Future of Four Seasons in Maine and the Maine Data Literacy Project
Does a rainy summer mean a snowy winter is coming?
Background: There are lots of “urban legends” and people predicting how snowy or cold
a winter will be based on things like caterpillar coloration, the Old Farmer’s Almanac, or
the weather in the year leading up to winter. A Washington, D.C. meteorologist decided
to test out a few hypotheses about whether winter weather is related to the weather
during the rest of the year.
The graph below shows summer rainfall (called “JJA Rainfall”, meaning June, July, and
August) in blue diamonds, and “Winter Snowfall” in red squares from the late 1800’s
to the most recent year of data, for Washington D.C.
Data Source: http://www.washingtonpost.com/blogs/capital-weather-gang/wp/2013/07/09/doesa-wet-summer-in-d-c-lead-to-a-snowy-winter/
1. Describe what the graph shows about how rainy summers and snowy winters are
related.
Purpose here is to elicit description of what the graph shows. Sample response: The symbols for rainy
summers don’t track the snowy winters well at all – there doesn’t seem to be a pattern or trend in either
time series (JJA rainfall or winter snowfall))
2. I interpret the graph to mean… (Purpose here is to elicit an explanation (e.g. of the pattern or
variability) or interpretation of the meaning in terms of the context of the question. Sample response:
The graph isn’t showing any correlation between rainy summers and snowy winters. There’s a lot of
variability, but it’s not clear that when summer (JJA) rainfall is high, winter snowfall for that year is also
high. You can’t see any signal (pattern) within the noise (variability).)
Extensions:
See the blog that originated this chart for a longer discussion of these data and investigation of other
possible drivers of snow winters. Read more here: http://www.washingtonpost.com/blogs/capitalweather-gang/wp/2013/07/09/does-a-wet-summer-in-d-c-lead-to-a-snowy-winter/
This graph is awkward in that the question is about correlation, and the graph looks like a scatterplot,
but really it is two overlaying time series with dots arranged in linear time. Would it be easier to see if
there’s a correlation or not if the meteorologist had created a true scatterplot, with JJA rainfall on the Xaxis and winter snowfall for the following winter on the Y-axis? Reading the blog, the author gives an Rsquared value of -0.02, which is essentially no correlation (remember, these range from 0 to 1 (absolute
value), where 1 is the strongest correlation (a one-to-one relationship). We have to take his word for it,
since we can’t see from the graph a proper scatter of the data.