New research may explain why serious thunderstorms and tornados are less prevalent on the weekends

Dec 22, 2011 by Bob Yirka report
Image credit: AGU

(PhysOrg.com) -- For much of the last century, people in parts of the United States have come to notice that just as they got the weekends off to relax, so too did it seem, did serious weather. Big booming thunderstorms that produced large hail and/or tornados, seemed to strike at will during the week, but come the weekend, things grew quiet. While there have been many theories as to why this might be, mostly religion based, it hasn’t been until much more recently that researchers have begun to take a closer look.

Now, in a truly interesting study, Daniel Rosenfeld and Thomas Bell, a seemingly odd paring when you consider that Rosenfeld is with the Institute of Earth Sciences in Israel, while Bell is with NASA, have found that the phenomenon is apparently real, though it’s clearly not because of an otherworldly presence. As they describe in their paper published in the Journal of Geophysical Research, it has far more to do with more down to earth human activities.

Rosenfeld and Bell limited their study area to a specific region of the American Southeast, an area that receives a number of and tornados every summer, and has been seen over the years as being particularly sensitive to the weekend syndrome.

Image credit: AGU

Because prior research by other groups showed that there were indeed more serious type storms in the area during the week, the two suspected it had something to do with air pollution. Using data (1995 to 2009) obtained from the EPA, which monitors air quality, they found that during the three summer months of June, July and August, there was a clear correlation between certain days of the week and the amount of aerosols in the atmosphere. As an example, they found that aerosols hit peak concentrations on Tuesdays, while bottoming out on weekends; evidence of the human work week, which causes much less particulate matter to be spewed when people are home relaxing on weekends.

Then, because aerosol concentrations are known to cause small water droplet formation in the air, causing clouds to form, they theorize that the addition of more aerosols leads to smaller droplets in those clouds because there is only so much water in the air to cling to. Smaller droplets means less regular type rain because the droplets are lighter and get carried up higher into the atmosphere. When those lighter droplets do eventually condense into rain, they release a lot of upper atmospheric energy, which creates even more updrafts, which can pull hail upwards over and over increasing their size and produce more dramatic lightning and wind. The end result is a storm that is more powerful than it would have been were it not for the addition of extra aerosols.

New research may explain why serious thunderstorms and tornados are less prevalent on the weekends
Image credit: AGU

But that’s still only part of the story. For such storms to spawn tornados, there has to be a shift of sorts or a tilt in the clouds to create so-called supercells, which occur when cool air is allowed to drop rapidly down though the lower clouds without affecting the warm air just next to it. This may come about, the researchers suggest, due to small raindrops evaporating as they fall through the lower clouds, causing the and surrounding larger droplets, to cool. At that point, all it would take is for certain twists and turns to cause changes in the cloud structure causing supercells to form and then the generation of tornados.

Thus, it all boils down to the fact that it’s our own human endeavors that are impacting the weather in ways that make sense when researchers look close enough.

Explore further: Lava creeps toward road on Hawaii's Big Island

More information: Why do tornados and hailstorms rest on weekends? JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, D20211, 14 PP., 2011
doi:10.1029/2011JD016214

Abstract
This study shows for the first time statistical evidence that when anthropogenic aerosols over the eastern United States during summertime are at their weekly mid-week peak, tornado and hailstorm activity there is also near its weekly maximum. The weekly cycle in summertime storm activity for 1995–2009 was found to be statistically significant and unlikely to be due to natural variability. It correlates well with previously observed weekly cycles of other measures of storm activity. The pattern of variability supports the hypothesis that air pollution aerosols invigorate deep convective clouds in a moist, unstable atmosphere, to the extent of inducing production of large hailstones and tornados. This is caused by the effect of aerosols on cloud drop nucleation, making cloud drops smaller and hydrometeors larger. According to simulations, the larger ice hydrometeors contribute to more hail. The reduced evaporation from the larger hydrometeors produces weaker cold pools. Simulations have shown that too cold and fast-expanding pools inhibit the formation of tornados. The statistical observations suggest that this might be the mechanism by which the weekly modulation in pollution aerosols is causing the weekly cycle in severe convective storms during summer over the eastern United States. Although we focus here on the role of aerosols, they are not a primary atmospheric driver of tornados and hailstorms but rather modulate them in certain conditions.

via Arstechnica

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User comments : 19

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SleepTech
5 / 5 (18) Dec 22, 2011
The obvious answer to this is we need longer weekends
CapitalismPrevails
1.9 / 5 (13) Dec 22, 2011
Far streeeetched to me.
DrSki
1 / 5 (4) Dec 22, 2011
Hopefully the researchers removed those storms/tornadoes associated with hurricanes and tropical storms prior to their correlations (unless positing some 'Action at a Distance'). Then time of day might need to be addressed since residual particulate matter from Friday activity could spill over to Saturday unless nocturnal storms/tornadoes have been accounted for separately. Otherwise VERY intriguing, especially for late afternoon/early evening storms when daytime warming might time very well with increased late afternoon/early evening particulates. The above considerations would only STRENGTHEN their theory.
rawa1
1.7 / 5 (6) Dec 22, 2011
it's proven statistically, people are making weather warmer and drier on per week basis - we can just extrapolate these weekend trends to decades of years. During last warming periods the rise of carbon dioxide followed warming with delay of many decades with compare to present situation - so we can see this argument of many sceptics rather as another evidence for man-made origin of global warming. In addition, we can consider for example September 11, 2001 climate impact study. Measurements showed that without contrails, the local diurnal temperature range (difference of day and night temperatures) was about 1 degree Celsius higher than immediately before.

http://www.greend...man-made
http://news.bbc.c...6120.stm
http://facstaff.u...ar04.pdf
Xbw
1.9 / 5 (9) Dec 22, 2011
Wow this is a blast from the past. I remember the 80s. HAIRSPRAY IS PUTTING A HOLE IN THE OZONE! Everyone in New Zealand is getting cancer!
rawa1
1 / 5 (4) Dec 22, 2011
Wow this is a blast from the past. I remember the 80s
The aerosols are causing droughts in well understood mechanism: they serve as a condensation nuclei for water droplets. If you introduce too many nuclei into wet air, then the water vapour will condense into fog, instead of more sparse but heavier rain droplets and it will not fall down to Earth. Whereas I don't believe in antropogenic origin of global warming very much, the global droughts can be of antropogenic origin quite easily and we should always distinguish these two aspects of recent climatic changes.
Isaacsname
not rated yet Dec 22, 2011
Well sh*t, trees put out terpenes that are responsible for cloud seeding, particulates in exhaust are larger, no ?
LKD
3 / 5 (8) Dec 22, 2011
Clearly there is a God. And God wants us to enjoy our weekends. XD
LKD
2.7 / 5 (3) Dec 22, 2011
...about 1 degree Celsius higher than immediately before.


The only thing the study returned with was the very interesting notion that contrails can act as an insulatory factor in the atmosphere.
Cave_Man
2 / 5 (5) Dec 22, 2011
...about 1 degree Celsius higher than immediately before.


The only thing the study returned with was the very interesting notion that contrails can act as an insulatory factor in the atmosphere.


Old news though, there was a study on this site not long ago that contrails have contributed more to an increasing average yearly temperature. If you take all the contrail effect and all the vehicular activity since they were both invented the contrails effect is greater on temperature disruption and increase.

I can't respect anyone who still doesn't believe in obvious facts like Anthropocentric Climate Change.

Everyone here has driven their cars around the circumference of the planet between 4 and 12 times with just 300million people in america all that exhaust is being emitted relatively confined. If you live in a big city (over 1-2mil) then you are brain damaged from CO, Benz and many other neurotoxins in exhaust, that's not an insult it's a fact of biology
Nanobanano
3.7 / 5 (3) Dec 22, 2011
I find this humorous, since we actually noticed the exact OPPOSITE phenomena when I was in school and college.

When it was a weekday, and you wer at school, the weather was perfect. When it was a weekend and you wanted to go outside and play basketball in the driveway, or baseball, etc, it always seemed to be raining.

Could be an observers bias, whereby you "remember" the rain screwing up your plans more often than the days it did not screw up your plans, but just saying, it definitely happened.

My brother and neighbours used to talk about it all the time, "it always seems to be raining on the weekends".

That was mid and late 90's, so it's overlapping the time of this supposed study.

Now of course, local trends don't always coincide with the average of a regional trend, so I understand that.

Just find it odd that personal experience is the exact opposite of the study...
rawa1
2.5 / 5 (4) Dec 23, 2011
I find this humorous, since we actually noticed the exact OPPOSITE phenomena when I was in school and college.
It's not so strange, because the wind and hailstorms aren't necessarily related to wet, rainy weather. They're rather related to atmospheric instabilities. From the same reason the extreme weather but no rain is attributed to global warming.
Ojorf
1 / 5 (1) Dec 25, 2011
I find this humorous, since we actually noticed the exact OPPOSITE phenomena when I was in school and college.


Yeah, it's called confirmation bias.
Forestgnome
1 / 5 (1) Dec 25, 2011
Wow this is a blast from the past. I remember the 80s
The aerosols are causing droughts in well understood mechanism: they serve as a condensation nuclei for water droplets. If you introduce too many nuclei into wet air, then the water vapour will condense into fog, instead of more sparse but heavier rain droplets and it will not fall down to Earth. Whereas I don't believe in antropogenic origin of global warming very much, the global droughts can be of antropogenic origin quite easily and we should always distinguish these two aspects of recent climatic changes.

That's interesting, because when we intentionally seed clouds to alter weather the effect almost immeasurable.
Callippo
1 / 5 (3) Dec 25, 2011
when we intentionally seed clouds to alter weather the effect almost immeasurable
In general, the seeding of atmosphere during drought has no effect, because there is no water to condense. The experiments with seeding of clouds to bring rain during periods of drought actually make the problem even worse, because they increase the number of nuclei, thus prohibiting the formation of larger droplets, which would fall down in form of rain. The smaller the droplets are, the more difficult they coalesce due their increased surface curvature and charge. The attempts with charging of clouds just makes the clouds more stable (compare the oleum fog, which is very stable being composed of very tiny charged dropplets). What would work instead is the discharging of clouds with using of basic chemicals in such a way, the already existing droplets will coalesce mutually.
Dug
1 / 5 (1) Dec 25, 2011
In my neighborhood, at 5PM on Friday - the smells of charcoal and BBQ saturate the atmosphere - along with the faint odor of beer fizz - all aerosols not normally experienced anywhere near that level during the weekdays. Continues into late Sunday night. Consequently, I have a counter theory of at least the same empirical and scientific quality as the one in the article - high lipid BBQ aerosols in combination with beer fizz/high alcohol vapor levels counters bad weather.
Callippo
1 / 5 (1) Dec 25, 2011
Actually terpenes evaporating from certain trees are believed to cause hailstorms as their radicals formed during oxidation can ionize water droplets, increase the conductivity of air and to attract lightning. So your hypothesis can still have some scientific merit.
gwrede
1 / 5 (1) Dec 25, 2011
While there have been many theories as to why this might be, mostly religion based, it hasnt been until much more recently that researchers have begun to take a closer look.

Oh, geez!!! This country is going down like the Titanic!
Howhot
1 / 5 (1) Dec 26, 2011
You know what would be interesting to know is what is West of the south-east data points. It would be interesting to know what is upwind of the data collection points. If they are coal plants releasing lots of soot during the week, then surely the EPA would be interested in regulating those upwind plants.

Nobody likes a tornado or hail storm being caused from a coal/oil plant upwind from them. But of course AGW CO2 is also emitted from these same plants. The Southeast might actually have fewer tornadoes and hail with an Obama implemented a CO2 cap-n-trade model like the EU. At least we need to close down the soot emitters by EPA at a minimum.