Blogging on EARTH: Controversy over the weekend effect

by Meg Marquardt
Thursday, January 5, 2012

It’s Friday: Is it raining where you are?

For several decades, humans have been blamed for climate phenomena, including the so-called weekend effect. The term, which is completely unrelated to the financial trend of the same name, applies to a theoretical week-long human-induced weather pattern that starts with the Monday drive to work. Air pollution released during the week due to the work commute leads to the buildup of clouds and heat, meaning warmer, wetter days as the week progresses. But on the weekend, when fewer vehicles are on the road, the atmosphere resets, so come Monday the weather is clear and cool.

But a question remains: Does the weekend effect even exist, or is it the result of a handful of papers attributing human influence to natural weather patterns? The answer is not clear.

The hunt for the weekend effect can be traced back to at least 1974, when it was known as the Sunday effect. A paper published that year in Science compared daily air pollution measurements to solar radiation levels in New York and New Jersey. The results showed that there was more sunlight on Sundays than during the work week and that aerosol concentrations were at their lowest on Sundays. Their hypothesis behind the connection was the same as it is today: Cars and industrial operations are the main source of aerosols. When cars are off the road and factories are quiet, the air cleans up, allowing more sunlight to shine through. Air pollution decline on the weekends has been well-documented since the 1950s.

The catch-all word “aerosol” refers to a wide-range of solid particulates found in gas. When it comes to air pollution, aerosols are often a mix of carbon dioxide, oxides of nitrogen or chlorofluorocarbons (CFCs), along with particulate matter. As aerosols drift into the upper atmosphere, they can act as cloud condensation nuclei, the seeds around which clouds can form. Thus, the more aerosols, the more clouds.

So, by that definition: More people equals more aerosols equals more clouds. Thus it’s no surprise that as populations have boomed, the weekend effect has gathered increasing attention.

Now, another question has been thrown into the mix: What if the weekend effect has nothing to do with humans at all? Is there a natural seven-day cycle in the atmosphere that’s responsible for our rainy Fridays? One scientist thinks so, and his research is causing a buzz in the atmospheric research world.

A recent study in Geophysical Research Letters by Kwang-Yul Kim of Seoul National University in South Korea suggests that once you have removed the human element from all of the data, the seven-day pattern of weather remains, meaning there must be some natural component to the weekend effect. He points toward Rossby waves, atmospheric rivers in high-altitude winds that are known to play a major role in weather.

Other researchers aren’t so sure. Ronald Cohen, an atmospheric chemist at the University of California at Berkeley, says that the atmosphere is full of waves that have countless patterns, so finding a seven-day pattern at random is not surprising. What would be surprising, however, is if that random pattern is constantly driving a weather phenomenon that lines up exactly with our work week, he says.

Unlike papers in the past, such as the 1974 Science paper that focused on New York and New Jersey, Kim’s study looked at the entirety of the United States instead of a specific urban area. And covering so much land may be why a “natural component” of the weekend effect suddenly emerged, Cohen says. The amount of space taken up by urban areas in the United States is minuscule compared to the huge swaths of rural and agriculture land. So, he says, there would be a “huge bias toward a natural component” rather than a human one. If the model were to focus solely on an urban setting like Chicago, it is likely that the human domination would return.

Researchers didn’t see a strong human influence on weather until the 1990s, says Thomas Bell, a former climate researcher for NASA who is still actively publishing in his retirement. And even with that, not all researchers agree that a human-caused weekly weather influence even exists, he adds. They point to suspect experimental designs and inconclusive statistics as evidence that no one has clearly shown a link.

Bell thinks there probably is such a link, however, as his own research has shown that lightning is just as sensitive to pollution as rain clouds; over the course of 10 years' worth of data, there is a significant pattern of increased lightning strikes during the middle of week in the southeastern United States, he says. Nonetheless, he recognizes that the question as to whether there is a weekend effect and whether it is caused by humans will not likely be answered soon.

After all, if it were really so cyclic, meteorologists wouldn’t have to work so hard on their forecasts.