Global warning is upending weather maps. With temperatures reaching a near-record high of 119°F (48°C) in Arizona earlier this week, meteorologists ran out of colors to indicate extreme temperatures in southwestern regions of the US. In a June 21 projection of the Phoenix metropolitan area by WeatherBELL Analytics, the hottest temperature on the spectrum is oddly denoted by a cool green.
Green for hot? The irony didn’t escape meteorologist Mark Torregrossa, who elaborated on the color conundrum in an article for the Michigan news channel MLive. “All of the orange and red shades are used. The next stage of heat, depicted by what I would call violet colors, is blown right through on this temperature map. I guess finally you get so hot you turn green,” he wrote.
Torregrossa explains that weather maps typically use shades of warm, yellow-based colors (oranges, reds, earthy browns) to denote warm weather, but with near-record temperatures in California, Nevada, and Arizona this week have blown through their “color box.” Australian meteorologists faced a similar problem during a punishing heatwave in 2013, when the Australian Bureau of Meteorology decided to add bright purple and hot pink to their spectrum to indicate areas cooking in 125°–129°F (52°–54°C) temperatures.
There are no universally defined color standards for weather maps; it falls to weather graphics providers to establish a logical color scale. The US National Weather Service, for example, uses the rainbow (ROYGBIV) spectrum on its maps, while the Weather Channel follows a 17-color scale, with a pale lemon yellow for the hottest temperatures.
WeatherBELL’s high-resolution projection of the Arizona region has 39 colors, with a new hue introduced for every 6° temperature change. It’s notable that the color for the hottest temperature is similar to the forest greens in sub-zero zones. Quartz reached out to WeatherBELL about its color selection process but didn’t hear back.