Despite all the planning in the world unless you live in a country that has perfect weather all year around you are always going to be at the mercy of the gods.
Fortunately most wedding planners, and the various professionals involved in arranging the different elements of the big day, are well used to the unreliability of sunshine and know all the tricks and alternatives that a bride and groom will ever need.
Wedding photographers will be well aware of the various options at your chosen destination. If it is too wet in the beautiful walled garden they will know an alternative, it is part of their job after all.
Never worry when you have paid someone to do that for you. Part of being an experienced photographer includes being versatile and making do with what you have.
If your mode of transport is an open top car, be aware of a drier alternative. Even elaborate horse carriages will have a cover just in case. They will have learn from experience and that is what you pay for and why you choose a reliable company to take care of this part of the day.
For those that live in a country or area prone to rain then the sensible option is to mainly keep things indoor and if you find that the sun does decide to shine, most wedding venues will accommodate a foray into the outdoors with your entourage on tow.
The big hotels, which are now very popular for weddings, cater for everything and they won’t be letting a bit of rain spoil anything. Their reputation depends on you, and your friends and family, having the best day possible.