The previous record of 5,184 metres was set on July 25 last year.
A hot spell enveloping Europe has pushed the zero-degree line -- the altitude at which the temperature dips into the minus -- to a record height of nearly 5,300 metres (17,400 feet) in Switzerland.
The zero-degree line is determined by meteorologists using weather balloons that take off twice a day from Payerne in western Switzerland.
MeteoSwiss said the new height was clocked overnight from Sunday to Monday at 5,298 metres, "which constitutes a record since monitoring began in 1954".
The previous record of 5,184 metres was set on July 25 last year.
"The area known as the zero-degree isotherm is the threshold between air layers with temperatures above 0 degrees Celsius at lower altitudes and those with temperatures below freezing at higher altitudes," MeteoSwiss said.
"Among other things, the zero-degree isotherm affects vegetation, the snow line and the water cycle, and so has a considerable impact on the habitats of humans, animals and plants alike," it added, calling the marker "an integral part of weather forecasts in the Alpine region".
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