Merge pull request #3851 from dcamron/qvec-example

Fix Q Vector example stability and scaling

Author: dopplershift

examples/calculations/QVector.py

@@ -12,6 +12,7 @@
 Dataset and plotting using Matplotlib.
 """
 import matplotlib.pyplot as plt
+import numpy as np
 
 import metpy.calc as mpcalc
 from metpy.cbook import example_data
@@ -26,7 +27,7 @@
 # Calculate the q-vectors. Passing in a fixed value of static stability, but could also
 # use `mpcalc.static_stability()`.
 u_qvect, v_qvect = mpcalc.q_vector(ds.uwind, ds.vwind, ds.temperature, 850 * units.hPa,
-                                   static_stability=0.02 * units('J / kg / Pa^2'))
+                                   static_stability=2e-6 * units('J / kg / Pa^2'))
 
 # start figure and set axis
 fig, ax = plt.subplots(figsize=(5, 5))
@@ -41,15 +42,22 @@
                  cmap=plt.cm.bwr, alpha=0.75)
 plt.colorbar(cf, pad=0, aspect=50)
 
+# calculate a scale length quantity of our vectors based on their mean
+scale = (np.hypot(u_qvect, v_qvect).mean() * np.sqrt(u_qvect.size)).data
+
 # plot Q-vectors as arrows, every other arrow
 qvec = ax.quiver(ds.lon.values[::2], ds.lat.values[::2],
-                 u_qvect[::2, ::2] * 1e13, v_qvect[::2, ::2] * 1e13,
-                 color='black', scale=1000, alpha=0.5, width=0.01)
+                 u_qvect[::2, ::2], v_qvect[::2, ::2],
+                 color='black', scale=scale.m, alpha=0.5, width=0.01)
+
+# calculate representative arrow for key
+key = scale / 10
 
-qk = ax.quiverkey(qvec, 0.8, 0.9, 200, r'$200 m^2/kg/s$', labelpos='E',
+# add key for arrow length
+qk = ax.quiverkey(qvec, 0.65, 0.905, key.m, f'{key:0.2~P}', labelpos='E',
                   coordinates='figure')
 
 ax.set(xlim=(260, 270), ylim=(30, 40))
-ax.set_title('Q-Vector Calculation')
+ax.set_title('Q-Vector Calculation', loc='left')
 
 plt.show()