Information Graphics

Color-Coding the Weather

To understand how unprecedented and extreme Hurricane Harvey was, consider this: The National Weather Service ran out of colors on its rainfall scale and had to add two more shades. The old rain gauge had 13 colors, starting with light green to indicate 0.1 inch of precipitation and ending with dark purple, to indicate 15 inches, the most rainfall it could envision in a single storm event. The over 40 inches of rain that Harvey dropped on portions of southeast Texas was unfathomable – until it happened.

The NWS quickly added two new shades of purple to its rainfall maps, but even the two new colors only show rainfall that exceeds 30 inches. Harvey has been called a once in a millennium rainfall event (let’s hope so).

Color-coded alerts have long been considered the fastest way to explain the severity of events. They can be understood intuitively no matter the spoken language, age group, or education level. The weather service actually uses 122 colors to convey weather. There are colors for watches, warnings and advisories. Tornados are shown in yellow, blizzards in scarlet, heavy smog in grey. More recently, the NWS has been weighing whether its watches and warnings have become so nuanced that they were confusing people. Judging from recent weather events, it seems the opposite is true. Maybe it’s time to adopt additional graphic symbols to advise people. Example: One star for light mist, five stars for umbrella and slicker, ten stars for pack your bags and head for higher ground.