Extreme heat wave strains U.S. power grids during July Fourth weekend

The United States government declared an energy emergency on Thursday as a powerful heat wave pushed temperatures above 40°C across much of the country. Around 160 million people in 30 states received extreme heat warnings. In cities including Washington D.C. and New York, it felt as hot as 45°C when humidity was included.

Power companies struggled to meet record electricity demand, partly because high temperatures cause power lines to sag and generating plants to work less efficiently. Amtrak cancelled more than 20 trains in the northeast and slowed others during the hottest hours of the day. An outdoor fair in Washington D.C. shut early on Friday for safety reasons.

The crisis arrived during the Fourth of July holiday weekend, when the country was celebrating its 250th anniversary.