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Yesterday I saw these day and night average temperature forecasts in my local TV weather report (see image below). I kept wondering if there are 'standard day hours' and 'standard night hours' for calculating these averages in the meteorological world, or are day and night hours determined solely by the forecast provider?

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I don't think a universal standard exists.

The US Weather Prediction Center, which is a part of the US National Weather Service, has a standard for its reporting of nationwide daily high and low temperatures. The WPC deems the daily high to be the highest measured / forecast temperature that occurs between one local midnight and the next, while the daily low is the lowest measured / forecast temperature between one local noon and the next. If the low occurs prior to midnight it is reported as the low for the following day.

On the other hand, other parts of the National Weather Service (in this case the Weather Service Forecast Office for Wichita, KS) deem the time from one local midnight to the next to suffice for both the daily high and the daily low. Many media in the US (e.g., the Chicago Tribune and WGN TV similarly use midnight to midnight for both the daily high temperature and the daily low temperature.

Yet others use one sunrise to the next as the delineation. Weather forecasts in the media do use the term overnight low, typically referring to the low temperature between sunset and sunrise.

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  • $\begingroup$ Both the high and low on climate reports are midnight to midnight for the NWS... see weather.gov/ict/f6decode $\endgroup$
    – JeopardyTempest
    Dec 4, 2021 at 21:20
  • $\begingroup$ @JeopardyTempest But see wpc.ncep.noaa.gov/discussions/hpcdiscussions.php?disc=nathilo , which denotes the law as the lowest temperature from local noon to local noon. $\endgroup$
    – David Hammen
    Dec 5, 2021 at 1:10
  • $\begingroup$ To be honest, I've never seen that product before. I take it as some special product WPC issues... it also shows to change 24 hour precip by the time of issuance, which isn't how climatology does it of course. But as far as I've ever known longterm official data is midnight to midnight. And most forecasts are usually either midnight to midnight or morning low, daytime high (indicating any abnormally timed temperatures with arrows or words like falling and rising). Will try to watch for the next big front to see how NWS does web forecasts, but I'm pretty sure it matches such. $\endgroup$
    – JeopardyTempest
    Dec 5, 2021 at 5:04
  • $\begingroup$ It's disappointing such data are so very inconsistent... recently came to find after a lot of discussion that a local "climate" site had a large change with the 10 year recalculations... because the site only previously had 20 years of precipitation data (didn't use threadex, and the site had only added precip records a couple decades ago). How highs and lows are determined should be right up front on historic data pages, and NOAA should have sites much more straightforward for station data downloads if they want to show reliability and clarity in data :( $\endgroup$
    – JeopardyTempest
    Dec 5, 2021 at 5:06
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    $\begingroup$ @JeopardyTempest Shall do. I should have said NOAA (which is the parent organization of the NWS). I'll also add references, including yours. I too find it a bit disturbing that (a) different organizations use different approaches, and (b) unless my google-fu has failed me, finding the approaches used by different organizations was nontrivial. $\endgroup$
    – David Hammen
    Dec 5, 2021 at 6:33

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