Overnight fires are emerging in North America with previously unknown drivers and implications. This notable phenomenon challenges the traditional understanding of the ‘active day, quiet night’ model of the diurnal fire cycle^1,2,3 and current fire management practices^4,5. Here we demonstrate that drought conditions promote overnight burning, which is a key mechanism fostering large active fires. We examined the hourly diurnal cycle of 23,557 fires and identified 1,095 overnight burning events (OBEs, each defined as a night when a fire burned through the night) in North America during 2017–2020 using geostationary satellite data and terrestrial fire records. A total of 99% of OBEs were associated with large fires (>1,000 ha) and at least one OBE was identified in 20% of these large fires. OBEs were early onset after ignition and OBE frequency was positively correlated with fire size. Although warming is weakening the climatological barrier to night-time fires⁶, we found that the main driver of recent OBEs in large fires was the accumulated fuel dryness and availability (that is, drought conditions), which tended to lead to consecutive OBEs in a single wildfire for several days and even weeks. Critically, we show that daytime drought indicators can predict whether an OBE will occur the following night, which could facilitate early detection and management of night-time fires. We also observed increases in fire weather conditions conducive to OBEs over recent decades, suggesting an accelerated disruption of the diurnal fire cycle.

北美正在出现夜间火灾，其驱动因素和影响此前未知。这一值得注意的现象挑战了对昼夜火灾周期“白天活跃，夜间安静”模式的传统理解^1,2,3和当前的火灾管理实践^4,5。在这里，我们证明干旱条件会促进夜间燃烧，这是助长大规模活跃火灾的关键机制。我们利用地球静止卫星数据和地面火灾记录，研究了 2017 年至 2020 年期间北美发生的 23,557 起火灾的每小时昼夜周期，并确定了 1,095 起夜间燃烧事件（OBE，每个事件定义为火灾燃烧整夜的夜晚）。总共 99% 的 OBE 与大火（> 1,000 公顷）有关，其中 20% 的大火中至少发现了一种 OBE。OBE 在着火后较早发生，OBE 频率与火势大小呈正相关。尽管变暖正在削弱夜间火灾的气候屏障⁶，但我们发现近期大型火灾中 OBE 的主要驱动因素是累积的燃料干燥度和可用性（即干旱条件），这往往会导致在一段时间内连续发生 OBE。单场野火持续数天甚至数周。至关重要的是，我们表明白天干旱指标可以预测第二天晚上是否会发生 OBE，这可以促进夜间火灾的早期发现和管理。我们还观察到近几十年来有利于 OBE 的火灾天气条件有所增加，这表明昼夜火灾周期的加速破坏。