Another way is to use a String array and save the data at the end calling saveStrings. If you are saving each hour, what you could do is to load the data with loadStrings, add your new value by appending to the array obtained from loadStrings, then call saveStrings. You have to do this as there is no way to append to a file in Processing. You have to load the file and then save it again with the new or updated values.
For data folder creation, you can check how Processing manages the folder stamp creation when it creates new files using the Calendar object:
Kf