San Diego measures annual rainfall for a year beginning July 1 and ending June 30, which has always seemed a bit odd to me. Finally, a few days ago, I read the explanation for that and I guess it actually makes a pretty good sense after all.
In most of the country rainfall is pretty much evenly distributed throughout the year, but not so here. We have rainfall concentrated from November to May and usually have something close to zero precipitation in between. San Diego is actually desert, and we have a lower average annual rainfall than Tucson, Arizona in the Sonora Desert.
So if we measured our annual rain amount on a calendar year we would get parts of two rainy seasons. By starting it in the middle of the calendar year as we do here, we are tracking the amount that falls during a single, and complete, rainy season.
I'm not entirely sure why it's important to do that, but it does seem logical.