Monday was the warmest day recorded on Earth. But how do scientists actually measure that?

In the past week, Earth’s record for the hottest day was broken twice. Sunday July 21 was declared Earth’s hottest day since records began, when average surface temperature reached 17.09°C. On Monday the record fell again, when average temperatures reached 17.16°C – and Tuesday was almost as hot.

The declarations were made by Copernicus, the European climate change service. They made international headlines – especially in the northern hemisphere, which has been experiencing extreme summer heat.

Determining the global average temperature on any given day is complex. It involves thousands of observations using high-tech equipment and in some cases, sophisticated computer models.