Python - ufunc 'add' did not contain a loop with signature matching types dtype('<U77') dtype('<U77') dtype('

import numpy as np

import urllib.request as urllib2

from bs4 import BeautifulSoup

año = np.arange(1960, 2017,1)

mes = np.arange(1,13)

dia = np.arange(1,32)

hora = np.arange(1,25)

stn = '78954' #estación meteorológica

for i in  np.nditer(año):

    for j in  np.nditer(mes):

        for k in  np.nditer(dia):

            url = 'http://weather.uwyo.edu/cgi-bin/sounding?region=naconf&TYPE=TEXT%3ALIST&YEAR='+año+'&MONTH='+mes+'&FROM='+dia+hora+'&TO='+dia+hora+'&STNM='+stn

            content = urllib2.urlopen(url).read()

            soup = BeautifulSoup(content)

            data_text = soup.get_text()

            splitted = data_text.split("\n",data_text.count("\n"))

            Sounding_filename = str(stn)+'.'+str(año)+str(mes)+str(dia)+str(hora)+'.txt'

            f = open(Sounding_filename,'w')

            for line in splitted[4:]:

                f.write(line+'\n')

                f.close()

TypeError                                 Traceback (most recent call last)

<ipython-input-9-33d4330c2f14> in <module>()

      2     for j in  np.nditer(mes):

      3         for k in  np.nditer(dia):

----> 4             url = 'http://weather.uwyo.edu/cgi-bin/sounding?region=naconf&TYPE=TEXT%3ALIST&YEAR='+año+'&MONTH='+mes+'&FROM='+dia+hora+'&TO='+dia+hora+'&STNM='+stn

      5             content = urllib2.urlopen(url).read()

      6             soup = BeautifulSoup(content)

TypeError: ufunc 'add' did not contain a loop with signature matching types dtype('<U77') dtype('<U77') dtype('<U77')