TO_NM * SG_FEET_TO_METER * 3600;
 532 #endif
 533 }
 534
 535 void
 536 FGEnvironment::_recalc_ne ()
 537 {
 538   double speed_fps =
 539     wind_speed_kt * SG_NM_TO_METER * SG_METER_TO_FEET * (1.0/3600);
 540
 541   wind_from_north_fps = speed_fps *
 542     cos(wind_from_heading_deg * SGD_DEGREES_TO_RADIANS);
 543   wind_from_east_fps = speed_fps *
 544     sin(wind_from_heading_deg * SGD_DEGREES_TO_RADIANS);
 545 }
 546
 547 // Intended to help with the interpretation of METAR data,
 548 // not for random in-flight outside-air temperatures.
 549 void
 550 FGEnvironment::_recalc_sl_temperature ()
 551 {
 552
 553 #if 0
 554   {
 555     SG_LOG(SG_GENERAL, SG_DEBUG, "recalc_sl_temperature: using "
 556       << temperature_degc << " @ " << elevation_ft << " :: " << this);
 557   }
 558 #endif
 559
 560   if (elevation_ft >= ISA_def[1].height) {
 561     SG_LOG(SG_GENERAL, SG_ALERT, "recalc_sl_temperature: "
 562         << "valid only in troposphere, not " << elevation_ft);
 563     return;
 564   }
 565
 566 // Clamp: temperature of the stratosphere, in degrees C:
 567   double t_strato = ISA_def[1].temp - atmodel::freezing;
 568   if (temperature_degc < t_strato) temperature_sea_level_degc = t_strato;
 569   else temperature_sea_level_degc =
 570       temperature_degc - elevation_ft * ISA_def[0].lapse;
 571
 572 // Alternative implemenation:
 573 //  else temperature_sea_level_inhg = T_layer(0., elevation_ft * foot,
 574 //      pressure_inhg * inHg, temperature_degc + freezing, ISA_def[0].lapse) - freezing;
 575 }
 576
 577 void
 578 FGEnvironment::_recalc_sl_dewpoint ()
 579 {
 580                                 // 0.2degC/1000ft
 581                                 // FIXME: this will work only for low
 582                                 // elevations
 583   dewpoint_sea_level_degc = dewpoint_degc + (elevation_ft * .0002);
 584   if (dewpoint_sea_level_degc > temperature_sea_level_degc)
 585     dewpoint_sea_level_degc = temperature_sea_level_degc;
 586 }
 587
 588 void
 589 FGEnvironment::_recalc_alt_dewpoint ()
 590 {
 591                                 // 0.2degC/1000ft
 592                                 // FIXME: this will work only for low
 593                                 // elevations
 594   dewpoint_degc = dewpoint_sea_level_degc + (elevation_ft * .0002);
 595   if (dewpoint_degc > temperature_degc)
 596     dewpoint_degc = temperature_degc;
 597 }
 598
 599 void
 600 FGEnvironment::_recalc_sl_pressure ()
 601 {
 602   using namespace atmodel;
 603 #if 0
 604   {
 605     SG_LOG(SG_GENERAL, SG_ALERT, "recalc_sl_pressure: using "
 606       << pressure_inhg << " and "
 607       << temperature_degc << " @ " << elevation_ft << " :: " << this);
 608   }
 609 #endif
 610   pressure_sea_level_inhg = P_layer(0., elevation_ft * foot,
 611       pressure_inhg * inHg, temperature_degc + freezing, ISA_def[0].lapse) / inHg;
 612 }
 613
 614 // This gets called at frame rate, to account for the aircraft's
 615 // changing altitude.
 616 // Called by set_elevation_ft() which is called by FGEnvironmentMgr::update
 617
 618 void
 619 FGEnvironment::_recalc_alt_pt ()
 620 {
 621   using namespace atmodel;
 622 #if 0
 623   {
 624     static int count(0);
 625     if (++count % 1000 == 0) {
 626       SG_LOG(SG_GENERAL, SG_ALERT,
 627            "recalc_alt_pt for: " << elevation_ft
 628         << "  using "  << pressure_sea_level_inhg
 629         << "  and "  << temperature_sea_level_degc
 630         << " :: " << this
 631         << "  # " << count);
 632         ///////////////////////////////////raise(SIGUSR1);
 633     }
 634   }
 635 #endif
 636   double press, temp;
 637   make_tuple(ref(press), ref(temp)) = PT_vs_hpt(elevation_ft * foot,
 638         pressure_sea_level_inhg * inHg, temperature_sea_level_degc + freezing);
 639   temperature_degc = temp - freezing;
 640   pressure_inhg = press / inHg;
 641 }
 642
 643 void
 644 FGEnvironment::_recalc_density ()
 645 {
 646   double pressure_psf = pressure_inhg * 70.7487;
 647
 648   // adjust for humidity
 649   // calculations taken from USA Today (oops!) at
 650   // http://www.usatoday.com/weather/basics/density-calculations.htm
 651   double temperature_degk = temperature_degc + 273.15;
 652   double pressure_mb = pressure_inhg * 33.86;
 653   double vapor_pressure_mb =
 654     6.11 * pow(10.0, 7.5 * dewpoint_degc / (237.7 + dewpoint_degc));
 655   double virtual_temperature_degk = temperature_degk / (1 - (vapor_pressure_mb / pressure_mb) * (1.0 - 0.622));
 656   double virtual_temperature_degr = virtual_temperature_degk * 1.8;
 657
 658   density_slugft3 = pressure_psf / (virtual_temperature_degr * 1718);
 659   _recalc_density_tropo_avg_kgm3();
 660 }
 661
 662 // This is used to calculate the average density on the path
 663 // of sunlight to the observer for calculating sun-color
 664 void
 665 FGEnvironment::_recalc_density_tropo_avg_kgm3 ()
 666 {
 667   double pressure_mb = pressure_inhg * 33.86;
 668   double vaporpressure = 6.11 * pow(10.0, ((7.5 * dewpoint_degc) / (237.7 + dewpoint_degc)));
 669
 670   double virtual_temp = (temperature_degc + 273.15) / (1 - 0.379 * (vaporpressure/pressure_mb));
 671
 672   double density_half = (100 * pressure_mb * exp(-altitude_half_to_sun_m / 8000))
 673       / (287.05 * virtual_temp);
 674   double density_tropo = (100 * pressure_mb * exp((-1 * altitude_tropo_top_m) / 8000))
 675       / ( 287.05 * virtual_temp);
 676
 677   density_tropo_avg_kgm3 = ((density_slugft3 * 515.379) + density_half + density_tropo) / 3;
 678 }
 679
 680 void
 681 FGEnvironment::_recalc_relative_humidity ()
 682 {
 683   double vaporpressure = 6.11 * pow(10.0, ((7.5 * dewpoint_degc) / ( 237.7 + dewpoint_degc)));
 684   double sat_vaporpressure = 6.11 * pow(10.0, ((7.5 * temperature_degc)
 685       / ( 237.7 + temperature_degc)) );
 686   relative_humidity = 100 * vaporpressure / sat_vaporpressure ;
 687 }
 688
 689
 690
 691 ////////////////////////////////////////////////////////////////////////
 692 // Functions.
 693 ////////////////////////////////////////////////////////////////////////
 694
 695 static inline double
 696 do_interp (double a, double b, double fraction)
 697 {
 698     double retval = (a + ((b - a) * fraction));
 699     return retval;
 700 }
 701
 702 static inline double
 703 do_interp_deg (double a, double b, double fraction)
 704 {
 705     a = fmod(a, 360);
 706     b = fmod(b, 360);
 707     if (fabs(b-a) > 180) {
 708         if (a < b)
 709             a += 360;
 710         else
 711             b += 360;
 712     }
 713     return fmod(do_interp(a, b, fraction), 360);
 714 }
 715
 716 void
 717 interpolate (const FGEnvironment * env1, const FGEnvironment * env2,
 718              double fraction, FGEnvironment * result)
 719 {
 720     result->set_visibility_m
 721         (do_interp(env1->get_visibility_m(),
 722                    env2->get_visibility_m(),
 723                    fraction));
 724
 725     result->set_temperature_sea_level_degc
 726         (do_interp(env1->get_temperature_sea_level_degc(),
 727                    env2->get_temperature_sea_level_degc(),
 728                    fraction));
 729
 730     result->set_dewpoint_degc
 731         (do_interp(env1->get_dewpoint_sea_level_degc(),
 732                    env2->get_dewpoint_sea_level_degc(),
 733                    fraction));
 734
 735     result->set_pressure_sea_level_inhg
 736         (do_interp(env1->get_pressure_sea_level_inhg(),
 737                    env2->get_pressure_sea_level_inhg(),
 738                    fraction));
 739
 740     result->set_wind_from_heading_deg
 741         (do_interp_deg(env1->get_wind_from_heading_deg(),
 742                        env2->get_wind_from_heading_deg(),
 743                        fraction));
 744
 745     result->set_wind_speed_kt
 746         (do_interp(env1->get_wind_speed_kt(),
 747                    env2->get_wind_speed_kt(),
 748                    fraction));
 749
 750     result->set_elevation_ft
 751         (do_interp(env1->get_elevation_ft(),
 752                    env2->get_elevation_ft(),
 753                    fraction));
 754
 755     result->set_turbulence_magnitude_norm
 756         (do_interp(env1->get_turbulence_magnitude_norm(),
 757                    env2->get_turbulence_magnitude_norm(),
 758                    fraction));
 759
 760     result->set_turbulence_rate_hz
 761         (do_interp(env1->get_turbulence_rate_hz(),
 762                    env2->get_turbulence_rate_hz(),
 763                    fraction));
 764 }
 765
 766 // end of environment.cxx