Add a few new WeatherHelper functions

src/devices/Common/Iot/Device/Common/WeatherHelper.cs

@@ -331,5 +331,84 @@ public static Pressure CalculateBarometricPressure(Pressure measuredPressure, Te
         }
 
         #endregion
+
+        /// <summary>
+        /// Simplified air density (not taking humidity into account)
+        /// </summary>
+        /// <param name="airPressure">Measured air pressure</param>
+        /// <param name="temperature">Measured temperature</param>
+        /// <returns>Approximate standard air density</returns>
+        public static Density CalculateAirDensity(Pressure airPressure, Temperature temperature)
+        {
+            double gasConstant = 287.058; // J / (kg * K), for dry air
+            var result = airPressure.Pascals / (gasConstant * temperature.Kelvins);
+            return Density.FromKilogramsPerCubicMeter(result);
+        }
+
+        /// <summary>
+        /// Calculates the air density
+        /// </summary>
+        /// <param name="airPressure">Measured air pressure</param>
+        /// <param name="temperature">Measured temperature</param>
+        /// <param name="humidity">Measured relative humidity</param>
+        /// <returns>Approximate standard air density at sea level</returns>
+        /// <remarks>From https://de.wikipedia.org/wiki/Luftdichte </remarks>
+        public static Density CalculateAirDensity(Pressure airPressure, Temperature temperature, RelativeHumidity humidity)
+        {
+            double rs = 287.058;
+            double rd = 461.523;
+            var pd = CalculateSaturatedVaporPressureOverWater(temperature);
+            // It's still called "constant" even though it's not constant
+            double gasConstant = rs / (1 - (humidity.Percent / 100) * (pd.Pascals / airPressure.Pascals) * (1 - (rs / rd)));
+            var result = airPressure.Pascals / (gasConstant * temperature.Kelvins);
+            return Density.FromKilogramsPerCubicMeter(result);
+        }
+
+        /// <summary>
+        /// Calculates the wind chill temperature - this is the perceived temperature in (heavy) winds at cold temperatures.
+        /// This is only useful at temperatures below about 20°C, above use <see cref="CalculateHeatIndex"/> instead.
+        /// Not suitable for wind speeds &lt; 5 km/h.
+        /// </summary>
+        /// <param name="temperature">The measured air temperature</param>
+        /// <param name="windSpeed">The wind speed (measured at 10m above ground)</param>
+        /// <returns>The perceived temperature. Note that this is not a real temperature, and the skin will never really reach
+        /// this temperature. This is more an indication on how fast the skin will reach the air temperature. If the skin
+        /// reaches a temperature of about -5°C, frostbite might occur.</returns>
+        /// <remarks>From https://de.wikipedia.org/wiki/Windchill </remarks>
+        /// <exception cref="ArgumentOutOfRangeException">The wind speed is less than zero</exception>
+        public static Temperature CalculateWindchill(Temperature temperature, Speed windSpeed)
+        {
+            if (windSpeed < Speed.Zero)
+            {
+                throw new ArgumentOutOfRangeException(nameof(windSpeed), "The wind speed cannot be negative");
+            }
+
+            double va = temperature.DegreesCelsius;
+            if (windSpeed < Speed.FromKilometersPerHour(1))
+            {
+                // otherwise, the result is complete crap, because the second and third terms of the equation are 0
+                windSpeed = Speed.FromKilometersPerHour(1);
+            }
+
+            double wct = 13.12 + 0.6215 * va + (0.3965 * va - 11.37) * Math.Pow(windSpeed.KilometersPerHour, 0.16);
+            return Temperature.FromDegreesCelsius(wct);
+        }
+
+        /// <summary>
+        /// Calculates the wind force on an object.
+        /// </summary>
+        /// <param name="densityOfAir">The denisty of the air, calculated using one of the overloads of <see cref="CalculateAirDensity(UnitsNet.Pressure,UnitsNet.Temperature)"/></param>
+        /// <param name="windSpeed">The speed of the wind</param>
+        /// <param name="pressureCoefficient">Pressure coefficient for the shape of the object. Use 1 for a rectangular object directly facing the wind</param>
+        /// <returns>The Pressure the wind applies on the object</returns>
+        /// <remarks>From https://de.wikipedia.org/wiki/Winddruck </remarks>
+        public static Pressure CalculateWindForce(Density densityOfAir, Speed windSpeed, double pressureCoefficient = 1.0)
+        {
+            double v = windSpeed.MetersPerSecond;
+            double rho = densityOfAir.KilogramsPerCubicMeter;
+
+            double wd = pressureCoefficient * rho / 2 * (v * v);
+            return Pressure.FromNewtonsPerSquareMeter(wd);
+        }
     }
 }

src/devices/Common/tests/WeatherTests.cs

@@ -185,5 +185,53 @@ public void GetRelativeHumidityFromActualAirTemperature(double inTemp, double in
 
             Assert.Equal(outHumidityExpected, result.Percent, 3);
         }
+
+        [Theory]
+        [InlineData(1013.25, 35, 1.1455)]
+        [InlineData(1013.25, 0, 1.2922)]
+        [InlineData(1013.25, -25, 1.4224)]
+        public void CalculateAirDensitySimple(double inPressure, double inTemp, double expected)
+        {
+            var result = WeatherHelper.CalculateAirDensity(Pressure.FromMillibars(inPressure),
+                Temperature.FromDegreesCelsius(inTemp));
+
+            Assert.Equal(expected, result.KilogramsPerCubicMeter, 4);
+        }
+
+        [Theory]
+        [InlineData(1013.25, 35, 0, 1.1455)]
+        [InlineData(1013.25, 35, 50, 1.1335)]
+        [InlineData(1013.25, 35, 100, 1.1214)]
+        public void CalculateAirDensity(double inPressure, double inTemp, double inHumidity, double expected)
+        {
+            var result = WeatherHelper.CalculateAirDensity(Pressure.FromMillibars(inPressure),
+                Temperature.FromDegreesCelsius(inTemp), RelativeHumidity.FromPercent(inHumidity));
+
+            Assert.Equal(expected, result.KilogramsPerCubicMeter, 4);
+        }
+
+        [Theory]
+        [InlineData(0, 0, 1.75)] // This is to be expected, see WP article
+        [InlineData(10, 5, 9.8)]
+        [InlineData(-10, 20, -17.9)]
+        [InlineData(-15, 40, -27.4)]
+        public void CalculateWindchill(double temperature, double windSpeed, double expected)
+        {
+            var result = WeatherHelper.CalculateWindchill(Temperature.FromDegreesCelsius(temperature),
+                Speed.FromKilometersPerHour(windSpeed));
+
+            Assert.Equal(expected, result.DegreesCelsius, 1);
+        }
+
+        [Theory]
+        [InlineData(20, 38.5, 1013, 68.83)]
+        [InlineData(20, 88.9, 1013, 367.0)]
+        public void CalculateWindForce(double temperature, double windSpeed, double pressure, double expected)
+        {
+            Density airDensity = WeatherHelper.CalculateAirDensity(Pressure.FromHectopascals(pressure),
+                Temperature.FromDegreesCelsius(temperature));
+            var density = WeatherHelper.CalculateWindForce(airDensity, Speed.FromKilometersPerHour(windSpeed), 1.0);
+            Assert.Equal(expected, density.NewtonsPerSquareMeter, 1);
+        }
     }
 }