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In this BBC News article, there is a chart labelled "Hottest day on record globally - Daily average air temperature, 1940-2023". It shows temperatures that are higher in summer and lower in winter for the northern hemisphere, which leads me to wonder what exactly this chart is showing. If it were a global average, would the temperature in summer (or winter) not be offset by the fact that the other side of the globe has its winter (or summer) at the same time?

So what is the method used in this chart? Is it just for the northern hemisphere? Is it the temperature over the land mass but not over the sea? Is the data for the southern hemisphere offset by half a year? Or is the world actually hotter in July because there's more land mass in the nothern hemisphere?

Daily average air temperature, 1940-2023

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    $\begingroup$ @njuffa I suspect it may have been averaged over the landmass, which is also concentrated in the north. On the other hand, do we not have temperatures for Antarctica since 1940? $\endgroup$
    – user29329
    Jul 22 at 20:57
  • $\begingroup$ I asked this related (same?) question: How is the "hottest day ever on Earth" determined?. $\endgroup$
    – Rebecca J. Stones
    Jul 28 at 4:16

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As stated in your linked article, the graph shows the global average temperature based on ERA5 reanalysis data.

Typically for graphs like this you integrate the surface temperature over the whole domain (surface of earth) and normalise the result with Earth's surface area.

A short explanation for the seasonal cycle is that the northern hemisphere has more land surface area (less water surface area) compared to the southern hemisphere. Water surfaces heat much slower than land surfaces, which introduces a phase lag with respect to the solar heating. Very simplified think about it like this: High solar radiation in the southern hemisphere leads to high temperatures a few weeks/months later, while in the northern hemisphere high temperatures coincide with high radiation levels. This leads to the sinusoidal pattern you observed in the graph.

In the introduction of this article you can find many nice references if you are interested in some more details. Apparently, this has been textbook knowledge since at least 1903.

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    $\begingroup$ This refers to the article I linked to. Reanalysis is a difficult subject but in principle works like this: You take oberservational data at time x and feed it into a numerical weather prediction model. You run the model until time y for which you also have observational data and constrain it, such that the model output will match the observational data at time y. $\endgroup$
    – Joscha Fregin
    Jul 22 at 21:38
  • $\begingroup$ Thanks for your suggestions. I will keep the answer as it is. The link uses a doi which will refer to the article even if the link "rots away". $\endgroup$
    – Joscha Fregin
    Jul 22 at 21:51

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