fext = open('external.csv', 'a')
fint = open('internal.csv', 'a')

for df in pd.read_csv('todo.csv', parse_dates=['Measured At'],
                  low_memory=False, chunksize=500000):
    dfExt = df[df['Temperatura Exterior'].notnull()]
    dfInt = df[df['Temperatura Exterior'].isnull()]
    dfExt.to_csv(fext,header=False)
    dfInt.to_csv(fint,header=False)

fext.close();fint.close()