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An Interactive Heat Map Points to a Hot and Humid Future

Climate Central is a platform for research and reporting about our changing climate and its impact on the American public. We’ve worked with their team of scientists and journalists to create interactives that show how summers are getting hotter and winters are getting warmer. We also helped them think about how to tell a data story about increasing precipitation. So when the Climate Central team came to us with a slew of new data about how cities across the U.S. are experiencing a higher frequency of days when the combination of heat and humidity (or heat index) reaches dangerous levels, we were excited to start sketching out ideas.

One of the interesting factors nested in Climate Central’s data is that dangerously hot days are particularly risky for vulnerable populations of adults over 65 and children under 5 who live below the poverty line. This is partly a function of physical susceptibility. but it also relates to access to the types of infrastructure that can shield people from the effects of hot days–particularly air conditioning. This intrigued us, so we started to envision an interactive experience that highlighted cities where it could become perilous to retire or raise children. Unfortunately, an interactive can only be as specific as its underlying data, and in this case we concluded that we didn’t have adequately comprehensive longitudinal data about vulnerable populations or infrastructure. This is the kind of false start we occasionally run into when getting a handle on data and determining the parameters of the story that we can realistically tell. In consultation with Climate Central, we shifted our focus to the more conspicuous story: while the number of extremely hot days has increased steadily since the 1950s, Climate Central’s data projects a rapid proliferation of the most dangerous days–when the heat index tops 105º–in the next four decades.

A sticking point in many of our discussion was how to portray the data without creating yet another “map with dots.” In this case, we followed several digressions before circling back to our stubborn original premise: the best way to show how temperature affects cities across the U.S. is to use a map. So we started thinking about how we could make this map distinctive. Could we create something that looked and behaved like an infrared heat map? What if the dots changed color and shape, depending on the severity of the temperature in each city? We experimented with different criteria to determine the size and color of the dots representing each city. After considerable back-and-forth, we decided to represent the number of “danger days” (when the heat index tops 105º)  using a halo that magnifies the impact on the affected city, and the number of “extreme caution days” (when the heat index tops 90º) using color to reflect impact, with red representing the highest number of such days.

Once we zeroed in on how our heat map would work, there were lots of details to fill in. We created a hover-over popup for each city to give a data detail for each time period of the study. We built an animated timeline to show users how the map “heated up,” and made the timeline interactive so users could focus on decades past, present, and future. By design, the timeline helps to emphasize the shocking proliferation of dangerously hot weather projected over the next 40 years. In order to keep the map clean yet comprehensive, we tweaked the thresholds that defined the size and color of each city so that the map would animate in an impactful way while remaining legible and uncluttered. Although we were limited to data culled from the 2010 census, we also displayed vulnerable population figures for each city to give users additional context about impact. Finally, to create more real estate for a map that would be embedded at widths as small as 550 pixels, we used a hide/reveal tab for accessing the legend used to explain the color and size representations.

Based on our UX, UI, designs and guidance, Climate Central’s development team built the interactive on a remarkably tight timeline. As their accompanying blog post notes, the map highlights how dangerously hot and humid days will hit the southern and southeastern U.S. hardest in the next few decades. If snowbirds consult this interactive data visualization, they may decide to fly in a different direction for winters to come.