Functions and procedures - where to use which

As we have seen Functions return one output and update anything passed in as a parameter or keyword. Procedures just update anything passed in as a parameter or keyword. So, you can use either for anything, but it's neat to use functions when you are calculating one thing (e.g. calculating potential temperature from temperature and pressure) and procedures when you are doing lots of things (e.g. reading in a series of files, calculating products from them and generating some plots).