Saturday, January 27, 2007

Los Angeles Rainfall

Heard on KPCC (Pasadena Public Radio) this morning, "LA only received x (I forgot, but it was ~ 1.5 inches) or rain this year when we should have gotten 7 inches."

What does she mean, we should have gotten y amount of rain? Rain falls or it doesn't fall. There is no should about it. After all, we are not owed rainfall.

I believe she meant that we have received quite a bit less rain than the average rainfall. That is quite true. But average rainfall in the arid west is highly misleading. Take a look at the monthly rain statistics from 1921-2006. LA rainfall is highly variable. In fact, there doesn't seem to be an average year.

Luckily, Charles Fisk has plotted up the LA rain data so I don't have to. The top plot shows deviation of rainfall from the mean (~15"), the bottom shows deviation from the median (~12.5"). Immediately, one can see how few years have close to the mean rainfall. We have mostly dry years (relative to the mean) with a few exceptionally wet years that skew the mean. (When the median and the mean vary by such a relatively large amount, one suspects that the data is skewed, in the statistical sense.)

For the latest rain year data, see the California Nevada River Forecast Center page for data for the most recent two years. For the past 6-24 hours.

Capecho news
The capecho continues to grow. I completed 2 swatch pentagons while trying different construction techniques. Then I settled upon a technique and completed 2 more. Remember, I wanted a ring of 6 pentagons. I can't use the 2 swatch ones; the reason will be clear when I get the photos up. So I have 4 more pentagons left to knit.

1 comment:

  1. I'm glad you've pointed out the slippery nature of the rainfall statistics: I've probably allowed my concern over global warming and my garden to color my concern over our dry winter season.

    My latest blog has a related lesson learned from the field of space weather.


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