We had a very outdoorsy July wedding in a barn (DH's idea), and we had ceremony in a clearing next to it - due to it being a Jewish wedding, the license was with the rabbi rather than the venue so we could be more flexible. I insisted on having a marquee over the clearing as I thought the barn itself would be a bit too tight for the ceremony in the event of rain.
And thank God, because it was summer 2007 and basically there were floods all over the shop. The venue claimed they'd never had a day 'rained off', but it did rain the whole day before, although the forecast didn't look too bad the next day.
Just as the ceremony was due to start, and I was sitting in my dad's car, the heavens opened with torrential rain. I thought 'OK, it won't rain this hard for that long.'
But it kept going and I thought 'This isn't funny anymore'... then I saw everyone scurrying around with umbrellas and then I thought 'Sod it, it is funny actually'.
Just as we were ready to begin, it stopped, and it stayed dry albeit cloudy the rest of the day. The rabbi said that traditionally rain is thought of as very auspicious for Jewish weddings.
Honestly unless you've planned the whole thing to be outdoors (always unwise with a British wedding) the weather really won't matter. The rain actually made ours even more memorable I think.