Merge pull request #46 from mbirth/feature_add_cloud_base

Added cloud base as suggested in Google group

src/pywws/conversions.py

@@ -269,6 +269,23 @@ def apparent_temp(temp, rh, wind):
         17.27 * temp / (237.7 + temp))
     return temp + (0.33 * vap_press) - (0.70 * wind) - 4.00
 
+def cloud_base(temp, hum):
+    """Calculate cumulus cloud base in metres, using formula from
+    https://en.wikipedia.org/wiki/Cloud_base or
+    https://de.wikipedia.org/wiki/Kondensationsniveau#Konvektionskondensationsniveau
+    """
+    if temp is None or hum is None:
+        return None
+    dew_pt = dew_point(temp, hum)
+    spread = float(temp) - dew_pt
+    return spread * 125.0
+
+def cloud_ft(m):
+    "Convert cloud base from metres to feet."
+    if m is None:
+        return None
+    return float(m) * 3.28084
+
 def _main(argv=None):
     global _winddir_text_array
     # run some simple tests
@@ -287,6 +304,10 @@ def _main(argv=None):
     for pts in range(16):
         print winddir_text(pts),
     print
+    print 'Cloud base in m and ft:'
+    for hum in range(25, 75, 5):
+        print "%8.3f m / %8.3f ft" % (cloud_base(15.0, hum), cloud_ft(cloud_base(15.0, hum)))
+    print
 
 if __name__ == "__main__":
     _main()

src/pywws/examples/templates/6hrs.txt

@@ -27,6 +27,7 @@
   <col align="char" char="m" />
   <col align="char" char="." />
   <col align="char" char="," />
+  <col align="char" char="m" />
   <tr>
     <th colspan="2" rowspan="2">Date &amp; time</th>
     <th rowspan="2">Temp</th>
@@ -35,6 +36,7 @@
     <th colspan="3">Wind</th>
     <th rowspan="2">Rain</th>
     <th rowspan="2">Pressure</th>
+    <th rowspan="2">Cloud base</th>
   </tr>
   <tr>
     <th>dir</th>
@@ -57,6 +59,7 @@
     <td>#wind_gust "%.0f mph" "" "wind_mph(x)"#</td>
     <td>#rain "%0.1f mm"#</td>
     <td>#rel_pressure "%.1f hPa"#, #pressure_trend "%s" "" "pressure_trend_text(x)"#</td>
+    <td>#calc "cloud_base(data['temp_out'], data['hum_out'])" "%.1f m"#</td>
   </tr>
 #jump 1#
 #endloop#