How could they be observed thousands of years ago? If we see how they are formed now, it doesn't necessary mean that everything has gone the same way all the time.
Sorry, I don't believe that. But, I also meant with rain, water coming in any form, show, rain or sleet. Sorry that I was not more accurate. In my defense I want to say, in Finnish, rain can mean water in any form.
And you think it changes exactly when the year ends 31.12?
Is it not possible that every time it snows or rains, it can have slightly different acid concentration?