Icons and Uncertainty

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Icons and Uncertainty

• Compiled by Cliff Mass

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What Does Probablility of Precipitaiotn Mean?
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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.

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• 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 10-20
• Chance 30-50
• Likely 60-70
• no term used 80-100
• The icons text use CHANCE from 30 to 70!!
• The icons are the same for 20 to 70 !!
• Doesnt the slight chance or chance icons look
  VERY, VERY WET? Completely useless in
  communicating the chance category

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Even worsethey 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
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• Just as bad are their cloudiness iconslook at
  mostly cloudy belowdoes that icon fit the
  description? No, of course. And look at partly
  cloudyalmost the sameboth have the sun out!!
• Also note that the POP is 20 on the rightno
  chance of rain indicated.

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And a slight chance of freezing drizzle reminds
one of a trip to Antarctica
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Intensity vs Probabilities

• Need better icons and other tools to communicate
  the distinction between them

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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.

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Other Graphics
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