Are fires at night time always Arson?
The idea that fires that start at night are always the result of arson frequently comes up in public debate whenever there are large fires in Portugal.
This was also the meaning of the statement made by João Tilly , a member of parliament for the Chega party, at the Parliamentary Inquiry Committee into the Rural Fires Business , last Tuesday.
“The fires that break out at night and in the early morning are all arson. All of them, 100%,” stated the Chega party deputy.
In his response, researcher Joaquim Sande Silva, a former member of the independent technical commission for the 2017 fires and of the Independent Technical Observatory, downplayed the “arson myth”.
“Beyond being a teacher, I am also a researcher, so I try to be guided by facts, things that are properly proven. It is not properly proven that all fires that break out at night are intentionally set,” said the expert, before listing scenarios in which a fire can break out at night.
Let’s look at the data.
The available official data – namely from the Institute for Nature Conservation and Forests (ICNF) and entities within the fire management system – show that the generalization made by João Tilly has no factual basis .
First of all, it is important to separate two different issues: when the fires start and what causes them . The ICNF reports exhaustively analyze the causes of ignitions, but do not establish any direct relationship that would allow us to conclude that the nighttime period is predominantly associated with arson . On the contrary, the evidence points to a more complex reality.
The most recent data indicate that arson accounts for a substantial portion of fires. In the last provisional report of 2025 , for example, the ICNF (Institute for Nature Conservation and Forests) states that approximately 31% of the fires investigated were intentionally set .
In turn, burning and controlled burns account for almost a quarter (23%) of the total causes identified , a trend that is not isolated. Negligence is historically the main cause of fires: between 50% and 60% of occurrences originate from the improper use of fire (controlled burns, campfires, burning of garbage), while arson typically accounts for 25% to 40% of cases, although it has a greater impact on the area burned.
Furthermore, there are other relevant causes that dismantle the idea that everything boils down to deliberate criminal acts : rekindling of poorly extinguished fires, equipment or power line failures, machinery accidents and, although residual, natural phenomena such as lightning strikes.
The report prepared last year by the Agency for Integrated Rural Fire Management also underlines this diversity. In 2024, for example, only about 70% of the fires investigated had a determined cause, which means that a significant portion remains undetermined, making any simplistic conclusion about motivations impossible.
There is also an essential point that directly contradicts the statement in question: the fact that a fire starts at night does not allow us to infer its cause. Many negligent ignitions occur precisely outside the hottest hours: for example, controlled burns carried out at the end of the day that are poorly extinguished and reignite during the early morning, or agricultural work that prolongs thermal effects for hours. Reignitions are, moreover, a category of their own in official statistics and explain a portion of the occurrences.
The legislation itself imposes time restrictions on the use of machinery on days with a high risk of fire – “from 11 am until sunset”, according to Article 69 of Decree-Law No. 82/2021 – which pushes these activities into the cooler hours (night/early morning), increasing the likelihood of accidents during those time windows.
Furthermore, meteorological and physical factors also play a significant role in the evolution of fires during the night . The decrease in temperature and the increase in humidity can slow the progression, but they do not prevent latent fires from turning into active fires hours later, without any further human intervention.
In an in-depth analysis of one of the most recent major fire tragedies in Portugal, the report by the Independent Technical Commission on the October 2017 fires points to a ” very strong correlation between the number of ignitions and the exceptional weather conditions ” and specifies: “The fires of October 15 were dominated by the influence of Hurricane Ophelia . Initially, it was the wind strength and low humidity that allowed the fires to grow. However, it is the set of pyro-convective phenomena [ascending and descending movements in the atmosphere associated with fires] that subsequently develop, and not solely the wind, that is responsible for the large fire episode on the afternoon of October 15 and the night of October 16.”
In short, official data from the ICNF and other entities show that arson is only one of several causes of fire and that there are multiple additional origins (accidental, rekindled, unknown), with no evidence linking nighttime to arson .
SIC Verifica that it is…
The statement that “fires that break out at night and in the early morning are all arson” lacks factual support. It is a generalization unsupported by official data, ignoring the diversity of causes and the actual ignition mechanisms of rural fires in Portugal.
