But the definition can vary by region; in the Northeast, it is defined as three straight days in the 90s or above. Heat waves begin when high pressure in the atmosphere moves in and pushes warm air toward the ground. That air warms up further as it is compressed, and we begin to feel a lot hotter.

What is causing the heat wave?

Heat waves occur naturally in the summer, but climate change is exacerbating these events because emissions of heat-trapping greenhouse gases are causing average temperatures to increase.

Why are heat waves becoming more common?

Unusually hot days and multi-day heat waves are a natural part of day-to-day variation in weather. As the Earth's climate warms, however, hotter-than-usual days and nights are becoming more common (see the High and Low Temperatures indicator) and heat waves are expected to become more frequent and intense.