Red alerts for extreme heat are in place in most of Italy's main cities as a heatwave intensifies in Europe.Temperatures are expected to peak on Wednesday, with 23 cities on high alert - from Trieste in the north-east to Messina in the south-west.The warnings mean the heat poses a threat to everybody, not just vulnerable groups.Wildfires are also raging across the continent, including in Greece and the Swiss Alps.Millions of people in the northern hemisphere are being affected by scorching temperatures.It is being caused by a high pressure system bringing warmer, tropical air, south of a jet stream currently stuck over central Europe.