From the L.A. Times: Why L.A. is having such a wet winter after years of drought conditions. (Warning: they’ll let you look at their site for a while, then cut you off like a barkeep when closing time approaches.) Haven’t looked at the article yet, but I’ll fall off my chair if the answer doesn’t contain global warming/climate change.
But I have some ideas of my own. Historical data on seasonal rainfall totals for Los Angeles over the last 140+ year is readily available on the web. I took that data, and did a little light analysis.
Average seasonal rainfall in L.A. is 14.07″. 60% of the time, rainfall is below average; 40% above. Percentage of seasons with:
- less than 75% of average rain: 32.62
- between 75% and 125%: 39.01
- over 75%: 28.37
“Normal” rainfall covers a pretty wide range, one would reasonably suppose. Getting a lot or a little seems somewhat more likely than getting somewhere around average. This fits with my experience growing up in L.A. (18 year sample size, use with caution.)
The last 20 years look like:
|Season (July 1-June 30)||Total Rainfall, Inches||Variance from Avg|
14 years out of 20 (70%) are under average; 6 above. Those 5 years in a row stand out, as does the 9 out of 11 years under from 2005-2006 to 2015-2016. (That 22.55 inches in 2004-2005 also stands out – very wet year by L.A. standards.)
Wow, that does look bad. So does this stretch, with 7 out of 8 under:
And this one, with 10 out of 11:
Or this, with 6 out of 7:
This last cherry-picked selection is also like the most recent years in that annual rainfall is not just under, but way under. This last sample shows more than 6″ under, in 5 out of 6 years. In the recent sample, 5 out of the last 7 years prior to this year were more than 6″ under, and one over 5″ under.
How often does L.A. get rainfall 6″ or more under average? About 22% of the time. So, hardly unusual, and, given a big enough sample (evidently not very big), you would expect to find the sorts of patterns we see here, even if, as it would be foolish to assume, every year’s rainfall is a completely independent event from the preceding year or years. It would make at least as much sense to think there are big, multi-year, multi-decade, multi-century and so on cycles – cycles that would take much larger samples of seasonal rainfall to detect. And those cycles could very well interact – cycles within cycles.
Problem is, I’ve got 141 years of data, so I can’t say. I suspect nobody can. Given the poorly understood cycles in the oceans and sun, and the effect of the moon on the oceans and atmosphere, which it would be reasonable to assume affect weather and rainfall, we’re far from discovering the causes of the little patterns cherry picking the data might present to us. They only tell us that rainfall seems to fall into patterns, where one dry year is often followed by one or two or even four or five more dry years. And sometimes not.
L.A. also gets stretches such as this:
Not only are 7 out of 10 years wetter than average, the 3 years under average are only a little short. This would help explain why it is so often raining in Raymond Chandler stories set in L.A. – this sample of years overlaps most of his masterpieces.
The L.A. Times sees something in this data-based Rorschach test; I see nothing much. Let’s see what the article says:
Nothing. The headline writer, editor and writer evidently don’t talk to each other, as the article as published makes no attempt to answer or even address the question implied in the headline. It’s just a glorified weather report cobbled together from interviews from over the last several months. Conclusion: things seem OK, water system wise, for now, but keep some panic on slow simmer, just in case. Something like that.
Oh, well. You win some, you lose some. That *thunk* you hear is me falling out of my chair.