make requested changes

src/metpy/calc/thermo.py

@@ -1654,11 +1654,18 @@ def wetbulb_potential_temperature(pressure, temperature, dewpoint):
     r"""Calculate wet-bulb potential temperature.
 
     This calculation must be given an air parcel's pressure, temperature, and dewpoint.
-    The implementation uses the formula outlined in [DaviesJones2008]_:
-
+    The implementation uses the formula outlined in [DaviesJones2008]_.
     First, theta-e is calculated
     then use the formula from [DaviesJones2008]_
 
+    .. math:: \theta_w = \theta_e -
+        exp(\frac{a_0 + a_1 x + a_2 x^2 + a_3 x^3 + a_4 x^4}
+            {1 + b_1 x + b_2 x^2 + b_3 x^3 + b_4 x^4})
+
+    where :math:`x = \theta_e / 273.15 K`.
+
+    When :math:`\theta_e <= -173.15 K` then :math:`\theta_w = \theta_e`.
+
     Parameters
     ----------
     pressure: `pint.Quantity`
@@ -1676,11 +1683,16 @@ def wetbulb_potential_temperature(pressure, temperature, dewpoint):
         wet-bulb potential temperature of the parcel
 
     """
-    theta_e = equivalent_potential_temperature(pressure, temperature, dewpoint).magnitude
-    x = theta_e / 273.15
-    a = 7.101574 - 20.68208 * x + 16.11182 * x ** 2 + 2.574631 * x ** 3 - 5.205688 * x ** 4
-    b = 1 - 3.552497 * x + 3.781782 * x ** 2 - 0.6899655 * x ** 3 - 0.5929340 * x ** 4
-    return units.Quantity(theta_e - np.exp((a) / (b)), 'kelvin')
+    theta_e = equivalent_potential_temperature(pressure, temperature, dewpoint)
+    x = theta_e.m_as('kelvin') / 273.15
+    x2 = x * x
+    x3 = x2 * x
+    x4 = x2 * x2
+    a = 7.101574 - 20.68208 * x + 16.11182 * x2 + 2.574631 * x3 - 5.205688 * x4
+    b = 1 - 3.552497 * x + 3.781782 * x2 - 0.6899655 * x3 - 0.5929340 * x4
+
+    theta_w = units.Quantity(theta_e.m_as('kelvin') - np.exp(a / b), 'kelvin')
+    return np.where(theta_e <= units.Quantity(173.15, 'kelvin'), theta_e, theta_w)
 
 
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