Britain is turning into a pressure cooker as hot, steamy air and intense sunshine roasts the country and kicks off the first serious heatwave of summer.

The Met Office expects a high of 30C (86F) today, but warns that later in the week it could climb to 32C (90F) or even higher. And there could be many more uncomfortable nights ahead as thick clouds help to trap the day’s heat and humidity, with night temperatures of 18C (64F).

A Level 2 health warning has been issued by the Met Office for the week with fears that if temperatures continue to rise it will be forced to issue an emergency Level 4 warning. This is triggered when a heatwave is classed as severe and prolonged and where “its effects extend outside health and social care, such as power or water shortages, and/or where the integrity of health and social care systems is threatened”.

The heatwave has been fuelled by a mass of hot, sticky air feeding in from Europe, where some explosive thunderstorms have been sparked off and huge deluges have flooded large parts of the Czech Republic, Austria, Germany, Romania and Poland.

As that hot air mass has drifted over the Channel, it has been reheated over ground, baked by intense sunshine, to produce scorching temperatures. When the hot air collided with much cooler air aloft it detonated some big thunderstorms, such as the one in Small Heath Park, Birmingham, on Saturday in which teenagers were struck by lightning. A 17-year-old boy who suffered a heart attack in the lightning strike remained in a critical but stable condition in Heartlands Hospital, police said.

Five friends aged 16 and 17 were under observation. “I think the boys had been playing cricket and then were standing under a tree, sheltering from the rain,” Vasan Maniam, a park keeper, told the BBC.

The heatwave is set to be particularly intense in cities where heat is soaked up by buildings, roads and the rest of the urban landscape and rereleased like a storage heat radiator, making for uncomfortable conditions, especially at night. Another problem in cities is the build up of air pollution as traffic fumes are cooked by the sunshine into a toxic brew of ozone and other pollutants.

At the Level 2 health warning, the NHS issues advice on how to cope with the heat, such as staying out of the hottest times of the day between 11am-3pm, wearing a hat outdoors, staying cool indoors using fans, using wet sponges, taking baths and showers, blocking out sunlight with blinds or curtains and drinking plenty of water.

These warnings were borne out of the deadly heatwave of August 2003, when Britain’s temperature record was broken, 38.5C (101.3F) at Brogdale, Kent, and more than 2,000 people died of the heat across the country.

Will the temperature record be broken this year? If it happens it will probably strike in late July or early August. This is when the ground has had plenty of chance to heat up and the soil has baked dry and behaves like a furnace. A feed of hot air from Europe can then send temperatures soaring. The very young, elderly or infirm are the most vulnerable to heatrelated problems — not heat stroke or heat exhaustion as often assumed, but complications caused by a thickening of the blood and stress on the heart and circulation system, as well as respiratory problems.