Heatwaves in the United States are generally defined as periods of abnormally hot weather lasting at least two consecutive days, often with temperatures exceeding 90°F (32°C) or higher thresholds depending on the region. However, the typical duration is longer than the minimum.
Average Duration
• Nationally, the average heatwave lasts 3 to 4 days. This has increased over time: in the 1960s, the average was about 3 days, rising to 4 days by the 2010s and 2020s due to climate change influences.
• Three- and four-day heatwaves are the most common, occurring roughly twice per year in many areas.
Variations and Extremes
• Regional differences: In the Midwest and Northeast, heatwaves might last 2–3 days with high humidity, while in the Southwest (e.g., Arizona or California), they can extend 5+ days due to drier conditions and persistent high-pressure systems.
• Extreme cases: Rare but intense events can persist for 1–2 weeks or more, such as the 2021 Pacific Northwest heat dome (up to 7 days) or the 1936 North American heatwave (over 2 weeks in some areas).
• Trends: Durations are lengthening overall, with the annual heatwave season (from first to last event) expanding from ~24 days in the 1960s to ~73 days in the 2020s.
These patterns are tracked by agencies like the National Weather Service and EPA, which issue heat advisories for 2–3 days of risk and warnings for 4+ consecutive days. Climate change is accelerating both frequency and length, making prolonged events more common.