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?