Temperatures were set to soar across Europe beginning on Wednesday, with authorities from Paris to Warsaw issuing heat alerts, canceling events and bracing for potentially record heat across the region.

Meteorologists say the temperatures, a result of a warm air mass traveling north from Africa, could climb well above the typical June average to 40 degrees Celsius, or more than 104 degrees Fahrenheit, in large stretches of the continent.

Temperatures hit 98 degrees Fahrenheit in Berlin, 96 degrees in Vienna, 95 degrees in Warsaw and 92 degrees in Paris, and it was likely to get worse, not better, in many places.

Heat waves like these are becoming more frequent and prolonged, with hotter temperatures appearing earlier in the season, weather experts say. Here’s what to know as the mercury rises.