def _temp_dew_dirint(temp_dew, times):
if temp_dew is not None:<EOL><INDENT>w = pd.Series(np.exp(<NUM_LIT> * temp_dew - <NUM_LIT>), index=times)<EOL><DEDENT>else:<EOL><INDENT>w = pd.Series(-<NUM_LIT:1>, index=times)<EOL><DEDENT>return w<EOL>
Calculate precipitable water from surface dew point temp (Perez eqn 4),
or return a default value for use with :py:func:`_dirint_bins`.