A couple of slight modifications will help:
def open_temperature_file(filename):
full_text = []
csv_file = open(filename, 'r')
reader = csv.reader(csv_file)
for line in reader:
full_text.append(line)
return full_text[3:] # return lines, starting from line 3
On the return above, I used a list slice to discard the first couple of lines.