added dewpoint_array for deriving dewpoint

metobs/data/__init__.py

@@ -10,7 +10,7 @@ import math
 from datetime import timedelta
 
 import fpconst
-from numpy import array, average, zeros, radians, degrees, arctan2, sin, cos, ndarray
+from numpy import array, average, zeros, radians, degrees, arctan2, sin, cos, ndarray, log
 from numpy.ma import masked_array, average as masked_average, MaskedArray, masked_where, column_stack
 from pydap.client import open_url as dapopen
 
@@ -166,6 +166,19 @@ def dewpoint(tempC, relhum):
 
     return min(dp - 273.15, tempC)
 
+def dewpoint_array(tempC, relhum):
+    """Equivalent to the dewpoint function, but operates on numpy arrays.
+
+    :param tempC: temperature in celsius
+    :param relhum: relative humidity as a percentage
+    """
+    gasconst = 461.5
+    latheat = 2500800.0
+    
+    dp = 1.0 / ( 1.0 / ( 273.15 + tempC ) - gasconst * log( (0.0 + relhum) / 100 ) / \
+        ( latheat - tempC * 2397.5 ))
+    return numpy.minimum(dp - 273.15, tempC)
+
 def relhum(airTempK, dewpointTempK):
     """
     Algorithm derived by David Hoese from the above