Final answer:
The complement of the probability of a thunderstorm is 7/9.
Step-by-step explanation:
The complement of an event is the probability of the event not occurring. In this case, the complement represents the probability of it not thunderstorming today. Since the probability of a thunderstorm is 2/9, the probability of it not thunderstorming is 1 - 2/9 = 7/9.