The weekday -vs- weekend weather effect

Hailstorms and tornadoes are more common during the weekday due to human created aerosols.

By Dr. Roger Pielke Sr.

New Paper “Why Do Tornados And Hail Storms Rest On Weekends” By Rosenfeld and Bell 2011

There is a new paper which further documents the diversity of human climate forcings that is presented in

Pielke Sr., R., K. Beven, G. Brasseur, J. Calvert, M. Chahine, R. Dickerson, D. Entekhabi, E. Foufoula-Georgiou, H. Gupta, V. Gupta, W. Krajewski, E. Philip Krider, W. K.M. Lau, J. McDonnell,  W. Rossow,  J. Schaake, J. Smith, S. Sorooshian,  and E. Wood, 2009: Climate change: The need to consider human forcings besides greenhouse gases. Eos, Vol. 90, No. 45, 10 November 2009, 413. Copyright (2009) American Geophysical Union

and in the American Meteorological Society statement on

Inadvertent Weather Modification (Adopted by the AMS Council on 2 November 2010)

The new paper is

Rosenfeld, D., and T. L. Bell (2011), Why do tornados and hail storms rest on weekends?, J. Geophys. Res., doi:10.1029/2011JD016214, in press.

The abstract reads [highlights added]

This study shows for the first time statistical evidence that when anthropogenic aerosols over the eastern USA 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 USA.”

Excerpts from the paper read

“The results are in agreement with our previous reports of similar weekly cycles in the rainfall [Bell et al., 2008] and lightning [Bell et al., 2009a] over the USA. The cycle was ascribed there to aerosols invigorating deep convective clouds in a warm, moist atmosphere. It is therefore not too surprising to find that the invigorated clouds also produce more hail and tornados.”

“This study has shown a clear correspondence between the weekly cycle of anthropogenic aerosols and the occurrences of severe convective storms, which is highly unlikely to be a result of natural variability. The observed associations cannot serve as proof for causality. However, the results are consistent with the hypothesis that air pollution aerosols invigorate deep convective clouds in moist and unstable atmosphere,  and the possibility that they can even induce the storms to produce large hail and tornados. This is also consistent with the hypothesis that the severe storms are better organized and violent because aerosols increase the hydrometeor size, decreasing their evaporation and so weakening the negative buoyancy of the downdrafts, thereby preventing the gust front from outrunning and undercutting the updraft in the feeder clouds.  Anthropogenic emissions have caused large enhancements of aerosol loads even over the remote continents, with typical enhancements of 50–300% over remote regions of Asia, North America, and South America (Wilson et al., 2001; Chin et al., 2004; Park et al., 2006; Stier et al., 2006).Regarding this increase, it is worth pointing out that if a roughly 10% weekly variation in pollution levels is resulting in a similar change in severe storm activity, then the “background” aerosol level, which is elevated with respect to the pre-industrial level even during weekends, is also likely to be changing the storm frequency that we experience today.”

source of image

0 0 votes
Article Rating
Newest Most Voted
Inline Feedbacks
View all comments
Robert M
August 26, 2011 10:13 pm

I don’t know about aerosols, but that is one wicked looking storm.

Dr A Burns
August 26, 2011 10:21 pm

This would imply a very short life cycle for aerosols.

August 26, 2011 10:40 pm

Don’t have any aerosol observation on the NW Ca region. What does hold up is each winters run of storms cycles with a particular phase range of the Moon, lasting 10-14 days. In some really bad years, you get a double whammy, with (for example) a storm series going from New Moon to Full, followed by another one from Full Moon to New Moon, or no cycles (76-77 drought).

August 26, 2011 10:55 pm

Maybe they’ve found statistical evidence that people pray for good weather for the weekend.

August 26, 2011 11:04 pm

Sounds nice an neat but the devil is in the details and the details are not given. I cringe every time I see statistically significant and no values. Sorry but we see way to much of this stuff and in the end it is effectively meaningless.

Roger Knights
August 26, 2011 11:17 pm

This implies that some increases in extreme weather can’t be reflexively blamed on CO2.

August 26, 2011 11:32 pm

I assume this a joke to demonstrate how correlation proves nothin

Lisa K
August 26, 2011 11:35 pm

I live in a Colorado resort community. It always seems we get a lot of storms on Thursday evening. Convenient, conicidence, effective cloud seeding programs or just that I have band practice on Thursday evenings? For the resorts the timing couldn’t be better. Everyone in the Denver metro area wakes up on Friday morning already thinking about the weekend and then they get a great snow report. Yes, some of the resorts waste millions on cloud seeding programs.

Rob Z
August 26, 2011 11:43 pm

just by chance, over 70% of the time the storm will occur on a weekday. Reminds me of the manager’s complaint against a worker. “You’re a slacker, kid, whenever your sick, 40% of the time it’s on a Monday or a Friday”

August 26, 2011 11:49 pm

Hit me in the head with a frying pan BUT it seems to me we would need hot and cold fronts to achieve heavy weather to begin the process. Now to me at least it is worthwhile then to think aerosols are in the atmosphere at somewhat the same level and with storms that do not produce severe weather the vast majority of the time. That is unless humans in the weather path all watch the sky and whenever they see clouds roll in everyone starts spraying their favorite fragrance in a continuous fashion resulting in human induced heavy weather. Yeah – that seems on a par with aliens invading due to human global warming universal concerns not to mention weren’t we all responsible for the holes in the north and south poles due to aerosols that seems to have disappeared from the radar lately. Look, I get it – we go crazy with global warming – so crazy we’re branching out and blowing up stars these days. The aliens have to be crazier than us taking earthlings on ’cause we’ll blow up their star next if they mess with us. Excuse me for asking – but what are we spraying to cause the droughts?

August 26, 2011 11:59 pm

Is this saying humans can have an influence on the weather?

August 27, 2011 12:29 am

This also implies that humans are cooling the earth (land)?

August 27, 2011 12:31 am

Maybe the anthropically generated aerosols [i]add[/i] to the bad weather but are not necessarily [i]causative[/i]. The blame for the 7-day week, if my recollection serves me well, is attributed to the Babylonians. They were highly observant astronomers and consequently well aware of the length of the lunar month but they could just as easily have established 7 4-day weeks or even 2 14 day weeks, as 4 7-day ones. Perhaps a 7-day weather cycle even back then, was the deciding factor …

Disko Troop
August 27, 2011 1:03 am

Now I understand. Now I understand! I thought fly spray killed flys, but because I use an aerosol fly spray, that causes it to rain and the drops hit the flys and kill them. That is why there are fewer flys around in wet weather!
Seriously, I agree with Dr A Burns. This implys that aerosols are so short lived as to have a 48 to 96 hour life yet they are also travelling all the way to remote regions? Could it not be that activity causes more heat, hence more convection etc etc. What’s up with that?

August 27, 2011 1:05 am

The paper includes the following statement
the “background” aerosol level, which is elevated with respect to the pre-industrial level even during weekends, is also likely to be changing the storm frequency that we experience today.
I was under the impression that the level of storms has fallen over the last 50 years, relative to the previous 50 years.
Am I mistaken?

George Lawson
August 27, 2011 1:07 am

As there happens to be five weekdays and only two weekend days in any week, isn’t it statistically obvious that any weather will be two and a half times more during the week than at the weekend!

August 27, 2011 1:11 am

Weather is controlled by cock crows. That’s settled common knowledge I was told already by my granny. The really haunting problems are elsewhere. Lack of education? See for example
If we don’t solve that soon, anything may happen. Anything! I tell you.

August 27, 2011 1:24 am

‘Science’ plumbs new depths. The old saying has never rung so true, “There are limits to everything, except stupidity”

John Marshall
August 27, 2011 1:32 am

Roger Sen. seems confused as to the difference between climate and weather. All these events described are weather. Humans would have to do far more than produce a few aerosols to affect climate.

August 27, 2011 1:35 am

On reflection, this is a shocking finding and just emphasises that our worst fears are being realised (or something).
Is it possible to slap a tax on those wicked people who keep shooting dirty airy-solles into the air?
Or is it level airoplanes or what?
Anyway, it’s a disgrace and summthin shud be dun abit.
There – I’ve had my sayeeee.

August 27, 2011 1:58 am

Nice sounding paper. I’m tempted to believe it actually went through some sort of pal-review.

Derek Cummings
August 27, 2011 2:10 am

There is the answer to drought hit lands. Give everybody in the land an aerosol and tell them to al spray together, into the air at 1pm. Resulting shortly after in one hell of a storm, and lots of rain. Mind, is that going over the top. We don’t want to create a flood.
In short though with the article. What a load of tosh as we call it here. I see someone else looking for more funding coming out the woodwork.

Dodgy Geezer
August 27, 2011 2:17 am

Completely off-topic, but it cries out for a wider audience!
[snip – and here is not the place for it – post in Tips & Notes ~jove, mod]

Julian in Wales
August 27, 2011 3:34 am

If there is no wind do the storms stay for the weekend? If during the week, when the wind is from the sea, do the storms go away? Are Sundays “statistically” less stormy than Saturdays (because there must be a lingering period for the aerosols to dissipate) ? And how about Monday – Tuesday – surely it takes time for the aerosols to build up after a weekend of low emissions?

August 27, 2011 5:18 am

This reminds me of a great film called drowning by numbers in which Tuesday’s were said to be the day on which the most violent deaths occurred

August 27, 2011 5:24 am

Not O/T, I have the most fascinating navel…

Lars P
August 27, 2011 5:30 am

So people going to work are causing the bad weather? And I though it is always bad weather on weekends. Maybe grilling in the garden creates too many aerosols, or only the plan to grill some sausages is enough?
Or maybe it is another man made effect – possibly in the way that there were less people on duty during the week-end to record data?
Interesting, the forecast for this weekend in New-York is storm & rain for Saturday & Sunday with clear sky for most of the coming week but that must be only weather or they work only on week-ends in New-York?:

August 27, 2011 5:43 am

Why wouldnt Friday have the maximum Aerosol level? (aka midweek maximum)
Have Wednesdays been measured to have maximum Aerosol or is this just assumed.

August 27, 2011 5:46 am

George Lawson says:
August 27, 2011 at 1:07 am
As there happens to be five weekdays and only two weekend days in any week, isn’t it statistically obvious that any weather will be two and a half times more during the week than at the weekend!

August 27, 2011 5:47 am

Just like those mercury ridden low watt light bulbs. If we all used them it would be as effective as throwing a sugar cube into Loch Ness to make the water sweeter. Ir depending on wind turbines only for electricity when the wind don’t bloweth and solar panels when dat sun donna shine.

August 27, 2011 5:48 am

Did a bit of research on this my self.
Of the last 1000 storms 715 occurred on week days and only 285 on weekends.
I could not believe my findings. There are 250% more storms on week days compared to weekends. This research must be correct. Unbelievable.
No wait 715/5 = 143, 285/2 = 142.5
Yes the original data is correct. There is 0.5 more storms per day through the week as compared to a weekend.
A grant is needed to research this further.

August 27, 2011 5:49 am

Well, I’m doing my part to mitigate the weekday aerosols.
My weekday commuting is offset by my weekend lawnmowing, leaf blowing, motocross, and barbequeing.
My efforts however, and those of countless others, may be fruitless because invisible aliens may be altering weather patterns without us realizing it. Someone should apply for an NSF grant to study this.
Michael Mann has graciously prepared the graph for the final report in advance for us.

Tom in South Jersey
August 27, 2011 6:25 am

Clearly the researchers haven’t spent anytime in shore traffic on the weekend.

August 27, 2011 6:58 am

This looks like another attempt to shut down an entire industry.

August 27, 2011 7:08 am

Very interesting — I assume that they found a small, but statistically significant difference in the frequency per day, and it’s plausible that this is caused by human-generated aerosols.
But even so, are the aerosols just triggering severe storms that would have occurred randomly otherwise, so that the number of severe storms is about the same as it would have been without the aerosols?
Or, are they causing severe storms to be triggered earlier than they otherwise would be, so that they are actually less extreme than they would be if they were allowed to build up naturally to full force?

August 27, 2011 7:12 am

Correlation does not prove causation.

August 27, 2011 7:19 am

Fork over my stimulus grant money Mr. Biden!!

tom T
August 27, 2011 7:26 am

Headline: Research shows that aerosols cause weekdays.

Jim Barker
August 27, 2011 8:29 am

What type of aerosol is widely used in this studied area? Can hairspray be blamed:-)

August 27, 2011 8:45 am

Mid-week isn’t the cumulative maximum for aerosols–it would be Friday or Saturday (the accumulation of 5 days even if a significant number of people curtail business activity on Friday). I’d say this study is suspect if they find mid-week (Wednesday) to be the day of maximum storm activity.

Barbara Skolaut
August 27, 2011 9:20 am

Holy @&*%! That is an amazing picture.
Interesting paper, too.

Reed Coray
August 27, 2011 9:30 am

I want to know what happened to the photographer who took the picture of the storm. Is he still with us?

August 27, 2011 10:03 am

It was quite common back in the late 1960s for the summers to be clear during the weekdays and then rain on the weekends. Basically, it took 4-5 days for the aerosols to build up and then rain would form and clean the air.
This pattern was truly a pain in the ass as I was dating a girl at the NJ shore and studying in Rochester, NY. I would drive down on Friday and back Sunday and had 10 weeks of rainy weekends—did not go to the beach once! The movies back then did not turn over that rapidly and there were few places to eat.

August 27, 2011 10:03 am

@ Gator says: August 27, 2011 at 5:24 am. The word that you are looking for is omphaloskepsis.

Joe Crawford
August 27, 2011 10:04 am

Dr. Pielke Sr. has been saying for years that increased CO2 is not man’s only influence on weather/climate, and that man’s other changes to the physical environment have at least as much, if not greater effect. On the other side we have the CAGW ‘Team’ that, in dozens of published papers, has been trying to ‘prove’ scary increases in the both frequency and strength of hurricanes, tornadoes, and other weather events, all caused by increasing CO2. However, upon closer examination, the statistics of the Team seem to always fall apart.
If it stands, this paper will, as far as I know, be one of the few to actually support the theory that man has any effect at all on weather/climate on anything other than a very localized scale. And, it supports Dr. Pielke Sr.’s side of the argument over the ‘Team’s’.

August 27, 2011 10:24 am

Really good point about the residence time of aerosols if you can see a mid-week to weekend difference in hail storms (causative). And what does “statistically significant” mean? That if you don’t use computers and algorithms you won’t see the correlation?
That’s the second “good” point. Only math says there is a signal inside natural variability. And the residence time is so short that the other dumb idea, that the aerosols from Chinese coal-powered power plants stopped global warming 10 years ago.
Speaking of which: China has been building and putting on-stream coal-fired plants every month for about 18 months now, with at least 48 in the line-up. If China really is affecting the weather, the global temperatures will actually DROP over the next few years.
Again, it is not natural. Global warming, at least the Chinese type, will result in global cooling.
Unfalsifiable again.

Brian D
August 27, 2011 11:24 am

They really need to do indepth day to day met analysis before they can come to that conclusion. Especially when they say…”The observed associations cannot serve as proof for causality.” Are there more in the way of naturally strong fronts during this period due to the natural rhythms of weather patterns we can fall into?

August 27, 2011 1:17 pm

14 years * 20 weeks( or whatever the number of weeks that are considered summer storm weeks) does not provide many cycles to analyze.

August 27, 2011 1:49 pm

“possibly in the way that there were less people on duty during the week-end to record data?”
Lars, you cracked me up man.

John Robertson
August 27, 2011 2:51 pm

This report is nothing more (or less) than saying that extra aerosols injected into the atmosphere tend to lead to increased storm activity – at least with respect to hail and tornadoes. What is the difference between this and Cloud seeding ? which is known to have a slight increase in inducing rain or storm activity. What the authors appear to be saying is pollution does produce similar results. Saying that man can have no affect on weather is disingenuous in the least – after all people have been trying to affect local weather for generations. Humans can indeed effect changes in local climate – cities are warming than the countryside, deforestation can lead to desertification of areas (Cedars of Lebanon – where are they now?), and so on.
This has little to do with GHG or other claims by the warmists, but does show that man can have an effect and these must be studied to understand how to account for them in local weather and climate.
Personally I think the juxtaposition of this study and the recent CERN experiments with cloud creation are fascinating and should be accepted (if non-falsafiable) as yet another data in the understanding of our environment and how we interact with it.

August 27, 2011 3:00 pm

I lived in the California Bay Area and have long noted that in summer the weekends are almost always cooler and windier than the middle of the week. I suspect that this corresponds with the development of a updraft around Sacramento that pulls cooler air, and offshore fog, in through the Bay Area (you can watch it coming over the hills). It would make sense that the updraft may be the result of a short term forcing from the weekday traffic in Sacramento. There does not seem to be any correspondence to the bloviating in the legislature.

Dr A Burns
August 27, 2011 3:08 pm

Heavy industry like oil refineries, power stations etc work 7 days. It must be those office workers who are to blame.

August 27, 2011 3:24 pm

The observed associations cannot serve as proof for causality.
That’s a beautiful sentence. I wish more people would use it.

August 27, 2011 3:37 pm

Not all aerosols peak during the week. It appears that elemental (also called black) carbon, mainly from diesels, is at its lowest concentrations on Sunday and Monday in rural middle America. Natural dusts are also lower on Sundays and Mondays — perhaps also due to less driving, less stirring up of dust? Nitrates (mainly but not exclusively a vehicular emission) exhibit the same weekly cycle. Surprisingly, power plant particle emissions do not, although sulfur dioxide levels (a gas) are lower on weekends. See this link:
Here are parts of the Abstract:
“Data from the Interagency Monitoring of Protected Visual Environments (IMPROVE) network of aerosol samplers and NOAA monitoring sites are examined for weekly cycles. At remote and rural sites, fine particle elemental carbon, crustal elements, and coarse particle mass had pronounced (up to 20%) weekly cycles with minima on Sunday or Monday. Fine particle organic carbon and mass had smaller amplitude cycles, also with Sunday or Monday minima. There was no statistically significant weekly cycle in fine particle sulfate despite a 5 to 15% weekly cycle in power plant SO2 emissions. Although results for nitrate may be more susceptible to sampling artifacts, nitrate also showed a pronounced weekly cycle with an amplitude similar to elemental carbon…[snip]…These results support a large role of diesel emissions in elemental carbon aerosol over the entire United States and suggest that a large fraction of the airborne soil dust is anthropogenic. They also suggest that studies of weekly cycles in temperature, cloudiness, precipitation, or other meteorological variables should look for causes more in light-absorbing particles and possible ice nucleation by dust rather than sulfate or total aerosol.

Power Grab
August 27, 2011 6:58 pm

How about electromagnetic “pollution”? Take a look at .
Back O/T: Are the human-induced storms located near the centers of aerosol pollution?

John W
August 27, 2011 7:55 pm

Global Atmosphere Watch (GAW)
“Note that also mineral aerosol components (Ca2+, Mg2+ and
Na+) show this weekly periodicity although they are typically
not attributed to human activities.”
So if there is a natural weekly cycle for mineral aerosol components why does a weekly cycle of other aerosols have to be induced from human activity?

August 27, 2011 8:24 pm

To John W: The only explanation I can see for a weekly cycle in which mineral aerosol components drop is that with less traffic on weekends, less road dust is put into the air. Your citation comes to very similar conclusions as mine (above) does:
in that they both find traffic aerosols lower on weekends than weekdays.

John W
August 27, 2011 8:48 pm

Here’s a similar “study” from 2008 (details available)
From their summary:
“3) there is a strong tendency for this weekly variation
to show up in afternoon data in most (but not all) summers;”
“5) the effect, which clearly must be anthropogenic,
extends over the nearby Atlantic, and is almost as strong
there, but is reversed in sign;”
They’re not looking for a natural cause, it simply must be anthropogenic even though it doesn’t show up every summer and the effect reverses over the Atlantic. Ignore what doesn’t fit!

Brian D
August 27, 2011 9:03 pm

If air pollution was worse prior to this time, were the storms worse as well? It’s too bad we don’t have a clean, natural period with plenty of data to compare to now. I’m sure human aerosols have some kind of effect, much more so than any trace gas.

John W
August 27, 2011 9:57 pm
“To look for signs of the influence of weekly variations
in human activity on precipitation, averages for each
day of the week based on 8 years (1998–2005) of data”
Spring Equinox
1998 – 2005
Monday 1 13%
Tuesday 1 13%
Wednesday 1 13%
Thursday 0 0%
Friday 2 25%
Saturday 1 13%
Sunday 2 25%
Anthropogenic suppression of spring equinox!

August 29, 2011 9:21 am

AusieDan said:
“I was under the impression that the level of storms has fallen over the last 50 years, relative to the previous 50 years.
Am I mistaken?”
Tornadoes F3 and stronger have actually shown a slight long term decline during the 20th century.
It appears to me the most likely explanation is global warming, affecting the Arctic more than other regions of the world. This reduces the north-south temperature gradient in North America, where most of the world’s tornadoes form. This temperature gradient powers the wind sheer that major tornadoes need to form.
Although I see existence of AGW, I am seeing its magnitude being about 40-50% of that proposed by most proponents of existence of AGW.
As for effects of weekly cycle of aerosols on overall storm trends – this could very easily
be very minor. The article bringing this up shows this effect in summertime in the highly populated eastern USA, while the worst of severe thunderstorm and tornado activity is in springtime in central and south-central USA. In other regions and at other times of the year, this effect may not be significant. And, the major historic tornado outbreaks occurred when large weather systems favorable to their formation existed – likely independently from air pollution.

Brian H
August 29, 2011 11:15 pm

Joe Crawford says:
August 27, 2011 at 10:04 am
Dr. Pielke Sr. has been saying for years that increased CO2 is not man’s only influence on weather/climate,

If it were, there would be little or none at all!

%d bloggers like this:
Verified by MonsterInsights