import numpy

%pylab inline

ifile = open('sample.txt', 'r')
lines = ifile.readlines()

lines

lines[1]    

lines[1][59:62]

a = lines[1][59:62]

a == '   '

len(lines)

data = numpy.empty((16,12))

data.fill(9999)
data = data.astype('|S4' )

data

for year, line in enumerate(lines):
    data[year,0] = line[12:16]
    data[year,1] = line[18:22]
    data[year,2] = line[24:28]
    data[year,3] = line[30:34]
    data[year,4] = line[36:40]
    data[year,5] = line[42:46]
    data[year,6] = line[48:52]
    data[year,7] = line[54:58]
    data[year,8] = line[60:64]
    data[year,9] = line[66:70]
    data[year,10] =  line[72:76]
    data[year,11] = line[78:82]

data

data[data=='    '] = '-999'
Теперь массив выглядит так:
data

data = data.astype('float')

data

plot(data[0,:])

plot(data[1,:])

data_miss = numpy.ma.masked_equal(data, -999)

plot(data_miss[1,:])

data_miss[1,:].mean()

import pandas
pandas.__version__

pd = pandas.read_fwf('sample.txt',colspecs=[(6,10),(12,16), (18,22), (24,28),
                                            (30,34), (36,40), (42,46), (48,52), 
                                            (54,58), (60, 64), (66,70), (72, 76), 
                                            (78,82) ], names=['Year', 'JAN', 'FEB',
                                            'MAR', 'APR', 'MAY', 'JUN', 'JUL', 'AUG',
                                            'SEP', 'OCT', 'NOV', 'DEC'], 
                                            parse_dates=['Year'], index_col=['Year'] )

pd

pd.JAN.plot()

pd.ix['1980':'1982'].plot(kind='barh')

pd.ix[1].plot()