People might use cars or public transport instead of a bike to commute if there is heavy rain or snow. This is an important consideration in Bergen which is notoriously the rainiest city in Europe (it rains more than half the time).
https://en.wikipedia.org/wiki/List_of_countries_by_average_a...
Also, "volume of rain" might be misleading because it doesn't give you a feeling for how often it rains. The Highlands in particular can have a light rain that falls for weeks at a time (or at least seems like it if you are in a tent).
I remember one time a few years back when I lived in Stavanger, I went to the post office in the city center there in a bright nice summer day and came out to torrential rain and rivers in the street!