This lovely picture from stevewall shows that rain is not only necessary, but can also be beautiful. When looking at meteorological data, it’s hard to find anything beautiful in the usual definition of the word. But it can be both fun and interesting. In this post, I’ll look at the average monthly precipitation at a few locations in the United States. They are vastly different depending on where you are, and it’s a fun exercise to figure out why the distributions are the way they are.

I’m not a true weather weenie, but I’ll offer some comments on the yearly cycle. Hopefully I won’t say anything that’s terribly wrong. The data is from the USHCN data available on the NOAA website. The thick black line is the mean monthly values, and the light grey area surrounding the line represents the 1 standard deviation errors. I could have used this page, but then I wouldn’t have been able to include the “error bars”.

Atkinson is located in north-eastern Nebraska. It has most of their rainfall occurs during the summer months, with a peak in June. The large amount of rain in the summer almost reaches to 4 inches. This most likely is from the numerous summer storms that sweep across the region. There is very little rain in the winter months, which is typical of the upper plains regions.

Moving a little bit east is Hillsboro, Ohio. Located about halfway between Cincinnati and Columbus, one might expect the precipitation patterns to be similar to Nebraska. You’d be wrong. There is basically the same amount of precipitation in every month. They receive a lot of rain in the winter compared to Nebraska. Clearly, the winter mid-latitude storms bring Ohio a fair amount of precipitation.

Moving south to Everglades, Florida is a pattern similar to Atkinson, Nebraska. A lot of rain during the summer and not much during the winter. The summer precipitation is largely due to how the sea breeze and trade winds interact. However, notice the vertical axis. During the months of June, July, August and September Everglades receives a total of about 36 inches of rain. During the driest months, they still get around 2 inches of rain. Keep that figure of 2 inches in a month in mind, we’ll see a number like that later on.

Moving on to the West coast. with Centralia, Washington. Here we see the exact opposite pattern. They get a lot of their precipitation during the winter months, and are driest during the summer. Most of their precipitation is from extratropical cyclones than move in from the Pacific. As the number of storms changes throughout the year, so does their precipitation amounts.

Just to the south is Ojai, California. Not surprisingly, it has a similar pattern of precipitation as Washington. However, notice the grey error bands. They’re huge. For January, the 1 standard deviation range spans values less than zero (obviously not physically possible) up to 10 inches. Compare that to Washington where the error bands only range a more modest 4-10. Also, some months during the summer, they average very near zero precipitation.

Moving a little south and east gets us to Tucson, Arizona. Our only ‘desert’ location in this short trip around the United States. There is very little precipitation here. Duh. The highest monthly precipitation comes during July and August with a combined total of around 4 inches of rain. What’s interesting about Tucson is that there are two peaks in the precipitation. One corresponds to the summer ‘monsoon’, and the other is during the winter months.

Now it’s your turn to show off. Locate the city/state/region below based solely on its yearly precipitation cycle. The winner gets nothing besides a pat on the back. Extra credit bonus points for descriptions of the mechanisms for the precipitation patterns.

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