Large fires are more common on weekdays, a study of more than a billion such events has revealed. The aerosols given off by these fires shape our weather, and the discovery could help improve our understanding of climate dynamics and possibly even help us predict when it will rain.

Dr Nick Earl of the University of Melbourne used a database of fires observed from space between 2001 and 2013. Earl told IFLScience that NASA uses the Moderate Resolution Imaging Spectroradiometer, an infrared detector, to seek fire by spotting areas substantially hotter than their surroundings. In Geophysical Research Letters Earl reports that there were fewer fires seen worldwide on Sundays than any other day, and more on Tuesdays and Wednesdays. The effect is stronger in wealthier countries.

"Nature doesn't adhere to the weekly cycle, so this really highlights the influence we have on the planet when it comes to fires,” Earl said in a statement. The patterns indicate that a very high proportion of fires are being lit as part of Monday-Friday jobs, either to prevent larger fires by reducing fuel or for agricultural purposes.

House fires do not heat large enough areas to be detected from space, and Earl told IFLScience even burning skyscrapers or industrial sites might not make the register. Observed fires are rural, resulting from a mixture of lightning strikes, accidents, arson, and deliberate burning, the last of these either to reduce fuel before the wildfire season or to clear land for agriculture.

Earl notes that fires starting from an unattended campfire or dropped cigarette are most likely to occur on weekends, suggesting that legally lit fires are probably even more skewed towards weekdays than the eight percent overall gap between Sunday and Tuesday.

There is a cultural influence to the distribution as well. Predominantly Muslim regions of central Asia have the lowest numbers of fires on Thursdays and Fridays, when starting blazes or monitoring and controlling those lit the day before, would clash with the call to prayer. The Washington Post described Earl’s work as revealing the capacity to “see religion from space”. Unfortunately, Earl could detect no weekly cycle for Indonesia with destructive fires raging throughout the dry season.

^{The United States and southern Australia show particularly stark differences in fire numbers by week days, while Kazakstan has fewer fires on Fridays. Each bar graph starts with Sunday. Earl et al. Larger version can be viewed here.}

Earl is interested in the topic for what it reveals about the aerosols (airborne particles) released in smoke. Industrial aerosols have been shown to shape the weather of the United States, with working days wetter than weekends as a result of the additional particulates pumped into the air. It is suspected that smoke from the fires Earl has studied are having a similar effect, but the details are unclear.

Measuring aerosols from fires worldwide is a challenge. Earl pointed out to IFLScience that “some aerosols come down within a few minutes while others stay in the air for a week.” Nevertheless he hopes his work will lead to a better understanding of the way aerosols affect cloud formation, with benefits for predicting how the atmosphere will change on both long and short timescales.

The database also sheds light on the geographic and monthly distribution of fires, with the paper noting that 43 percent of fires large enough to be detected from space occur in Africa.