<p>The icons that are used are really just a visual way of describing the phrasing that is used in the forecast.  You can see an example of one set of icons offered by Weather Underground <a href="https://github.com/manifestinteractive/weather-underground-icons/blob/master/dist/icons/black/icon-preview.png" rel="nofollow noreferrer">here</a>.  And they have <a href="https://github.com/manifestinteractive/weather-underground-icons" rel="nofollow noreferrer">many different icon sets</a> that you could choose from.  The phrases themselves are linked to specific ranges of probability of precipitation, cloud cover, or other factor.  <a href="http://weather.gfc.state.ga.us/Info/WXexp.aspx" rel="nofollow noreferrer">This site</a> shows some nice tables that give some of those ranges (sample shown below):</p>
<p><a href="https://i.sstatic.net/aDyzv.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/aDyzv.png" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2017-01-10T20:52:55.950Z
2022-02-10T04:17:42.850Z
2022-02-10T04:17:42.850Z
f.thorpe
<p>Some historical standards formed due to instrumentation.  Cloud cover observations were setup based upon okta, which are eighth fractions of the sky covered overhead during the past hour (perhaps 8 observations were taken/recorded by early instruments/observers).  This lead to the United States National Weather Service <a href="http://www.weather.gov/bgm/forecast_terms" rel="nofollow noreferrer">using</a> this table for reporting and forecasting:</p>
<p><a href="https://i.sstatic.net/2DzqR.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/2DzqR.png" alt="NWS" /></a></p>
<p>Likewise, the <a href="https://en.wikipedia.org/wiki/Rain_gauge#Tipping_bucket_rain_gauge" rel="nofollow noreferrer">tipping bucket rain gauge</a> measured in 0.01 inch (0.25 mm) increments, so that influenced how precipitation categories were chosen.  <a href="http://web.archive.org/web/20060110140456/http://www.met.tamu.edu/class/METAR/metar-pg9-ww.html#DRIZZLE" rel="nofollow noreferrer">This journal article</a> states that:</p>
<blockquote>
<p>In  weather observations, drizzle is classified as (a) “very light”,
comprised of scattered drops that do  not  completely  wet  an
exposed  surface, regardless of duration; (b) “light,” the rate of
fall being  from  a  trace  to  0.25  mm  per  hour:  (c) “moderate,”
the rate of  fall being 0.25-0.50 mm per hour:(d) “heavy” the rate of
fall being more than  0.5  mm  per  hour.    When  the  precipitation
equals or exceeds 1mm per hour, all or part of the  precipitation  is
usually  rain;  however,  true drizzle  falling  as  heavily  as  1.25
mm  per  hour has been observed.  By convention, drizzle drops are
0.5mm or less in diameter”</p>
</blockquote>
<p>Showing that there is plenty of connection in the definitions of rain/drizzle intensities to the 0.25 mm baseline.</p>
<p>However, while many organizations may work from this framework, there's no reason other organizations must base their wording and icons on these historical criteria.  They may wish to have more or fewer categories in their forecasts.   For example, Wunderground's API table shows that [<a href="https://www.wunderground.com/weather/api/d/docs?d=resources/phrase-glossary&amp;MR=1" rel="nofollow noreferrer">current link</a> now access protected, original information <a href="https://web.archive.org/web/20180328124309/https://www.wunderground.com/weather/api/d/docs?d=resources/phrase-glossary&amp;MR=1" rel="nofollow noreferrer">here</a>]:</p>
<p><a href="https://i.sstatic.net/iAm1S.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/iAm1S.png" alt="enter image description here" /></a></p>
<p>They use just four cloud coverage icon categories, while their forecast wording options include clear, cloudy, mostly cloudy, mostly sunny, partly cloudy, partly sunny, and scattered clouds.  So they don't appear to match the standard NWS table.</p>
<p>So long story short: <strong>There are some historical standards, but sites can take liberties in reporting it how they wish.  Looking through some of the APIs, it would definitely be nice if they would better indicate the actual criteria they use!</strong></p>

CC BY-SA 3.0
2017-01-10T23:41:52.390Z
Nice answer!  Do you think I should delete mine?
f.thorpe
CC BY-SA 3.0
2017-01-10T23:47:58.647Z
Funny enough, originally I wrote it as an edit to yours to include the extra information.  But in the end, I felt I'd changed yours so much that it might make you unhappy that I'd altered your message :-)    If you'd rather, I can add it back to yours and delete, as you found the Wunderground references that really were the heart of the idea.
JeopardyTempest
CC BY-SA 3.0
2017-01-10T23:50:07.133Z
Let's just leave them be for now I guess.  Thanks for the insight.
f.thorpe
CC BY-SA 3.0
2017-01-10T23:50:46.057Z
No problem.  Good answer yourself :-)  The goal is to get good information out there, and I feel like we did a reasonable job.
JeopardyTempest
CC BY-SA 4.0
2022-02-09T14:07:15.330Z
The Weatherunderground link seems to be broken.
user
CC BY-SA 4.0
2022-02-09T15:34:29.380Z
@user thanks, changed it to an old Internet Archive result.
JeopardyTempest
CC BY-SA 4.0
2017-01-10T22:21:52.903Z
2022-02-09T15:33:58.033Z
2022-02-09T15:33:58.033Z
JeopardyTempest
<p>Weather APIs often provide weather icons which symbolize the state of the sky at a given point, or as an average.</p>

<p>Is there a <strong>standardized</strong> way to define what the icon should be, based on quantifiable data such as cloud coverage or rain level (the idea being that, say, with over 50% cloud coverage a "cloud" is used, then with > 1 mm rain, "drops" are added etc.) -- or is it left purely as an interpretation to the user? </p>

CC BY-SA 4.0
2022-02-10T07:03:48.833Z
The answer is no. Selecting weather icons is purely subjective.
Whir
<meteorology><weather-forecasting>
Is there a standardized way to define weather icons based on quantifiable data?
WoJ
# Is there a standardized way to define weather icons based on quantifiable data?

(Asked by: username_0 on 2017-01-10)

Weather APIs often provide weather icons which symbolize the state of the sky at a given point, or as an average.

Is there a **standardized** way to define what the icon should be, based on quantifiable data such as cloud coverage or rain level (the idea being that, say, with over 50% cloud coverage a "cloud" is used, then with > 1 mm rain, "drops" are added etc.) -- or is it left purely as an interpretation to the user?

**Tags:** meteorology, weather-forecasting

> **Comments:**
> **username_1**: The answer is no. Selecting weather icons is purely subjective.  

---

## 2 answers

### Answer by username_3 on 2017-01-10. Score: 7 | Accepted answer

Some historical standards formed due to instrumentation. Cloud cover observations were setup based upon okta, which are eighth fractions of the sky covered overhead during the past hour (perhaps 8 observations were taken/recorded by early instruments/observers). This lead to the United States National Weather Service [using](http://www.weather.gov/bgm/forecast_terms) this table for reporting and forecasting:

[![NWS](https://i.sstatic.net/2DzqR.png)](https://i.sstatic.net/2DzqR.png)

Likewise, the [tipping bucket rain gauge](https://en.wikipedia.org/wiki/Rain_gauge#Tipping_bucket_rain_gauge) measured in 0.01 inch (0.25 mm) increments, so that influenced how precipitation categories were chosen. [This journal article](http://web.archive.org/web/20060110140456/http://www.met.tamu.edu/class/METAR/metar-pg9-ww.html#DRIZZLE) states that:

> In weather observations, drizzle is classified as (a) “very light”,
> comprised of scattered drops that do not completely wet an
> exposed surface, regardless of duration; (b) “light,” the rate of
> fall being from a trace to 0.25 mm per hour: (c) “moderate,”
> the rate of fall being 0.25-0.50 mm per hour:(d) “heavy” the rate of
> fall being more than 0.5 mm per hour. When the precipitation
> equals or exceeds 1mm per hour, all or part of the precipitation is
> usually rain; however, true drizzle falling as heavily as 1.25
> mm per hour has been observed. By convention, drizzle drops are
> 0.5mm or less in diameter”

Showing that there is plenty of connection in the definitions of rain/drizzle intensities to the 0.25 mm baseline.

However, while many organizations may work from this framework, there's no reason other organizations must base their wording and icons on these historical criteria. They may wish to have more or fewer categories in their forecasts. For example, Wunderground's API table shows that [[current link](https://www.wunderground.com/weather/api/d/docs?d=resources/phrase-glossary&MR=1) now access protected, original information [here](https://web.archive.org/web/20180328124309/https://www.wunderground.com/weather/api/d/docs?d=resources/phrase-glossary&MR=1)]:

[![enter image description here](https://i.sstatic.net/iAm1S.png)](https://i.sstatic.net/iAm1S.png)

They use just four cloud coverage icon categories, while their forecast wording options include clear, cloudy, mostly cloudy, mostly sunny, partly cloudy, partly sunny, and scattered clouds. So they don't appear to match the standard NWS table.

So long story short: **There are some historical standards, but sites can take liberties in reporting it how they wish. Looking through some of the APIs, it would definitely be nice if they would better indicate the actual criteria they use!**

> **Comments:**
> **username_2**: Nice answer!  Do you think I should delete mine?  
> **username_3**: Funny enough, originally I wrote it as an edit to yours to include the extra information.  But in the end, I felt I'd changed yours so much that it might make you unhappy that I'd altered your message :-)    If you'd rather, I can add it back to yours and delete, as you found the Wunderground references that really were the heart of the idea.  
> **username_2**: Let's just leave them be for now I guess.  Thanks for the insight.  
> **username_3**: No problem.  Good answer yourself :-)  The goal is to get good information out there, and I feel like we did a reasonable job.  
> **username_4**: The Weatherunderground link seems to be broken.  
> **username_3**: @username_4 thanks, changed it to an old Internet Archive result.  

---

### Answer by username_2 on 2017-01-10. Score: 7

The icons that are used are really just a visual way of describing the phrasing that is used in the forecast. You can see an example of one set of icons offered by Weather Underground [here](https://github.com/manifestinteractive/weather-underground-icons/blob/master/dist/icons/black/icon-preview.png). And they have [many different icon sets](https://github.com/manifestinteractive/weather-underground-icons) that you could choose from. The phrases themselves are linked to specific ranges of probability of precipitation, cloud cover, or other factor. [This site](http://weather.gfc.state.ga.us/Info/WXexp.aspx) shows some nice tables that give some of those ranges (sample shown below):

[![enter image description here](https://i.sstatic.net/aDyzv.png)](https://i.sstatic.net/aDyzv.png)


---

