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Tornadoes Go Easy on the Weekends

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Tornadoes Go Easy on the Weekends

By Scott Johnson, Ars Technica

One of the classic sci-fi doomsday machines is the weather manipulator. What better way to bend the world to your will than taking control of the weather? It seems, however, that labor regulations may have beaten mad scientists to the punch.

Past studies have identified weekly cycles in a variety of weather phenomena, including rainfall, lightning, and storm heights. It’s called the weekend effect, and it’s thought to be be linked to the industrial air pollution associated with the five-day work week, though there has been a lot of discussion about the mechanics of that connection. These aren’t global analyses—many of these studies have focused on the southeastern United States during the summer months, though similar trends have been identified in other regions, as well. There’s a good reason for this. It seems that warm, moist conditions are a pre-requisite for the effect to manifest.

A new study published recently in the Journal of Geophysical Research adds to the list, finding strong evidence for weekly cycles in tornadoes and hail storms, and discusses the most likely mechanism behind them.

The researchers looked at the eastern half of the US (east of 100° W longitude) during the months of June to August. There’s a pretty sharp divide in average dewpoint temperature right along that longitude in the summer, with much higher dewpoints across most of the eastern United States. Data on the weekly pattern of atmospheric particulate matter (or aerosols) comes from EPA air quality monitoring. Summer aerosol concentrations seem to peak on Tuesdays (about 4-8 percent above the weekly average, depending on the particle size), and are lowest over the weekend (4-10 percent below average).

The group did some heavy-duty statistics to ensure a robust analysis, adjusting for things like long-term trends and seasonal patterns. In order to avoid the reporting bias that comes with improvements in weather-observing technology, the tornado and hail storm data only go back to 1995. In the end, they found a strong correlation between aerosol concentrations and the number of tornadoes and hail storms. The number of tornadoes was about 20 percent above average mid-week, and nearly 20 percent below average on the weekend. The hail storm pattern was nearly identical.

They repeated the analysis separately for each month and region of the eastern US to show that correlation is indeed strongest over the summer months in the southeast, and that no other significant correlation shows up anywhere. They also confirm that there is little difference in the correlation from year to year, and that no significant correlation exists for the western United States.

Aerosols and hail

So what is behind this apparent linkage between air pollution and violent weather events? Unlike the fabled inverse relationship between pirate population and global warming, there’s a good physical basis for the connection: it comes down to heat transport.

Aerosol particles are perfect condensation nuclei. More particles means more cloud droplets, but they’re competing for a limited pool of water vapor. Consequently, more cloud droplets also means smaller cloud droplets. The smaller the droplets, the less rain develops at low altitudes as warm air along a front rises and cools. Instead, the moisture is carried higher into the cloud before condensing.

Water vapor condensing into a liquid releases a lot of energy to the surrounding environment. By causing this energy release to occur higher in the cloud, aerosols strengthen the upward transport of heat that drives storm clouds—they push storm clouds closer to their maximum potential for severity.

Aerosols can stimulate hail formation by carrying cloud droplets above the freezing line. (Freezing a liquid, of course, releases even more energy.) Strong updrafts and plentiful hail stones are a potent mix for lightning. Those updrafts can also juggle hail back up above the freezing line repeatedly, building larger and larger hail stones. Even if the cloud is not cold enough or vigorous enough to produce hail, some cloud droplets will form small ice crystals, which are the very best seeds for raindrops. Paradoxically, by starting out with smaller cloud droplets (that reach greater heights) we end up with larger raindrops.

Giving tornadoes a push

All this has been indicated by extensive modeling as well as observations of weather systems affected by volcanic aerosols, but tornadoes are a bit different. Tornadoes require supercell-like conditions, where the storm cloud tilts like the Tower of Pisa, allowing cooler downdrafts to sink without interfering with rising warm air. Larger pools of cool air can bump into the rising column of warm air, disrupting the supercell state.

It should be clear that storm clouds are wild places for H2O, with freezing, melting, and evaporation accompanying large gusts of air that shift them from one place to another. As rain falls through the lower portion of the cloud, some of it evaporates. Since evaporation uses energy, this acts to cool that air—the mirror opposite of the effect of condensing cloud droplets at higher altitudes.

This evaporative cooling feeds the pool of cool air at the base of the cloud. Larger rain drops (aerosols help create these, too) provide less evaporative cooling than smaller rain drops. I’m guessing that’s essentially a surface area relationship—for the same amount of water falling as rain, smaller drops make for a much larger total surface area. Several modeling studies have shown that, for a storm cloud with the potential for producing tornadoes, simply increasing the rain drop size can push it over the edge.

Altogether, there seems to be a solid basis to conclude that anthropogenic aerosol emissions modulate certain types of weather events in areas where the atmospheric conditions are amenable.

You might want to keep an eye on severe weather in the states that recently passed legislation weakening labor unions. If storms have become accustomed to relaxing on the weekend, they may protest.

Source: Ars Technica

Image: National Oceanic and Atmospheric Administration

Citation: Journal of Geophysical Research, 2011. DOI: 10.1029/2011JD016214


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