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I was checking the weather forecast just now, and it is showing that it "feels like -999 °C". I never heard or saw -999 °C before. I searched other weather channels, and they were showing that it feels like 2 °C. What is the meaning of -999 °C, irrespective of the weather?

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    $\begingroup$ That means its cold. Really really cold. So cold that they really don't know how cold it actually feels. OR it just means that a programmer screwed up. $\endgroup$ – NotMe Dec 16 '14 at 0:24
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    $\begingroup$ @ChrisLively Or there was missing data in one of the input fields to their "feels like" algorithm. It probably isnt a straight wind chill and may depend on more than just temp and wind. $\endgroup$ – casey Dec 16 '14 at 2:37
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    $\begingroup$ Oh, the weather outside is frightful. $\endgroup$ – a CVn Dec 16 '14 at 14:25
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    $\begingroup$ @casey: a programmer is still supposed to take those types of things into account... $\endgroup$ – NotMe Dec 16 '14 at 16:07
  • $\begingroup$ This really sounds like a mistake to me! $\endgroup$ – L.B. Dec 18 '14 at 18:01
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The value -999 is likely the "fill value" used in the dataset when data is missing or is not being properly filtered or handled when displayed. In the specific case on the website you cite, it is likely a problem with the algorithm for wind chill (the "feels like" temperature this time of year).

It isn't a physical value and only means the value is missing. Furthermore, -999°C is not a possible value because absolute zero is –273.15°C, and it's not possible to be colder than this (at least not in any meaningful way, and certainly not because of wind chill). The coldest recorded temperature on earth is around –90°C.

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    $\begingroup$ Personally, I prefer -9e99 (or 9e99) as a dubious (and obvious) fill value. But yes, this is almost certainly what this is. $\endgroup$ – David Hammen Dec 16 '14 at 0:07
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    $\begingroup$ NASA often uses -9999... just to be sure they put the extra 9 $\endgroup$ – farrenthorpe Dec 16 '14 at 5:33
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    $\begingroup$ the data on that page comes from weather underground, see wunderground.com/weather/api/d/docs?d=resources/phrase-glossary 'values will = -9999 or -999 for Null or Non applicable (NA) variables' I also suspect that 'feels like' is the heat index value, not the wind chill, which is why it's undefined ( you're not going to get heat stroke there at the moment ) $\endgroup$ – Pete Kirkham Dec 16 '14 at 19:43
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    $\begingroup$ @PeteKirkham "feels like" is typically heat index in the warm season and wind chill in the cold season. Some provides use additional factors in their algorithm as well. $\endgroup$ – casey Dec 16 '14 at 19:45
  • $\begingroup$ Stupid languages that don't have an NA numeric value >:/ $\endgroup$ – naught101 Jan 12 '15 at 3:28

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