“Weathergami” charts paint new portrait of Minnesota climate

In a recent issue of the Bulletin of the American Meteorological Society, researcher Jonathan Kahl describes a novel way to visualize long-term weather data at a given location. Khan calls it “weathergami,” borrowing from the “scorigami” concept used to visualize sports scores.

Here at the Reformer we love data and we love weather stuff, so we decided to give Kahl’s technique a spin with some of the daily high and low temperature data maintained by the Minnesota Department of Natural Resources. Here’s what it looks like, using data from Minneapolis going back to 1938.

This plot contains more than 30,000 pairs of daily high and low temperature observations, with the lows on the horizontal axis and the highs on the vertical. 

If we just plopped down a dot for every observation we’d end up with an undifferentiated black blob, so the weathergami technique visualizes frequency of temperature pairs: we divide the chart area into a fine hexagonal grid, and then color each hexagon according to how many daily observations fall within it.

Dark blues indicate just one or two occurrences, while yellows show frequencies into the hundreds. See that area of concentrated yellow in the upper-right corner of the hex cloud? That shows that there have been a lot of summer days with a high temperature in the upper 70s or 80s and a low in the 60s.

Similarly, there’s a less intense mass of teal where highs are around 40 and lows near 25, a combination frequently seen in winter.

These charts look different depending on the weather trends in a given place. Kahl’s original paper includes a plot for Honolulu, Hawaii, that basically looks like a tight little bullseye because the temperature doesn’t change much there. But you can even see subtle variations across different cities in the same state.

Here, for instance, is Duluth. Notice that the cloud of hexes is shifted down and to the left, indicating generally cooler high and low temperatures. There’s also less intensity in the upper right corner of the cloud, indicating more variation during the summer.

Here’s Crookston, in the northwest corner of the state. There are a lot more hexes in the far lower left part of the chart, indicating lots of winter days where the temperature never rises above zero. There have been more than 900 such days in Crookston since 1938, compared to about 260 for Minneapolis.

See that one lonely hex separated from the others in the extreme bottom left? That was Feb. 29, 1996, with a high of -29 and a low of -43. Brr.

Another fun thing you can do with these charts is filter them by season. Here, for instance, is the Twin Cities plot but separated into four different charts, one for each season.

The thing that jumps out here is the relative spread of temperatures in the different seasons. Summer temperatures are tightly concentrated around that core of 80 degree highs. But spring and fall both kind of smear across the plot area, indicating the wide variety of temps in the shoulder seasons.

There’s a surprising amount of variation in the winter too, down at the colder end of the chart.

Finally, these charts can also give a sense of how the climate is changing over time. Here is a side-by-side of the Twin Cities, separated into the pre-1980 and post-1980 periods.

The thing that stands out is how there’s more summer intensity — less variation, in other words — in recent decades. The bulk of the hexes are also shifted ever so subtly up and to the right, reflecting the overall warming trend.

Overall these charts don’t necessarily tell us anything we didn’t already know, but it’s a lot of fun to pore over them and see what sorts of patterns strike the eye.

In the immortal words of Marge Simpson, “I just think they’re neat.”