Python: added Atmosphere function.

source/python/README.md

@@ -302,6 +302,19 @@ to iterate through consecutive alternating perigees and apogees.
 
 ---
 
+<a name="AtmosphereInfo"></a>
+### class AtmosphereInfo
+
+**Information about idealized atmospheric variables at a given elevation.**
+
+| Type | Attribute | Description |
+| --- | --- | --- |
+| `float` | `pressure` | Atmospheric pressure in pascals. |
+| `float` | `temperature` | Atmospheric temperature in kelvins. |
+| `float` | `density` | Atmospheric densitive relative to sea level. |
+
+---
+
 <a name="AxisInfo"></a>
 ### class AxisInfo
 
@@ -1317,6 +1330,28 @@ and the specified body as seen from the center of the Earth.
 
 ---
 
+<a name="Atmosphere"></a>
+### Atmosphere(elevationMeters: float) &#8594; [`AtmosphereInfo`](#AtmosphereInfo)
+
+**Calculates U.S. Standard Atmosphere (1976) variables as a function of elevation.**
+
+This function calculates idealized values of pressure, temperature, and density
+using the U.S. Standard Atmosphere (1976) model.
+1. COESA, U.S. Standard Atmosphere, 1976, U.S. Government Printing Office, Washington, DC, 1976.
+2. Jursa, A. S., Ed., Handbook of Geophysics and the Space Environment, Air Force Geophysics Laboratory, 1985.
+See:
+https://hbcp.chemnetbase.com/faces/documents/14_12/14_12_0001.xhtml
+https://ntrs.nasa.gov/api/citations/19770009539/downloads/19770009539.pdf
+https://www.ngdc.noaa.gov/stp/space-weather/online-publications/miscellaneous/us-standard-atmosphere-1976/us-standard-atmosphere_st76-1562_noaa.pdf
+
+| Type | Parameter | Description |
+| --- | --- | --- |
+| `float` | `elevationMeters` | The elevation above sea level at which to calculate atmospheric variables. Must be in the range -500 to +100000, or an exception will occur. |
+
+**Returns**: [`AtmosphereInfo`](#AtmosphereInfo)
+
+---
+
 <a name="BackdatePosition"></a>
 ### BackdatePosition(time: [`Time`](#Time), observerBody: [`Body`](#Body), targetBody: [`Body`](#Body), aberration: bool) &#8594; [`Vector`](#Vector)
 

source/python/astronomy/astronomy.py

@@ -5761,6 +5761,72 @@ def NextMoonQuarter(mq: MoonQuarter) -> MoonQuarter:
     return next_mq
 
 
+class AtmosphereInfo:
+    """Information about idealized atmospheric variables at a given elevation.
+
+    Attributes
+    ----------
+    pressure : float
+        Atmospheric pressure in pascals.
+    temperature : float
+        Atmospheric temperature in kelvins.
+    density : float
+        Atmospheric densitive relative to sea level.
+    """
+    def __init__(self, pressure:float, temperature:float, density:float) -> None:
+        self.pressure = pressure
+        self.temperature = temperature
+        self.density = density
+
+    def __repr__(self) -> str:
+        return 'AtmosphereInfo({} Pa, {} K, {})'.format(self.pressure, self.temperature, self.density)
+
+
+def Atmosphere(elevationMeters: float) -> AtmosphereInfo:
+    """Calculates U.S. Standard Atmosphere (1976) variables as a function of elevation.
+
+    This function calculates idealized values of pressure, temperature, and density
+    using the U.S. Standard Atmosphere (1976) model.
+    1. COESA, U.S. Standard Atmosphere, 1976, U.S. Government Printing Office, Washington, DC, 1976.
+    2. Jursa, A. S., Ed., Handbook of Geophysics and the Space Environment, Air Force Geophysics Laboratory, 1985.
+    See:
+    https://hbcp.chemnetbase.com/faces/documents/14_12/14_12_0001.xhtml
+    https://ntrs.nasa.gov/api/citations/19770009539/downloads/19770009539.pdf
+    https://www.ngdc.noaa.gov/stp/space-weather/online-publications/miscellaneous/us-standard-atmosphere-1976/us-standard-atmosphere_st76-1562_noaa.pdf
+
+    Parameters
+    ----------
+    elevationMeters : float
+        The elevation above sea level at which to calculate atmospheric variables.
+        Must be in the range -500 to +100000, or an exception will occur.
+
+    Returns
+    -------
+    AtmosphereInfo
+    """
+    P0 = 101325.0     # pressure at sea level [pascals]
+    T0 = 288.15       # temperature at sea level [kelvins]
+    T1 = 216.65       # temperature between 20 km and 32 km [kelvins]
+
+    if elevationMeters < -500.0 or elevationMeters > 100000.0:
+        raise Error('Invalid elevationMeters value: {}'.format(elevationMeters))
+
+    if elevationMeters <= 11000.0:
+        temperature = T0 - 0.0065*elevationMeters
+        pressure = P0 * (T0 / temperature)**(-5.25577)
+    elif elevationMeters <= 20000.0:
+        temperature = T1
+        pressure = 22632.0 * math.exp(-0.00015768832 * (elevationMeters - 11000.0))
+    else:
+        temperature = T1 + 0.001*(elevationMeters - 20000.0)
+        pressure = 5474.87 * (T1 / temperature)**(34.16319)
+
+    # The density is calculated relative to the sea level value.
+    # Using the ideal gas law PV=nRT, we deduce that density is proportional to P/T.
+    density = (pressure / temperature) / (P0 / T0)
+    return AtmosphereInfo(pressure, temperature, density)
+
+
 class IlluminationInfo:
     """Information about the brightness and illuminated shape of a celestial body.