Why So Many Weather Maps Are Rainbow-Colored (And Why They Shouldn’t Be)

Entertainment By Elena Boaghi |

Hurricane Harvey was a storm unlike anything the United States has ever seen–and so is Hurricane Irma, which threatens to bear down on Florida over the next week.

The storms herald a new era for weather, and new challenges for mapping it. As meteorologists raced to map Harvey’s rainfall, the National Weather Service realized that its current graphic standards couldn’t even communicate the extent of the deluge. “So much rain has fallen, we’ve had to update the color charts on our graphics in order to effectively map it,” the National Oceanic and Atmospheric Administration tweeted. At the epicenter of the storm, a very light purple was added to the deeply saturated rainbow color scale.

The decision has proven surprisingly controversial, at least by data science standards. That’s because rainbow color scales are a persistent burr in the saddle of the cartographic community–an issue that so irks some data scientists, it has its own pejorative catchphrase: End the Rainbow, or #endtherainbow or #endrainbow.

Their argument? Using a rainbow color scheme to explain science is exclusionary, and oftentimes unnecessarily misleading. And in a time when the truth about climate change and other scientific issues needs to be communicated forcefully and clearly, poor design is unacceptable.

Rainbow weather maps go all the way back to the early 1800s, when modern meteorology was emerging. In some early maps, ideas about modeling temperatures across the globe were conveyed in very broad strokes–just look at one of the earliest examples of a weather map that used color, drawn in 1823 by a Connecticut geographer named William Channing Woodbridge. It showed the rough regions of global temperature, using the spectrum to convey cold to hot–a sensible solution.

[Image: Wiki Commons]

When daily meteorological forecasts came to TV in the 1940s and ’50s, heralded by radar and other new technologies, the rainbow affordance endured. One 2009 study on rainbow graphics even suggests that people see rainbow color schemes as more scientific and authentic than graphics with fewer colors.

It stuck in scientific circles, too. Ed Hawkins, a climate scientist in the National Centre for Atmospheric Science, has pleaded for his peers to #endrainbow for years, making his case against the rainbow in Nature in 2015 (the journal currently recommends that authors avoid it altogether in submissions). So why do people keep using it? “Rainbow schemes are colorful and attractive,” explains Hawkins in an email. There are technical reasons too, he adds: “They are often the default in programming languages, though this is beginning to change.” 

There’s plenty of research that suggests the rainbow makes it harder for most of us to understand scientific data. We perceive the color spectrum not just in terms of red or blue, but through hue and brightness; some colors look lighter or darker to our eyes, meaning some colors look more different than others. Thanks to the distribution of different types of cones in our eyes, we’re pretty bad at detecting changes in color across the spectrum. For instance, as the authors of the same 2009 study explain, our eyes see yellow as more vibrant, so the yellow portion of a map will seem more dominant to us, even if the data it represents isn’t. “Transitions between some colors, green and red, for example, occur very rapidly, leading to false contrast,” NASA data visualizer and designer Robert Simmon notes in an essay. “Other transitions, especially green, are gradual, and there is a loss of detail.”

As a result, we’re prone to misinterpreting graphics that use rainbow colors; they’re confusing at best, and misleading at worst. The effects are even more severe for people who are colorblind–about 1 in 12 men and 1 in 200 women. For all of these reasons, scientists and cartographers have railed against rainbows for years, advocating for more thoughtful color scales that balance hue, saturation, and brightness.

Kenneth Field, senior cartographic product engineer at Esri, wrote a blog post heralding the New York Times‘ own Harvey rainfall map as “one of the best maps I have seen in a long while.” In contrast to the hyper-rainbow map of the National Weather Service, the Times uses shades of blue to map rainfall levels–a familiar affordance that’s easy to understand without looking at a key. It also uses symbols to show the chronology of the storm’s wracking waves. 

“The irony is that the NYT map uses the NWS data of the rainfall data to make their own version and prove that it’s perfectly possible to make terrific maps that communicate and which once again give us more reasons to #endtherainbow,” Field concludes on his blog. 

It might seem like choosing colors for maps is a relatively minor qualm–an insular debate in one corner of academia. But the stakes are huge. As climate science has become deeply politicized, the graphics that newspapers and websites publish play a huge role in how the public understands the science. “We need to emphasize that there is a ‘grammar of graphics,’” Hawkins writes. “More thought into following simple guidelines produces clearer visualizations, which will improve the communication of science to our peers and to the public.”

We’re living in an era when politicians at the highest levels of government are directly influenced by what they see on TV and in newspapers–a time when climate science is a national security issue. For better or worse, many of us get our understanding of science through graphics and maps, and the images we see about climate change deeply affect our perception of its severity. Poor graphic design isn’t just a problem in the science community: It’s a problem for everyone. 

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