“One of the more interesting takeaways from this summer is that many of the reliability concerns and demand records have occurred during weather that has been notably warm, but generally not historically extreme,” said Bradley Harvey, lead forecaster at Vaisala Xweather.

What has changed is humidity that makes high temperatures feel more oppressive, Vallee said. As the planet warms due to climate change, the air can hold more moisture, which inhibits overnight cooling and extends power demand later into the evening.