Does it rain all day in Paris?
Moreover, does it rain a lot in Paris?
It does rain a lot. In Paris the rain is light. It's not that heavy rain, with the big fat drops like in the south of France, or in many hot areas. The inconvenience with Paris is that there is only sidewalk and despite the drainage system there is a lot of water.
Similarly one may ask, what is the rainiest month in Paris?
You will see there is rainfall around the year in Paris. May is indeed the rainiest month, but statistics and averages don't have much to do with the actual weather you'll have!
You can wear a nice tee-shirt, buttoned-up shirt or blouse underneath a cotton or light wool sweater. Trench coats are a must in Paris: along with a small umbrella, they are perfect for rainy days. I also like to carry a light scarf in my bag, in case it gets colder in the evening.