There is a scientific reason why it always rains on weekends

If you are in the northeastern United States, it's likely that your weekend traffic makes it rain on Saturday and Sunday. In fact, most of us help control the weather in some way. Here's how you are making the sun shine, or bringing on a thunderstorm.

If you are in the northeastern United States, and yet another weekend has been ruined by rain, feel free to lean your head out a window and scream at a motorist. They are, for the most part, to blame. In 1998, a group of scientists at Arizona State University analyzed data going back to the 1940s, and found that a disproportionate amount of rain was hitting on weekend days. Since the seven day week isn't natural, they went looking for some unnatural causes of the wet weekends.


How Humans Make it Rain

Most studies that discuss human-made weather patterns mention "aerosols." Although we think of aerosols as the hairspray proprellants that are eating a hole in the ozone layer, in this case they are just little particles of gas, liquid, or dust that float around the air. They can be spewed out by volcanoes and dust storms, or cars and factories. Throughout the week, they are sprayed out by cars and factories.

These aerosols act as little seeds to which water droplets can cling. As the amount of aerosols in the atmosphere grows over the week, more liquid water condenses around these little particles. When enough water groups together, it starts to rain. As we drive, or otherwise churn out gas, dust, and liquid, we are seeding the clouds.


But it's not as simple as just cloud seeding. Later studies, which focused on the southern part of the eastern seaboard have found that, when it comes to massive thunderstorms and tornadoes, people need to brace themselves midweek during the summer.

The particulates in the air cause convection, the process by which warm air near the surface of the Earth rises up, letting cold air rush in to warm itself, and rise up in turn. High in the sky, due to the dust and liquid aerosols, larger-than-usual raindrops and hailstones form. These large drops, and large hailstones, are not just misfortunes for whoever they happen to hit. Large objects don't evaporate as quickly as smaller ones. Evaporation is a process that takes in heat, leaving the area around it colder. Less evaporation means less cold, and large cold spots tend to discourage the formation of tornadoes. So because of a build-up of particulates during the week in summer, tornadoes and hailstorms are more likely to strike.


Human-Made Weather All Over the World

And what about the rest of the world? We all influence the weather, but what we get depends on what kind of weather was to be had in the first place, the geography of the region, and our neighbors. Iceland, for example, is getting colder and wetter weekends. Germany gets wet weather towards the end of the working week.


The United Kingdom and France, on the other hand, are catching a break with warmer and clearer weekends every week. And then there is Spain, which has nice clear winter weekends, but is more likely to have cold, wet, miserable summer weekends.


Even the clearing of pollutants can be a problem, depending on where you live. North Atlantic tropical storms and hurricanes may have been suppressed by lots of pollution. Aerosols have been reflecting the sun's heat back up, away from the sea, causing less evaporation and wetness, and reducing hurricanes. As people have somewhat cleaned up their act, hurricanes have been increasing.

No matter what we do, we're weather gods.

[Via Nature, ABC Science, Scienceagogo, Why do Tornadoes and Hailstorms Rest on Weekends.]


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