Yes it was pretty easy, ds was very vocal about what had upset him, not understanding what the teacher was saying or what he was supposed to be doing and getting no work done. He had a 3 day build up to this ie telling me for 3 days that he doesnt know what to think of in Literacy, doesnt do any work, then gets a detention, before he broke down 
All i can do for the moment is a big hug and try to take his mind off of it, im in the process of trying to deal with the root cause, school, who are denying that he has any difficulties. Still thats another story!
But with your problem ds used to have a real water phobia too, not so much rain, but just water in general.
We went to a sensory swimming pool for 3 days, then swimming very slowly, some days we never even got into the pool! This summer ds had a paddling pool in the garden for the first time ever.
So gradually exposing ds more to water really helped.
Rain is very unpredictable isnt it and a sudden change? Could you check the weather report each day, so if it is going to rain, your dd expects it? This might help to reduce a bit of anxiety.
An umbrella that your dd knows the whereabouts of at all times, in case it does?
A written plan, ie we check the forecast, if it rains we stay inside if possible, if not we use the umbrella, if doesnt we dont kind of thing?