Washington, June 19 : White House spokesperson Karine Jean-Pierre paused her daily briefing on Tuesday to check on a reporter who passed out because of severe heat conditions sweeping through Washington and other states in the US.

Spring officially ends in the US on Thursday, June 20, but the summer is not waiting.

'It is hot in here,' Jean-Pierre said as she resumed the briefing and asked the reporters if anyone needed water. The intern was treated by the White House medical staff.

Temperatures are hovering in the upper to mid 90-degree Fahrenheit (around 32-33-degree Celsius). Outside the White House briefing room, the temperature was around 91 F. And it is expected to get hotter later in the week, touching 100 F.

The heat index -- a measure of how hot it really feels when relative humidity is factored in with the actual air temperature -- has touched 100 F in Florida in the south of the country right up to Maine, the northernmost state that borders Canada on the east coast.

An estimated 70 million Americans are caught in this heat wave.

States in the midwest were enveloped in punishing heat earlier in the week. Residents of Chicago flocked to the beaches and swimming pools that had opened for the summer on Monday, when the temperature climbed to 95 F. Cincinnati in the neighbouring state of Ohio had crossed the 100 F-mark to 102 F.

The heat wave has since shifted eastward, enveloping the region. Washington has declared an extended heat emergency until the end of the week and activated cooling centres, which can be day centres, indoor and outdoor swimming pools, senior wellness centres, public libraries, recreation and community centres, youth centres, and spray parks.

The meteorological phenomenon behind this intense heat wave is called a 'heat dome'.

It's a high-pressure system that develops high above in the atmosphere enclosing the region below in a pot-like heat build-up. The high-pressure system compresses hot air, which gets hotter over time with sustained pressure from above.

Heat wave is described as a rise in temperature and heat dome is a high-pressure system that traps heat. Heat domes are linked to climate change and global warming. A leading meteorologist told The New York Times that in the 1970s, there was a cold wave for every heat wave, while now there are more heat waves in the ratio of two to one cold wave.

/IANS