Rain love it or hate it we need it, without it, we would not survive! For me, it feeds the soul and of course Mother Nature as well. It does get a tat annoying when day after day the rain just keeps falling and garden pots with beloved herbs inside constantly have their feet in wet cold soil, some seem to like others not so much, those that don't, generally have several stones or broken pots to help with the draining of the relentless rain. As a fair-weather gardener, you catch me out in the garden doing very much. Although the power of the rain we must remember is magical, a powerful magical ingredient falling from the sky into your hands, you can use rainwater anywhere you might use regular water.
I will wash my doors and windows with water containing Rosemary and Sage and of course, on a full moon my crystals go out to say hello to the powerful energies of the full moon and I place a bowl out to make my full moon water too, often, I will add crystals to my moon water, but some crystals like Selenite don't hold up well to moisture, so be sure to double-check before giving any of your favourite crystals a moon bath if it raining! Your magical tools can also be cleared/cleansed that you know are safe to be cleaned with water. Rainwater may be magical, but it can cause harm to your beloved items, using a water butt collects this powerful element free of charge, always a bonus right!
Many witches like to keep representations of each element on their altar, and a small vessel of rainwater is perfect for bringing in the water element. You could use any rainwater for this if it's a general working altar, or you could collect rainwater under a certain astrological influence, like a phase of the moon or a solar season to suit it more specifically to your needs.
Of course, if you are brave enough and you don’t worry about what others think if you live in a build-up area, run out, and take that opportunity of a free shower, the summer rain would probably be better unless you don’t mind freezing your butt off!
I think there are types of rain that give us different energies for example: -
Water from a gentle, sunny rain shower: cleansing, purification, growth, joy.
Water from a moody downpour, no thunder/lightning: cleansing, dreams, sleep, intuition.
Water from a more aggressive storm lots of wind or thunder/lightning provides us with power/protection, sacred rage, confidence/motivation.
Your intuition will help you to decide what type of rain to use and when!