There is nothing in nature that occurs on a seven-day weekly cycle.
It means that any weekly pattern seen in weather parameters, be it temperatures or rainfall, is due to human activity, like generating electricity or using high-polluting vehicles.
Until now, no one has investigated whether fires occur on a weekly cycle. Establish a pattern, and establish human influence on fires throughout the world.
For this study at the University of Melbourne’s School of Earth Sciences, we analysed all the available fire data from the MODIS satellite from Nasa’s Earth Observatory website between 2001 and 2013.
Globally, the data shows large fires peak in the middle of the week with a Sunday minimum. All up over those 13 years, there were a total of 104 million Sunday fires.
That’s nine million (or eight per cent) less than on a Tuesday.
It’s a strong indication that there is a global weekly cycle of fires, heavily influenced by the working week and conversely, days of rest.
Some might say it offers a direct link to religion and its effect on the world.
Studies into weekly cycles go back as early as the late 1920s where James Reginald Ashworth “accidentally” discovered a weekly Sunday minimum when investigating rainfall in the industrial town of Rochdale in England.
He suggested that the weekly variations in smoke emissions from the local factories led to mid-week peaks in rainfall through the hot emissions during the week that caused instability, or to an aerosol effect on the clouds.
Since this insightful study, there have been many improvements in the quality and quantity of weather observations, with instruments like radar, satellite systems and automatic weather stations.
These technological advances have provided the opportunity for hundreds of more sophisticated studies into weekly cycles with focus on temperature, rainfall, wind, pressure and even lightning.
Many of these studies find strong signals on a week-long scale (eg. often with Sunday being the driest and coolest day of the week), from local studies in urban areas to the whole globe.
Our study, co-authored with fellow University of Melbourne Professor Ian Simmonds and Professor Nigel Tapper from Monash University, is important because it suggests the influence humans have on the climate, whether it be good or bad.
We know that fires are a major source of atmospheric aerosols, well known to affect the weather in many different (often conflicting) ways.
The overall effect that aerosols have on the weather varies a lot depending on where you are.
For example, aerosols over snow-covered areas absorb the sun’s radiation and reduce the amount of energy being reflected from the area by the bright surface, warming the surrounding area.
However, over dark surfaces like dense forest, these aerosols can actually reflect some of the sun’s radiation, reducing the amount being absorbed at the surface, cooling the atmosphere.
Around the world
The US’s fire weekly cycle is very strong, with close to twice as many fires occurring on Wednesdays compared to Sundays, influenced by the Monday-Friday working week.
The fire weekly cycle is also very strong for Australia, displaying a weekend minimum and a Tuesday peak.
When split into northern and southern Australia, it is clear that the weekly cycle is associated with areas in the south, whereas in the tropical north, there is no cycle.
The signal is strongest during the winter months because during the natural bushfire season (January and February), the controlled burning is dependent on the weather, burning during lower risk damp periods.
The governments of states that include areas in southern Australia encourage autumn/winter/spring burning for landowners, although in Victoria it becomes too wet during the winter months.
But the Saturday or Sunday minimum is not coherent throughout the world.
Areas with a higher Muslim population like the Kazakhstan region have minima on Thursday and Friday.
But it is worth noting that Friday is “the day of assembly” and prayer for the Muslim faith, so there is likely to be less industrial activity on these days.
This pattern indicates how days of rest can affect the large-scale weekly cycle of fires, with potentially major subsequent effects on the weather.
It is very likely that, on average, these days of rest experience cleaner air, cooler temperatures and less rain due to fewer aerosols from fire and other industrial activity.
So it suggests our weekly routine, originating from religion, has a major influence on the weather.
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