<p>The news is reporting a global climate event:</p>
<blockquote>
<p>The <strong>hottest day ever on Earth since records began</strong> happened on July 3, 2023.<br />
<sub><a href="https://www.nbcdfw.com/weather/weather-connection/earths-hottest-day-ever-was-recorded-on-july-3-2023/3289861/" rel="nofollow noreferrer">Earth's hottest day ever recorded was on July 3, 2023</a></sub></p>
</blockquote>
<p>Turns out, the record was broken again July 4:</p>
<blockquote>
<p><a href="https://i.sstatic.net/zIDZi.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/zIDZi.png" alt="image showing &quot;2m Temperature World&quot; from 1979 to July 4, 2023" /></a><br />
<sub><a href="https://climatereanalyzer.org/clim/t2_daily/" rel="nofollow noreferrer">Climate Reanalyzer</a>, University of Maine.</sub></p>
</blockquote>
<p>The above website gives the description:</p>
<blockquote>
<p>This page provides <strong>time series</strong> and <strong>map</strong> visualizations of daily mean <strong>2-meter air temperature</strong> from the NCEP Climate Forecast System (CFS) version 2 (April 2011 – present) and CFS Reanalysis (January 1979 – March 2011). CFS/CFSR is a numerical climate/weather modeling framework that ingests surface, radiosonde, and satellite observations to estimate the state of the atmosphere at hourly time resolution onward from 1 January 1979. The horizontal gridcell resolution is 0.5°x0.5° (~ 55km at 45°N). Temperature anomalies are in reference to 1979–2000 climatology for each specific day of the year.</p>
<p>The <strong>time series chart</strong> displays area-weighted means for the selected domain. For example, if World is selected, then each daily temperature value on the chart represents the average of all gridcells 90°S–90°N, 0–360°E and accounts for the convergence of longitudes at the poles. ...</p>
</blockquote>
<p>Alright, so it's averaged over selected points around the globe.  But...</p>
<p><strong>Question</strong>: How is the &quot;hottest day ever on Earth&quot; determined?</p>
<p>There are two issues here:</p>
<ul>
<li><p>&quot;day&quot; is not defined in the same way across the globe, and when it's daytime (and thus hot) in one place, it's simultaneously not daytime in another place; and</p>
</li>
<li><p>I don't see a reason why the northern hemisphere's summer is hotter on a global plot than the southern hemisphere's summer.</p>
</li>
</ul>

CC BY-SA 4.0
2023-07-06T06:49:04.393Z
They probably just take the analysis from a global forecasting model.
gerrit
CC BY-SA 4.0
2023-07-06T23:32:11.400Z
Northern hemisphere has much more land area and thus is hotter on average at the surface.
f.thorpe
CC BY-SA 4.0
2023-07-07T08:21:38.223Z
I would assume they calculate the mean temperature across all 0.5°x0.5° gridpoints over a 24-hour period in UTC time (as the CFSR/CFSv2 output is in UTC). So for example, the 6th July mean would be calculated from 2023-07-06 00:00 to 2023-07-06 23:00.
Ingvar Lukas
CC BY-SA 4.0
2023-07-07T11:03:01.053Z
A day in meteorological measurements: temperature, pressure, etc. is defined as 9 am to 9am for the local time zone. For a particular weather station recording for 7 July 2023 the measurements start at 9 am on that day & end at 8:59:59 am on 8 July 2023. To account for time changes due to daylight saving, times are recorded as UTC times.
Fred
<climate-change><climate>
How is the "hottest day ever on Earth" determined?
Rebecca J. Stones
# How is the "hottest day ever on Earth" determined?

(Asked by: username_0 on 2023-07-06)

The news is reporting a global climate event:

> The **hottest day ever on Earth since records began** happened on July 3, 2023.  
> [Earth's hottest day ever recorded was on July 3, 2023](https://www.nbcdfw.com/weather/weather-connection/earths-hottest-day-ever-was-recorded-on-july-3-2023/3289861/)

Turns out, the record was broken again July 4:

> [![image showing "2m Temperature World" from 1979 to July 4, 2023](https://i.sstatic.net/zIDZi.png)](https://i.sstatic.net/zIDZi.png)  
> [Climate Reanalyzer](https://climatereanalyzer.org/clim/t2_daily/), University of Maine.

The above website gives the description:

> This page provides **time series** and **map** visualizations of daily mean **2-meter air temperature** from the NCEP Climate Forecast System (CFS) version 2 (April 2011 – present) and CFS Reanalysis (January 1979 – March 2011). CFS/CFSR is a numerical climate/weather modeling framework that ingests surface, radiosonde, and satellite observations to estimate the state of the atmosphere at hourly time resolution onward from 1 January 1979. The horizontal gridcell resolution is 0.5°x0.5° (~ 55km at 45°N). Temperature anomalies are in reference to 1979–2000 climatology for each specific day of the year.
>
> The **time series chart** displays area-weighted means for the selected domain. For example, if World is selected, then each daily temperature value on the chart represents the average of all gridcells 90°S–90°N, 0–360°E and accounts for the convergence of longitudes at the poles. ...

Alright, so it's averaged over selected points around the globe. But...

**Question**: How is the "hottest day ever on Earth" determined?

There are two issues here:

* "day" is not defined in the same way across the globe, and when it's daytime (and thus hot) in one place, it's simultaneously not daytime in another place; and
* I don't see a reason why the northern hemisphere's summer is hotter on a global plot than the southern hemisphere's summer.

**Tags:** climate-change, climate

> **Comments:**
> **username_1**: They probably just take the analysis from a global forecasting model.  
> **username_2**: Northern hemisphere has much more land area and thus is hotter on average at the surface.  
> **username_3**: I would assume they calculate the mean temperature across all 0.5°x0.5° gridpoints over a 24-hour period in UTC time (as the CFSR/CFSv2 output is in UTC). So for example, the 6th July mean would be calculated from 2023-07-06 00:00 to 2023-07-06 23:00.  
> **username_4**: A day in meteorological measurements: temperature, pressure, etc. is defined as 9 am to 9am for the local time zone. For a particular weather station recording for 7 July 2023 the measurements start at 9 am on that day & end at 8:59:59 am on 8 July 2023. To account for time changes due to daylight saving, times are recorded as UTC times.  

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## 0 answers

