1. Icons and Uncertainty
Compiled by Cliff Mass

2. What Does Probablility of Precipitaiotn Mean?

3. ICONS are frequently used to communicate precip uncertainy
The use of icons are sloppy and inconsistent.
The icons are often inconsistent with the text under them.

4. The icons text use CHANCE from 30 to 70!!
Consider the term “chance of rain” for which the official NWS definitions are:
Uncertainty Terms
Indicate the percent chance of measurable (0.01 inches) precipitation occurring anywhere in the forecast zone or area.
Term Probability
Slight Chance %
Chance %
Likely %
no term used %
The icons text use CHANCE from 30 to 70!!
The icons are the same for 20% to 70% !!
Doesn’t the slight chance or chance icons look VERY, VERY WET? Completely useless in communicating the chance category

5. Even worse…they use the same icons for likely rain and rain as they do for chance rain. Also, they used “likely rain” for 70% on this page and “chance rain” for 70% in the example on the previous page

6. Just as bad are their cloudiness icons…look at mostly cloudy below……does that icon fit the description? No, of course. And look at partly cloudy…almost the same…both have the sun out!!
Also note that the POP is 20% on the right…no chance of rain indicated.

7. And a “slight” chance of freezing drizzle reminds one of a trip to Antarctica

9. Intensity vs Probabilities
Need better icons and other tools to communicate the distinction between them

10. Conclusions The current icons are a complete disaster:
They do not reflect that probabilities of the events they are trying to describe
They are not used in a consistent way between NWS offices or even at the same office from day to day.

11. Other Graphics