Corolla Start up in the rain

Hello I have a '99 Corolla VE, 75K on it. Bought the car a week ago, its been driving very well, starts up very nicely. However, today it was raining, I go to start the car but as I turned the key the engine did not turn (no sound from engine or starter). However, the a/c air comes on, I get a check engine&battery light. I was like O Crap :crying:. The key stays in the position it would stay when the car is on, and if I turn it off a/c air and the indications turn off.

I tried 3 more times without any luck. Took a look at the hook, battery looked dry. Finally I tried again, and it started right up. This is my first car so don't have much experience. Was wondering if guys can tell me what may be the issue, the usually (and still does) sound nicely when it starts up except for the time in the rain.