How a heatwave developsThe current warm weather spell has been created by a jet stream looping to the north of the UK, creating an area of ‘homegrown’ high pressure, say forecasters.“Long days, very still conditions and clear skies help June temperatures to get very intense,” explained Met Office forecaster Mark Foster.“The sun in June is relatively the highest it gets in the sky and heat can build up over successive days. People enjoy the sunshine in St James’s Park in LondonCredit:AFP Britain is currently sweltering in a heatwave with temperatures expected to reach a high of 33C (91.4F) by the end of the week.The UK is enjoying wall-to-wall sunshine and sizzled in the hottest day of the year so far on Monday, with temperatures rising to 30.1C (86.1F).The Met Office has issued a level two “yellow” health warning in response, with social and healthcare services at the ready to reduce harm during the extended period of hot weather. It has also sparked wildfires, resulted in train services being cancelled and warnings from police being issued about the dangers of swimming in open water.Bookmakers have now slashed the odds on next month being the hottest July on record, with the high temperatures expected to continue for the next few weeks. Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings. The Met Office explains high pressure systems are “slow moving and can persist over an area for a prolonged period of time such as days or weeks”.“They can occur in the UK due to the location of the jet stream, which is usually to the north of the UK in the summer,” it adds.The World Meteorological Organization defines a heatwave as “marked unusual hot weather over a region persisting at least two consecutive days”. “High pressure is going to dominate this week so we can expect very high levels of pollen and UV.” #Temperatures peaked at just over 30 degrees in the end at both Hampton Water Works in London and Teddington in Middlesex this afternoon, confirming that it’s been the warmest day of the year so far for the UK! #Heatwave 🌡️ pic.twitter.com/fw4KKRn81x— Met Office (@metoffice) June 25, 2018 The previous 2018 record was seen at St James’s Park in central London, which reached 29.1C (84.3F) on April 19.Meanwhile in Scotland, Achnagart in Ross and Cromarty boasted a temperature of 27.5C (81.5F) and the heat hit 25.3C (77.5F) in Castlederg in Co Tyrone, Northern Ireland, on Monday.The high temperatures are expected to be beaten later this week- before dropping off at the weekend. Mercury passes 30C – and it is due to get even hotterOn Monday, the hottest point of Britain was at Hampton, in London, where temperatures reached 30.1C (86.1F) – making it the hottest day of the year so far. He said the mercury could rise to a scorching 33C (91.4F) later in the week, soaring above the UK average for this time of year of 17.3C (63.1F).“At the moment it looks like that will be the general theme through much of July,” he said. “Generally it looks like it will stay drier and warmer than average.”Will it break records?The current heatwave will go close to breaking records for the hottest days and months ever recorded in the UK.The hottest June day on record in the UK was the 35.6C (96F) recorded during the scorching summer of 1976 in Southampton’s Mayflower Park.Last year, Britain basked in the hottest June day recorded in over 40 years when temperatures hit a peak 34.5C (94.1F) at Heathrow, west London, on 21 June.For July, the hottest day on record remains the 36.7C (98F), also recorded in west London in 2015. Met Office meteorologist Alex Burkill said the warm weather, with parts of the UK currently hotter than Athens in Greece and Rio in Brazil, looks set to continue into next month. The heatwave of 1976 produced the hottest summer average temperature in the UK since records began, when temperatures reached 32.2C (90F) for 15 consecutive days.Bookmaker Coral has also slashed the odds on next month being the hottest July on record and the UK’s hottest summer overall.“April, May and June have all been well above the average temperatures for the time of the year and with the current heatwave expected to continue we have had no choice but to slash the odds on the hottest July of all time,” said Coral’s Harry Aitkenhead.