Simplify and fix dewpoint calc

Beyond standardizing this calc on WMO8, dewpoint_from_specific_humidity
can be simplified to not require temperature as an argument. Add an
internal implementation to support this and preserve API compatibility.

src/metpy/calc/thermo.py

@@ -3871,21 +3871,16 @@ def static_stability(pressure, temperature, vertical_dim=0):
 
 
 @exporter.export
-@preprocess_and_wrap(
-    wrap_like='temperature',
-    broadcast=('pressure', 'temperature', 'specific_humidity')
-)
-@check_units('[pressure]', '[temperature]', '[dimensionless]')
-def dewpoint_from_specific_humidity(pressure, temperature, specific_humidity):
-    r"""Calculate the dewpoint from specific humidity, temperature, and pressure.
+def dewpoint_from_specific_humidity(*args, **kwargs):
+    r"""Calculate the dewpoint from specific humidity and pressure.
 
     Parameters
     ----------
     pressure: `pint.Quantity`
         Total atmospheric pressure
 
-    temperature: `pint.Quantity`
-        Air temperature
+    temperature: `pint.Quantity`, optional
+        Air temperature. Unused in calculation, pending deprecation.
 
     specific_humidity: `pint.Quantity`
         Specific humidity of air
@@ -3899,20 +3894,74 @@ def dewpoint_from_specific_humidity(pressure, temperature, specific_humidity):
     --------
     >>> from metpy.calc import dewpoint_from_specific_humidity
     >>> from metpy.units import units
-    >>> dewpoint_from_specific_humidity(1000 * units.hPa, 10 * units.degC, 5 * units('g/kg'))
-    <Quantity(3.73203192, 'degree_Celsius')>
+    >>> dewpoint_from_specific_humidity(1000 * units.hPa, 5 * units('g/kg'))
+    <Quantity(3.79314079, 'degree_Celsius')>
+
+    .. versionchanged:: 1.6
+       Made `temperature` arg optional, to be deprecated
 
     .. versionchanged:: 1.0
        Changed signature from ``(specific_humidity, temperature, pressure)``
 
     See Also
     --------
-    relative_humidity_from_mixing_ratio, dewpoint_from_relative_humidity
+    relative_humidity_from_mixing_ratio, dewpoint_from_relative_humidity, dewpoint
+
+    Notes
+    -----
+    Employs [WMO8]_ eq 4.A.6,
 
+    .. math:: e = \frac{w}{\epsilon + w} p
+
+    with
+
+    .. math:: w = \frac{q}{1-q}
+
+    where
+    * :math:`q` is specific humidity
+    * :math:`w` is mixing ratio
+    * :math:`\epsilon` is the molecular weight ratio of vapor to dry air
+
+    to calculate vapor partial pressure :math:`e` for dewpoint calculation input. See
+    :func:`~dewpoint` for additional information.
     """
-    return dewpoint_from_relative_humidity(temperature,
-                                           relative_humidity_from_specific_humidity(
-                                               pressure, temperature, specific_humidity))
+    pressure = kwargs.pop('pressure', None)
+    specific_humidity = kwargs.pop('specific_humidity', None)
+
+    return _dewpoint_from_specific_humidity(
+        pressure if pressure else args[0],
+        specific_humidity if specific_humidity else args[-1])
+
+
+@preprocess_and_wrap(
+    wrap_like='specific_humidity',
+    broadcast=('pressure', 'specific_humidity')
+)
+@check_units(pressure='[pressure]', specific_humidity='[dimensionless]')
+def _dewpoint_from_specific_humidity(pressure, specific_humidity):
+    r"""Calculate the dewpoint from specific humidity and pressure.
+
+    See :func:`~dewpoint_from_specific_humidity` for more information. This implementation
+    is provided internally to preserve backwards compatibility with MetPy<1.6.
+
+    Parameters
+    ----------
+    pressure: `pint.Quantity`
+        Total atmospheric pressure
+
+    specific_humidity: `pint.Quantity`
+        Specific humidity of air
+
+    Returns
+    -------
+    `pint.Quantity`
+        Dew point temperature
+
+    """
+    w = mixing_ratio_from_specific_humidity(specific_humidity)
+    e = pressure * w / (mpconsts.nounit.epsilon + w)
+
+    return dewpoint(e)
 
 
 @exporter.export

tests/calc/test_thermo.py

@@ -198,8 +198,8 @@ def test_moist_lapse_starting_points(start, direction):
     assert_almost_equal(temp, truth, 4)
 
 
-@pytest.mark.xfail(platform.machine() == 'aarch64',
-                   reason='ValueError is not raised on aarch64')
+@pytest.mark.xfail('arm' in platform.machine() or 'aarch' in platform.machine(),
+                   reason='ValueError is not raised on ARM architecture')
 @pytest.mark.xfail(sys.platform == 'win32', reason='solve_ivp() does not error on Windows')
 @pytest.mark.xfail(packaging.version.parse(scipy.__version__) < packaging.version.parse('1.7'),
                    reason='solve_ivp() does not error on Scipy < 1.7')
@@ -1816,6 +1816,33 @@ def test_dewpoint_specific_humidity_old_signature():
         dewpoint_from_specific_humidity(q, temperature, p)
 
 
+def test_dewpoint_specific_humidity_kwargs():
+    """Test kw-specified signature for backwards compatibility MetPy>=1.6."""
+    p = 1013.25 * units.mbar
+    temperature = 20. * units.degC
+    q = 0.012 * units.dimensionless
+    td = dewpoint_from_specific_humidity(
+        pressure=p, temperature=temperature, specific_humidity=q)
+    assert_almost_equal(td, 17.036 * units.degC, 3)
+
+
+def test_dewpoint_specific_humidity_mixed_args_kwargs():
+    """Test mixed arg, kwarg handling for backwards compatibility MetPy>=1.6."""
+    p = 1013.25 * units.mbar
+    temperature = 20. * units.degC
+    q = 0.012 * units.dimensionless
+    td = dewpoint_from_specific_humidity(p, temperature=temperature, specific_humidity=q)
+    assert_almost_equal(td, 17.036 * units.degC, 3)
+
+
+def test_dewpoint_specific_humidity_two_args():
+    """Test new signature, Temperature unneeded, MetPy>=1.6."""
+    p = 1013.25 * units.mbar
+    q = 0.012 * units.dimensionless
+    td = dewpoint_from_specific_humidity(p, q)
+    assert_almost_equal(td, 17.036 * units.degC, 3)
+
+
 def test_lfc_not_below_lcl():
     """Test sounding where LFC appears to be (but isn't) below LCL."""
     levels = np.array([1002.5, 1001.7, 1001., 1000.3, 999.7, 999., 998.2, 977.9,