Add K Index calculate function. (#1990)

* Add K Index calculation function.

docs/_templates/overrides/metpy.calc.rst

@@ -72,6 +72,7 @@ Soundings
       cape_cin
       critical_angle
       el
+      k_index
       lcl
       lfc
       lifted_index

docs/api/references.rst

@@ -83,6 +83,9 @@ References
 .. [Garratt1994] Garratt, J.R., 1994: *The Atmospheric Boundary Layer*. Cambridge
            University Press, 316 pp.
 
+.. [George1960] George, J.J., 1960: *Weather Forecasting for Aeronautics*. New York and
+           London Academic Press, 673 pp.
+
 .. [Hobbs1977] Hobbs, P. V., and J. M. Wallace, 1977: *Atmospheric Science: An
            Introductory Survey*. Academic Press, 350 pp.
 

src/metpy/calc/thermo.py

@@ -3266,6 +3266,55 @@ def lifted_index(pressure, temperature, parcel_profile):
     return lifted_index
 
 
+@exporter.export
+@preprocess_and_wrap()
+@check_units('[pressure]', '[temperature]', '[temperature]')
+def k_index(pressure, temperature, dewpoint):
+    """Calculate K Index from the pressure temperature and dewpoint.
+
+    K Index formula derived from [George1960]_:
+    K = (T850 - T500) + Td850 - (T700 - Td700)
+
+    where:
+
+    T850 is the temperature at 850 hPa
+    T700 is the temperature at 700 hPa
+    T500 is the temperature at 500 hPa
+    Td850 is the dewpoint at 850 hPa
+    Td700 is the dewpoint at 700 hPa
+
+    Calculation of the K Index is defined as the temperature difference between
+    the static instability between 850 hPa and 500 hPa, add with the moisture
+    at 850hPa, then subtract from the dryness of the airmass at 700 hPa.
+
+    Parameters
+    ----------
+    pressure : `pint.Quantity`
+        Pressure level(s), in order from highest to lowest pressure
+
+    temperature : `pint.Quantity`
+        Temperature corresponding to pressure
+
+    dewpoint : `pint.Quantity`
+        Dewpoint temperature corresponding to pressure
+
+    Returns
+    -------
+    `pint.Quantity`
+        K Index
+
+    """
+    # Find temperature and dewpoint at 850, 700 and 500 hPa
+    (t850, t700, t500), (td850, td700, _) = interpolate_1d(units.Quantity([850, 700, 500],
+                                                           'hPa'), pressure, temperature,
+                                                           dewpoint)
+
+    # Calculate k index.
+    k_index = ((t850 - t500) + td850 - (t700 - td700)).to(units.degC)
+
+    return k_index
+
+
 @exporter.export
 @add_vertical_dim_from_xarray
 @preprocess_and_wrap(

tests/calc/test_thermo.py

@@ -12,9 +12,9 @@
                         dewpoint_from_relative_humidity, dewpoint_from_specific_humidity,
                         dry_lapse, dry_static_energy, el, equivalent_potential_temperature,
                         exner_function, gradient_richardson_number, InvalidSoundingError,
-                        isentropic_interpolation, isentropic_interpolation_as_dataset, lcl,
-                        lfc, lifted_index, mixed_layer, mixed_layer_cape_cin, mixed_parcel,
-                        mixing_ratio, mixing_ratio_from_relative_humidity,
+                        isentropic_interpolation, isentropic_interpolation_as_dataset, k_index,
+                        lcl, lfc, lifted_index, mixed_layer, mixed_layer_cape_cin,
+                        mixed_parcel, mixing_ratio, mixing_ratio_from_relative_humidity,
                         mixing_ratio_from_specific_humidity, moist_lapse, moist_static_energy,
                         most_unstable_cape_cin, most_unstable_parcel, parcel_profile,
                         parcel_profile_with_lcl, parcel_profile_with_lcl_as_dataset,
@@ -1871,6 +1871,30 @@ def test_lifted_index():
     assert_almost_equal(li, -7.9176350 * units.delta_degree_Celsius, 2)
 
 
+def test_k_index():
+    """Test the K Index calculation."""
+    pressure = np.array([1014., 1000., 997., 981.2, 947.4, 925., 914.9, 911.,
+                         902., 883., 850., 822.3, 816., 807., 793.2, 770.,
+                         765.1, 753., 737.5, 737., 713., 700., 688., 685.,
+                         680., 666., 659.8, 653., 643., 634., 615., 611.8,
+                         566.2, 516., 500., 487., 484.2, 481., 475., 460.,
+                         400.]) * units.hPa
+    temperature = np.array([24.2, 24.2, 24., 23.1, 21., 19.6, 18.7, 18.4,
+                            19.2, 19.4, 17.2, 15.3, 14.8, 14.4, 13.4, 11.6,
+                            11.1, 10., 8.8, 8.8, 8.2, 7., 5.6, 5.6,
+                            5.6, 4.4, 3.8, 3.2, 3., 3.2, 1.8, 1.5,
+                            -3.4, -9.3, -11.3, -13.1, -13.1, -13.1, -13.7, -15.1,
+                            -23.5]) * units.degC
+    dewpoint = np.array([23.2, 23.1, 22.8, 22., 20.2, 19., 17.6, 17.,
+                         16.8, 15.5, 14., 11.7, 11.2, 8.4, 7., 4.6,
+                         5., 6., 4.2, 4.1, -1.8, -2., -1.4, -0.4,
+                         -3.4, -5.6, -4.3, -2.8, -7., -25.8, -31.2, -31.4,
+                         -34.1, -37.3, -32.3, -34.1, -37.3, -41.1, -37.7, -58.1,
+                         -57.5]) * units.degC
+    ki = k_index(pressure, temperature, dewpoint)
+    assert_almost_equal(ki, 33.5 * units.degC, 2)
+
+
 def test_gradient_richardson_number():
     """Test gradient Richardson number calculation."""
     theta = units('K') * np.asarray([254.5, 258.3, 262.2])