Cyclone Center’s Satellite Color Scheme
The Dvorak technique was developed in the 1970s and early 1980s. At that time, most satellite images were viewed on paper using black and white printers. To accommodate this medium, Dvorak developed the “BD Curve”. This curve assigned each satellite brightness temperature value to a specific shade of black, white, or gray.
The Dvorak technique relies on the analyst’s ability to identify each of these shades. Trained experts can usually do this relatively quickly. The BD Curve can be confusing, however, especially to newer analysts. Some colors are repeated, and it can be difficult to discern one shade of gray from another. We have developed a new full-color satellite enhancement for the Dvorak technique to address these issues. In addition to using this new color scheme for Cyclone Center, we plan to share it with tropical analysts around the globe.
The image above compares our color scheme with the BD Curve. Both schemes use gray shading to highlight clouds warmer than 9°C (48°F). The BD Curve then uses a second series of grays, while we give it a pink tint to help differentiate it from the warmer values.
Both color schemes use solid shades at varying intervals for temperatures colder than -30°C (-22°F). In our scheme, this begins with a dark red (which flows naturally from the pink). The colors become progressively less warm (orange, yellow, then shades of blue). Where the BD Curve is forced to repeat Medium Gray and Dark Gray shades, our colorized scheme is able to use unique colors throughout.
Note that the BD Curve uses black for temperatures from -63°C (-81°F) to -69°C (-92°F). This bold color marks a transition from moderate to tall clouds. This same transition is marked by the change from warm to cool colors in our scheme.
We have also included an additional color (white) for temperatures colder than -85°C (-121°F). This color is never used by the Dvorak technique, but it provides us additional information about the coldest clouds.
The images above show two views of Super Typhoon Gay (1992). The one on the left uses the BD curve; the one on the right is our color scheme. All features are identical in both color schemes, but we believe the colorized scheme makes them easier to identify.
We also wanted to ensure that our imagery could be easily interpreted by everyone, including people with color vision deficiencies. We were guided by the principles laid out by Light and Bartlein (2004). Specifically, we avoided any color scale that included both red and green. We also sought a scheme that varied in both hue and intensity. Our ultimate selection was inspired by the “RdYlBu” scheme from colorbrewer2.org.
The images above simulate how Super Typhoon Gay would appear to these users. These simulations are performed using vischeck.com. The one on the left simulates Deutarnopia, and the middle simulates Protanopia. These are both common forms of red/green deficiency. The image on the right simulates Tritanopia, a rare form of blue/yellow deficiency. These simulations suggest that any analyst, regardless of color deficiencies, would be able to identify the same features in our imagery.