Papers on weekly cycle in climate

This is a list of papers showing observations of weekly cycle in climate related variables. Weekly cycle in climate parameters is suggestive of anthropogenic control. Papers on so called “weekend effect” are also included. The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative.

UPDATE (February 5, 2012): Asmi (2012) and Pollack et al. (2012) added.
UPDATE (November 5, 2011): Rosenfeld & Bell (2011) and Tuttle & Carbone (2011) added.

Airborne and ground-based observations of a weekend effect in ozone, precursors, and oxidation products in the California South Coast Air Basin – Pollack et al. (2012) “Airborne and ground-based measurements during the CalNex (California Research at the Nexus of Air Quality and Climate Change) field study in May/June 2010 show a weekend effect in ozone in the South Coast Air Basin (SoCAB) consistent with previous observations. The well-known and much-studied weekend ozone effect has been attributed to weekend reductions in nitrogen oxide (NO_x = NO + NO₂) emissions, which affect ozone levels via two processes: (1) reduced ozone loss by titration and (2) enhanced photochemical production of ozone due to an increased ratio of non-methane volatile organic compounds (VOCs) to NO_x. In accord with previous assessments, the 2010 airborne and ground-based data show an average decrease in NO_x of 46 ± 11% and 34 ± 4%, respectively, and an average increase in VOC/NO_x ratio of 48 ± 8% and 43 ± 22%, respectively, on weekends. This work extends current understanding of the weekend ozone effect in the SoCAB by identifying its major causes and quantifying their relative importance from the available CalNex data. Increased weekend production of a VOC-NO_x oxidation product, peroxyacetyl nitrate, compared to a radical termination product, nitric acid, indicates a significant contribution from increased photochemical production on weekends. Weekday-to-weekend differences in the products of NO_x oxidation show 45 ± 13% and 42 ± 12% more extensive photochemical processing and, when compared with odd oxygen (O_x = O₃ + NO₂), 51 ± 14% and 22 ± 17% greater ozone production efficiency on weekends in the airborne and ground-based data, respectively, indicating that both contribute to higher weekend ozone levels in the SoCAB.” Pollack, I. B., et al. (2012), Airborne and ground-based observations of a weekend effect in ozone, precursors, and oxidation products in the California South Coast Air Basin, J. Geophys. Res., 117, D00V05, doi:10.1029/2011JD016772.

Weakness of the weekend effect in aerosol number concentrations – Asmi (2012) “Weekday related anthropogenic aerosol emissions have been suggested to affect regional climate via indirect aerosol effects. I studied the variability of potential cloud condensation nuclei using measurements of number size distributions of Cloud Condensation Nuclei (CCN) -sized aerosol particles and CCNs measured at several European regional background stations, located at a wide variety of environments. With notably rare exceptions, there were no statistically significant difference between concentrations on different weekdays. I further analysed the concentration timeseries of four long-period datasets in Germany and Finland with wavelet analysis. Outside of urban areas, very little weekday-connected variability was found. The lack of 7-day variability outside of cities is in contrast of earlier studies in this field, which used mostly particle mass as the representative measure of aerosol concentration. A time-scale and variability analysis showed that PM10 and PM2.5 are more sensitive for the weekly variation than CCN-sized particles. Using mass-based variations as a proxy for short-term variations of CCN particle numbers can thus overestimate the weekend effect for these particles. The results of this study do not support aerosol indirect effects from 50 to 500 nm diameter particles as a major contributor on potential weekday connected variations in European meteorology.” Ari Asmi, Atmospheric Environment, http://dx.doi.org/10.1016/j.atmosenv.2012.01.060.

Inferences of weekly cycles in summertime rainfall – Tuttle & Carbone (2011) “In several continental regions a weekly cycle of air pollution aerosols has been observed. It is usually characterized by concentration minima on weekends (Saturday and Sunday) and maxima on weekdays (Tuesday–Friday). Several studies have associated varying aerosol concentrations with precipitation production and attempted to determine whether or not there is a corresponding weekly cycle of precipitation. Results to date have been mixed. Here we examine a 12 year national composited radar data set for evidence of weekly precipitation cycles during the warm season (June–August). Various statistical quantities are calculated and subjected to “bootstrap” testing in order to assess significance. In many parts of the United States, warm season precipitation is relatively infrequent, with a few extreme events contributing to a large percentage of the total 12 year rainfall. For this reason, the statistics are often difficult to interpret. The general area east of the Mississippi River and north of 37°N contains regions where 25%–50% daily rainfall increases are inferred for weekdays (Tuesday–Friday) relative to weekends. The statistics suggest that western Pennsylvania is the largest and most likely contiguous region to have a weekly cycle. Parts of northern Florida and southeastern coastal areas infer a reverse-phase cycle, with less rainfall during the week than on weekends. Spot checks of surface rain gauge data confirm the phase of these radar-observed anomalies in both Pennsylvania and Florida. While there are indications of a weekly cycle in other locations of the United States, the degree of confidence is considerably lower. There is a strong statistical inference of weekday rainfall maxima over a net 8% of the area examined, which is approximately twice the area of cities. Future examination of lofted aerosol content, related condensation/ice nuclei spectra, and knowledge of the convective dynamical regime are needed in order to assess how anthropogenic aerosols may affect rainfall at urban and regional scales. If radar is the primary method of observation, it is also necessary to examine the effects of variable aerosol content on the parametric relationship between rainfall rate and radar reflectivity factor. Polarimetric radar observations could also serve to verify microphysical-dynamical hypotheses regarding precipitation production.” Tuttle, J. D., and R. E. Carbone (2011), J. Geophys. Res., 116, D20213, doi:10.1029/2011JD015819.

Why do tornados and hailstorms rest on weekends? – Rosenfeld & Bell (2011) “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.” Rosenfeld, D., and T. L. Bell (2011), J. Geophys. Res., 116, D20211, doi:10.1029/2011JD01.

An Empirical Study of Geographic and Seasonal Variations in Diurnal Temperature Range – Jackson & Forster (2010) “The diurnal temperature range (DTR) of surface air over land varies geographically and seasonally. The authors have investigated these variations using generalized additive models (GAMs), a nonlinear regression methodology. With DTR as the response variable, meteorological and land surface parameters were treated as explanatory variables. Regression curves related the deviation of DTR from its mean value to values of the meteorological and land surface variables. Cloud cover, soil moisture, distance inland, solar radiation, and elevation were combined as explanatory variables in an ensemble of 84 GAM models that used data grouped into seven vegetation types and 12 months. The ensemble explained 80% of the geographical and seasonal variation in DTR. Vegetation type and cloud cover exhibited the strongest relationships with DTR. Shortwave radiation, distance inland, and elevation were positively correlated with DTR, whereas cloud cover and soil moisture were negatively correlated. A separate analysis of the surface energy budget showed that changes in net longwave radiation represented the effects of solar and hydrological variation on DTR. It is found that vegetation and its associated climate is important for DTR variation in addition to the climatic influence of cloud cover, soil moisture, and solar radiation. It is also found that surface net longwave radiation is a powerful diagnostic of DTR variation, explaining over 95% of the seasonal variation of DTR in tropical regions.” Jackson, Lawrence S., Piers M. Forster, 2010, J. Climate, 23, 3205–3221, doi: 10.1175/2010JCLI3215.1.

Weekend effect: Anthropogenic or natural? – Kim et al. (2010) “Human activities have been suggested to result in weekly changes of meteorological variables, called the “weekend effect.” Recent debates on its statistical significance, however, reveal that there still remain huge uncertainties as to the anthropogenic origin of the weekend effect. We show that atmospheric Rossby waves induce the “natural” weekend effect, which is much stronger than the “anthropogenic” weekend effect. The “natural” weekend effect does not completely disappear even with averaging over 61 years of data; as a result, true “anthropogenic” weekend effect is obscured by the natural component. We attempted to remove the “natural” component in the diurnal temperature range and the resulting pattern is an overall positive weekend effect over North America.” Kim, K.-Y., R. J. Park, K.-R. Kim, and H. Na (2010), Geophys. Res. Lett., 37, L09808, doi:10.1029/2010GL043233. [full text]

The Detection of Weekly Preferential Occurrences with an Application to Rainfall – Marani (2010) “The detection of weekly preferential occurrences in atmospheric and hydrologic processes has recently attracted much attention as a way to identify the signature of anthropogenic climatic changes. The interpretation of previous analyses, however, is not unequivocal, in part as a result of a lack of widely accepted statistical criteria. Here, a general and exact method to detect the presence of weekly preferential occurrences is developed and applied to long rainfall observations in Marghera, Italy; Philadelphia, Pennsylvania; and Portland, Maine. The method makes use of the fact that, under the null hypothesis of stationarity, the process of event occurrence in the different days of the week is equivalent to the random distribution of a number of balls (the wet days) in a set of boxes (the days of the week). The departure from a homogeneous distribution is then characterized through the probability of the maximum number of balls in a box, which can be computed exactly with no ad hoc assumptions. The new method shows that (i) preferential rainfall weekly occurrences emerge in all cases in the most recent period analyzed (1990–2006), while they are absent—or are too weak to be detected—in previous years (before 1989); and (ii) the balls-in-boxes approach appears to be more sensitive than Pearson’s test when deviations from homogeneity are associated with just one day of the week, a common occurrence in connection with day-of-the-week effects. The results presented help to reconcile previous contrasting studies and to contribute compelling evidence that anthropogenic changes in the local climate have occurred over the past century in urban and industrial areas.” Marani, Marco, 2010, J. Climate, 23, 2379–2387, doi: 10.1175/2009JCLI3313.1.

Day-of-the-week variations of urban temperature and their long-term trends in Japan – Fujibe (2010) “Temperature differences among days of the week and their long-term trends were evaluated using 29 years of hourly data from the Automated Meteorological Data Acquisition System network of Japan. Stations were categorized with respect to the population density around each site, and an urban temperature anomaly (δT*) was defined as a departure from the spatial average of nearby rural stations. On Saturdays and holidays (Sundays and national holidays), δT* was lower than on weekdays by 0.2–0.25°C at Tokyo, by 0.1–0.2°C at Osaka, and by about 0.02°C at stations where the population density was 300 to 1,000 km^–2. Moreover, δT* showed a relative decreasing trend over the long term on Mondays and an increasing trend on Fridays, at a rate of about 0.05–0.1°C decade–1 at Tokyo and about 0.02°C decade–1 at stations where the population density was 100 to 1,000 km^–2, but no significant difference in δT* trends was observed between weekdays and weekend days.” Fumiaki Fujibe, Theoretical and Applied Climatology, Volume 102, Numbers 3-4, 393-401, DOI: 10.1007/s00704-010-0266-y.

Long-term changes in summer weekend effect over northeastern China and the connection with regional warming – Ho et al. (2009) “The 7-day cycle of human activities may lead to the “weekend effect” in climate variables and air pollutants. The weekend effect is defined as the average value (e.g., the diurnal temperature range) for Saturday through Monday minus the average value for Wednesday through Friday. A composite of the ground observations over northeastern China presents that, during 26-year (1980–2005) summers, the weekend effect in the diurnal temperature range increased by 1.2°C. Conversely, the weekend effects in the relative humidity, cloud amount, and light rain (≤5 mm day−1) events decreased. These changes are due to a shifted phase of the weekly cycle of the meteorological variables. The long-term change in weekend effects have a high correlation coefficient (∣r∣ ≈ 0.8) with the decrease in relative humidity over the region, which is likely induced by regional warming. The results suggest that regional warming is a possible factor in a transition of dominant aerosol effects in the weekend effect.” Ho, C.-H., Y.-S. Choi, and S.-K. Hur (2009), Geophys. Res. Lett., 36, L15706, doi:10.1029/2009GL039509.

Weekly periodicities of Aerosol Optical Thickness over Central Europe – evidence of an anthropogenic direct aerosol effect – Bäumer et al. (2008) “Statistical analyses of data from ground-based sun photometer stations in Central Europe are presented. All stations are part of the Aerosol Robotic Network (AERONET), and only data of the highest data quality level 2.0 has been applied. The averages by weekday of Aerosol Optical Thickness (AOT) at a wavelength of 440 nm of 12 of the 14 stations in the investigation area show a weekly periodicity with lowest values on Sunday and Monday, but greatest values from Wednesday until Saturday, that is significant at least on a 90% level. The stations in Germany and in Greater Paris show weekly cycles with ranges of about 20% on average. In Northern Italy and Switzerland this range is about 10% on average. By applying several checks, we exclude that the weekly cycles were caused by a maintenance effect or by different retrieval conditions as a consequence of a weekly cycle in cloud cover. The corresponding weekly cycle of anthropogenic gaseous and particulate emissions leads us to the conclusion of the anthropogenic origin of the weekly AOT cycle. Since these AOT patterns are derived from the reduction of the direct sun radiation by the columnar atmospheric aerosol, this result represents strong evidence for an anthropogenic direct aerosol effect on shortwave radiation. Furthermore, this study makes a first contribution to the understanding and explanation of recently observed weekly periodicities in meteorological variables as temperature in Germany.” Bäumer, D., Rinke, R., and Vogel, B., Atmos. Chem. Phys., 8, 83-90, doi:10.5194/acp-8-83-2008, 2008. [full text]

Winter “weekend effect” in southern Europe and its connections with periodicities in atmospheric dynamics – Sanchez-Lorenzo et al. (2008) “Winter weekly cycles of different climatic variables have been detected over Spain during the 1961–2004 period. The 13 analyzed series come from stations placed on different climatological and geographical areas with different level of urban influence. Therefore, the weekly cycles can hardly be related with local effects. Contrarily, we suggest that the weekly cycles may be related with changes in the atmospheric circulation over Western Europe, which may be due to some indirect effect of anthropogenic aerosols. Particularly interesting is the observed increase in Sea Level Pressure over Southern Europe during the weekends and consequently a decrease of anticyclonic conditions during the central weekdays.” Sanchez-Lorenzo, A., J. Calbó, J. Martin-Vide, A. Garcia-Manuel, G. García-Soriano, and C. Beck (2008), Geophys. Res. Lett., 35, L15711, doi:10.1029/2008GL034160.

Analysis of the weekly cycle of aerosol optical depth using AERONET and MODIS data – Xia et al. (2008) “Multi-year Aerosol Robotic Network (AERONET) and Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol optical depth (AOD) data are used to study AOD weekly variations at the global scale. A clear weekly cycle of AOD is observed in the United States (U.S.) and Central Europe. AOD during the weekday is larger than that during the weekend in 36 out of 43 AERONET sites in the U.S. The average U.S. weekend effect (the percent difference in AOD during the weekday and the weekend) is 3.8%. A weekly periodicity with lower AODs on Sunday and Monday and higher AODs from Wednesday until Saturday is revealed over Central Europe and the average weekend effect there is 4.0%. The weekly cycle in urban sites is greater than that in rural sites. AOD during the weekday is also significantly larger than that during the weekend in urban AERONET sites in South America and South Korea. However, a reversed AOD weekly cycle is observed in the Middle East and India. AODs on Thursday and Friday, the “weekend” for Middle East cultures, are relatively lower than AODs on other days. There is no clear weekly variation of AOD over eastern China. The striking feature in this region is the occurrence of much higher AOD on Sunday and this phenomenon is independent of season. The analysis of MODIS aerosol data is in good agreement with that of AERONET data.” Xia, X., T. F. Eck, B. N. Holben, G. Phillippe, and H. Chen (2008), J. Geophys. Res., 113, D14217, doi:10.1029/2007JD009604.

An unexpected pattern of distinct weekly periodicities in climatological variables in Germany – Bäumer & Vogel (2007) “Statistical analyses of data from 12 German meteorological stations meeting WMO standards in the period 1991–2005 are presented. These stations represent different local climate conditions in terms of both meteorology and pollution situation. For the average over data of all stations, we identified significant weekly periodicities in many variables such as temperature, daily temperature range, sunshine duration, cloud amount, precipitation, and precipitation frequency. Not only data of stations situated in congested urban areas, but also data of remote stations as e.g. on Mount Zugspitze 2960 m above sea level in the Alps showed significant in-phase weekly cycles. These weekly periodicities cannot be explained completely by local pollution effects or local heat emissions. We tend towards the hypothesis that the anthropogenic weekly emission cycle and the subsequent aerosol cycle interact with the atmospheric dynamics on a larger scale which leads to a forcing of a naturally existing 7-day period among the spectrum of atmospheric periods.” Bäumer, D., and B. Vogel (2007), Geophys. Res. Lett., 34, L03819, doi:10.1029/2006GL028559. [full text]

Weekly precipitation cycles? Lack of evidence from United States surface stations – Schultz et al. (2007) “Previous work has inferred a relationship between human activity and the occurrence and amount of precipitation through examining possible weekly cycles in precipitation. Daily precipitation records for 219 surface observing stations in the United States for the 42-year period 1951–1992 are investigated for weekly cycles in precipitation. Results indicate that neither the occurrence nor amount of precipitation significantly depends upon the day of the week.” Schultz, D. M., S. Mikkonen, A. Laaksonen, and M. B. Richman (2007), Geophys. Res. Lett., 34, L22815, doi:10.1029/2007GL031889. [full text]

Weekly cycle of aerosol-meteorology interaction over China – Gong et al. (2007) “Weekly cycles of the concentration of anthropogenic aerosols have been observed in many regions around the world. The phase and the magnitude of these cycles, however, vary greatly depending on region and season. In the present study the authors investigated important features of the weekly cycles of aerosol concentration and the covariations in meteorological conditions in major urban regions over east China, one of the most polluted areas in the world, in summertime during the period 2001–2005/2006. The PM10 (aerosol particulate matters of diameter < 10 μm) concentrations at 29 monitoring stations show significant weekly cycles with the largest values around midweek and smallest values in weekend. Accompanying the PM10 cycle, the meteorological variables also show notable and consistent weekly cycles. The wind speed in the lower troposphere is relatively small in the early part of the week and increases after about Wednesday. At the same time, the air temperature anomalies in low levels are positive and then become negative in the later part of the week. The authors hypothesize that the changes in the atmospheric circulation may be triggered by the accumulation of PM10 through diabatic heating of lower troposphere. During the early part of a week the anthropogenic aerosols are gradually accumulated in the lower troposphere. Around midweek, the accumulated aerosols could induce radiative heating, likely destabilizing the middle to lower troposphere and generating anomalously vertical air motion and thus resulting in stronger winds. The resulting circulation could promote ventilation to reduce aerosol concentrations in the boundary layer during the later part of the week. Corresponding to this cycle in anthropogenic aerosols the frequency of precipitation, particularly the light rain events, tends to be suppressed around midweek days through indirect aerosol effects. This is consistent with the observed anthropogenic weather cycles, i.e., more (less) solar radiation near surface, higher (lower) maximum temperature, larger (smaller) diurnal temperature range, and fewer (more) precipitation events in midweek days (weekend).” Gong, D.-Y., C.-H. Ho, D. Chen, Y. Qian, Y.-S. Choi, and J. Kim (2007), J. Geophys. Res., 112, D22202, doi:10.1029/2007JD008888. [full text, slide show]

Weekend effect in diurnal temperature range in China: Opposite signals between winter and summer – Gong et al. (2006) “Intense human activity can impact weather and climate in many ways. One possible important consequence is the weekly cycle (so-called weekend effect) in the diurnal temperature range (DTR). The weekend effect is defined as the average DTR for Saturday through Monday minus the average DTR for Wednesday through Friday. In the present study, the weekend effect in the DTR over east China combined with station observations of maximum and minimum temperatures, relative humidity, and total solar irradiance for the period 1955–2000 was analyzed. Results show that the weekend effect in the DTR has the opposite signal between winter (December, January, and February) and summer (June, July, and August). Wintertime DTR tends to have a positive weekend effect (i.e., larger DTR in weekend days compared to weekdays), in association with increased maximum temperature and total irradiance but decreased relative humidity. While summertime DTR displays a much stronger and significantly negative weekend effect (i.e., smaller DTR in weekend days), in association with decreased maximum temperature and total solar irradiance but increased relative humidity and a greater number of rainy days. This study indicates that the DTR difference between weekend and weekdays is predominantly related to weekly changes in the maximum temperature. The weekend effect in the DTR and maximum temperature is also found in the Reanalysis 2 data. The weekend effect in winter is supported by an analogous holiday (Spring Festival) effect. Since the late 1970s, the weekend effect has been enhanced in both winter and summer, concurrent with rapid development and enhanced human activity in China. The direct and indirect effects of human-related aerosols on radiation, cloud, precipitation, and so on, might play an important role in generating the opposite signal in the weekend effect for different seasons. During a dry winter, the reduction of aerosol concentrations may overwhelmingly impact on the DTR through a direct effect, i.e., by increasing total solar irradiance near the surface and raising the daytime temperature and maximum temperature and lowering relative humidity. By contrast, in summer the indirect effect of aerosols, i.e., reduction in precipitation efficiency caused by more numerous and smaller cloud droplets, would largely be responsible for the increased numbers of rainy days, the reduction of the total solar irradiance, and the lowering of the maximum temperature and DTR.” Gong, D.-Y., D. Guo, and C.-H. Ho (2006), J. Geophys. Res., 111, D18113, doi:10.1029/2006JD007068. [full text]

Comparison of 7 years of satellite-borne and ground-based tropospheric NO₂ measurements around Milan, Italy – Ordóñez et al. (2006) “Tropospheric NO₂ vertical column densities (VCDs) over the Lombardy region were retrieved from measurements of the Global Ozone Monitoring Experiment (GOME) spectrometer for the period 1996–2002 using a differential optical absorption method. This data set was compared with in situ measurements of NO₂ at around 100 ground stations in the Lombardy region, northern Italy. The tropospheric NO₂ VCDs are reasonably well correlated with the near-surface measurements under cloud-free conditions. However, the slope of the tropospheric VCDs versus ground measurements is higher in autumn-winter than in spring-summer. This effect is clearly reduced when the peroxyacetyl nitrate and nitric acid (HNO₃) interferences of conventional NO_x analyzers are taken into account. For a more quantitative comparison, the NO₂ ground measurements were scaled to tropospheric VCDs using a seasonal NO₂ vertical profile over northern Italy calculated by the Model of Ozone and Related Tracers 2 (MOZART-2). The tropospheric VCDs retrieved from satellite and those determined from ground measurements agree well, with a correlation coefficient R = 0.78 and a slope close to 1 for slightly polluted stations. GOME cannot reproduce the high NO₂ amounts over the most polluted stations, mainly because of the large spatial variability in the distribution of pollution within the GOME footprint. The yearly and weekly cycles of the tropospheric NO₂ VCDs are similar for both data sets, with significantly lower values in the summer months and on Sundays, respectively. Considering the pollution level and high aerosol concentrations of this region, the agreement is very good. Furthermore, uncertainties in the ground-based measurements, including the extrapolation to NO₂ VCDs, might be as important as those of the NO₂ satellite retrieval itself.” Ordóñez, C., A. Richter, M. Steinbacher, C. Zellweger, H. Nüß, J. P. Burrows, and A. S. H. Prévôt (2006), J. Geophys. Res., 111, D05310, doi:10.1029/2005JD006305. [full text]

Weekly periodicity of environmental variables in Phoenix, Arizona – Shutters & Balling (2006) “Though there is no known meteorological cause for weekly cycling of environmental variables, weekly cycles have been discovered at local to global scales, particularly in areas affected by human urbanization. To uncover such cycles in Phoenix, AZ, and to highlight possible mechanisms for their existence, data from several public domain sources were collected and analyzed for cycles in three categories of variables: meteorological, pollution, and human activity measured as vehicle traffic flows. Results indicated that many meteorological and pollution variables do exhibit weekly periodicity and that these cycles are likely due to the weekly pattern of human traffic flows. Atmospheric concentrations of O₃ and NO_X gases exhibit a high degree of negative correlation, supporting recent research that suggests anthropogenic NO_X gases are effective scavengers of ozone in urban cores. Results further suggest that vehicle-generated NO_X gases may be a significant generator of atmospheric nitrate particulates. Finally, both traffic flow and NO_X gas concentrations display a strong correlation with wind speed in the urban core, though this study does not speculate on a mechanism for this relationship.” Shade T. Shutters and Robert C. Balling Jr., Atmospheric Environment, Volume 40, Issue 2, January 2006, Pages 304-310, doi:10.1016/j.atmosenv.2005.09.037.

Urban aerosols and their variations with clouds and rainfall: A case study for New York and Houston – Jin et al. (2005) “Diurnal, weekly, seasonal, and interannual variations of urban aerosols were analyzed with an emphasis on summer months using 4 years of the NASA Earth Observing System (EOS) Moderate Resolution Imaging Spectroradiometer (MODIS) observations, in situ Aerosol Robotic Network (AERONET) observations, and in situ U.S. Environmental Protection Agency (EPA) PM2.5 data for one midlatitude city (New York) and one subtropical city (Houston). Seasonality is evident in aerosol optical thickness measurements, with a minimum in January and a maximum in April to July. The diurnal variations of aerosols, however, are detectable but largely affected by local and regional weather conditions, such as surface and upper-level winds. On calm clear days, aerosols peak during the two rush hours in the morning and evening. Furthermore, the anthropogenic-induced weekly cycles of aerosols and clouds are analyzed, which by themselves are weak, as the anthropogenic signal is mixed with noise of natural weather variability. In addition, corresponding cloud properties observed from MODIS demonstrate an opposite phase to the seasonality of aerosols. Nevertheless, no clear relationship was observed between monthly mean aerosols and rainfall measurements from NASA’s Tropical Rainfall Measuring Mission (TRMM), implying that in the summer the aerosol impact may not be the primary reason for the change of urban rainfall amount.” Jin, M., J. M. Shepherd, and M. D. King (2005), J. Geophys. Res., 110, D10S20, doi:10.1029/2004JD005081. [full text]

Atmospheric visibility trends in an urban area in Taiwan 1961–2003 – Tsai (2005) “Climatological observations made in Tainan urban area, southern Taiwan, between 1961 and June 2003 were analyzed along with critical air pollutants monitored from 1994 to June 2003 in order to establish the relationship between atmospheric visibility and major air pollutants and meteorological parameters in the urban area. The visibility discrepancy between weekend and week days was also examined. Average annual visibility in the complete period 1961–2003 was 12.4±4.2 km. However, during the early 1960s it was >20 km, against only 6–7 km between 2002 and 2003. This study revealed a correlation between PM₁₀ and NO_x levels, which were higher on weekdays than on weekends, and low weekday visibility. Furthermore, decreased visibility was related mostly to increases in PM₁₀. A weekend effect, in which weekend ozone concentrations exceed weekday concentrations, was also revealed. Mixing layer height is an most important meteorological parameter involved in visibility reduction. Principal component analysis demonstrated that increased vehicular emissions, road traffic dust and industrial activity markedly impacted visibility. Pollutant standard index (PSI) values >100 were characterized by concentrations of PM₁₀ and NO_x and atmospheric pressure higher than normal, but with wind speed lower than normal. Regression results for various empirical models of visibility demonstrated that higher PM₁₀ concentrations implied lower visibility ranges, and the parameter of ln[PM₁₀] represented the most significant impact on visibility. Because PM_2.5 has a significant impact on visibility, a targeted reduction of PM₁₀ would not completely improve the visual range. However, there is a strong association between presence of PM₁₀ and presence of PM_2.5, such that a targeted reduction in PM₁₀ is likely to lead to an increase in visibility.” Ying I. Tsai, Atmospheric Environment, Volume 39, Issue 30, September 2005, Pages 5555-5567, doi:10.1016/j.atmosenv.2005.06.012.

Variations in the diurnal character of tropical cyclone wind speeds – Cerveny & Balling (2005) It is not mentioned in the abstract, but apparently they also found a weekly cycle in their data. “A significant decline in diurnal temperature range (DTR) is identified along the Atlantic seaboard. Recent studies suggest that DTR changes demonstrate a human-created weekly cycle and may therefore be anthropogenic. In this study, we address whether there is a change in the diurnal variation in Tropical Cyclone (TC) wind speeds that is consistent with the DTR trend over land. Our analysis of 34 years of TC activity reveals variations such that the difference between day and night wind speeds of TCs has decreased over time. Our work gives limited support to the contention that TC diurnal wind speed differences are thermally driven and, hence, warmer night temperatures (a smaller DTR over time) are leading to stronger nighttime winds over time. Our study initiates investigation of potential influences of climate changes (such as DTR) on secondary climatic phenomena.” Cerveny, R. S., and R. C. Balling Jr. (2005), Geophys. Res. Lett., 32, L06706, doi:10.1029/2004GL021177.

Observations of a “weekend effect” in diurnal temperature range – Forster & Solomon (2003) “Using surface measurements of maximum and minimum temperatures from the Global Daily Climatological Network data set, we find evidence of a weekly cycle in diurnal temperature range (DTR) for many stations in the United States, Mexico, Japan, and China. The “weekend effect,” which we define as the average DTR for Saturday through Monday minus the average DTR for Wednesday through Friday, can be as large as 0.5 K, similar to the magnitude of observed long-term trends in DTR. This weekend effect has a distinct large-scale pattern that has changed only slightly over time, but its sign is not the same in all locations. The station procedures and the statistical robustness of both the individual station data and the patterns of DTR differences are thoroughly examined. We conclude that the weekend effect is a real short time scale and large spatial scale geophysical phenomenon, which is necessarily human in origin. We thus provide strong evidence of an anthropogenic link to DTR, an important climate indicator. Several possible anthropogenic mechanisms are discussed; we speculate that aerosol-cloud interactions are the most likely cause of this weekend effect, but we do not rule out others.” Piers M. de F. Forster and Susan Solomon, PNAS September 30, 2003, vol. 100 no. 20 11225-11230, doi: 10.1073/pnas.2034034100. [full text]

Weekly cycle of NO₂ by GOME measurements: a signature of anthropogenic sources – Beirle et al. (2003) “Nitrogen oxides (NO+NO2=NOx and reservoir species) are important trace gases in the troposphere with impact on human health, atmospheric chemistry and climate. Besides natural sources (lightning, soil emissions) and biomass burning, fossil fuel combustion is estimated to be responsible for about 50% of the total production of NOx. Since human activity in industrialized countries largely follows a seven-day cycle, fossil fuel combustion is expected to be reduced during weekends. This “weekend effect” is well known from local, ground based measurements, but has never been analysed on a global scale before. The Global Ozone Monitoring Experiment (GOME) on board the ESA-satellite ERS-2 allows measurements of NO2 column densities. By estimating and subtracting the stratospheric column, and considering radiative transfer, vertical column densities (VCD) of tropospheric NO2 can be determined (e.g. Leue et al., 2001). We demonstrate the statistical analysis of weekly cycles of tropospheric NO2 VCDs for different regions of the world. In the cycles of the industrialized regions and cities in the US, Europe and Japan a clear Sunday minimum of tropospheric NO2 VCD can be seen. Sunday NO2 VCDs are about 25-50% lower than working day levels. Metropolitan areas with other religious and cultural backgrounds (Jerusalem, Mecca) show different weekly patterns corresponding to different days of rest. In China, no weekly pattern can be found. The presence of a weekly cycle in the measured tropospheric NO2 VCD may help to identify the different anthropogenic source categories. Furthermore, we estimated the lifetime of tropospheric NO2 by analysing the mean weekly cycle exemplarily over Germany, obtaining a value of about 6 h in summer and 18-24 h in winter.” Beirle, S., Platt, U., Wenig, M., and Wagner, T., Atmos. Chem. Phys., 3, 2225-2232, doi:10.5194/acp-3-2225-2003, 2003. [full text]

A weekly cycle in atmospheric carbon dioxide – Cerveny & Coakley (2002) “We present a new statistic called the “Mean Symmetrized Residual” (MSR) for detection and quantification of a weekly cycle in measured daily atmospheric carbon dioxide (CO₂). At the Mauna Loa Observatory in Hawaii, we conclude that CO₂ concentrations, on average, are significantly lower (0.022 parts per million by volume, ppmv) on weekends (Saturday–Sunday) than during the rest of the week. Over the past twenty-five years, the variation of the mean values of MSR (as a function of day of the week) has been relatively stable. We speculate that the observed weekday/weekend variation in CO₂ at Mauna Loa is the result of anthropogenic emissions on Hawaii and nearby sources. We do not detect a weekly cycle in daily CO₂ concentration measured at South Pole, Antarctica. This methodology has applicability to a variety of datasets.” Cerveny, R. S., and K. J. Coakley (2002), Geophys. Res. Lett., 29(2), 1028, doi:10.1029/2001GL013952.

The influence of tropospheric ozone on the air temperature of the city of Toronto, Ontario, Canada – Beaney & Gough (2002) “Weekday/weekend variations in tropospheric ozone concentrations were examined to determine whether ground-level greenhouse gases have a significant impact on local climate. The city of Toronto, Canada, was chosen due to a high volume of commuter traffic and frequent exposure to high ozone episodes. Due to day-of-the-week variations in commuter traffic, ozone concentrations were shown to vary significantly between weekdays and weekends. During high ozone episodes weekend air temperatures were significantly higher than those observed on weekdays. As no meteorological phenomenon is known to occur over a 7 day cycle the observed temperature variations were attributed to anthropogenic activity.” Gary Beaney and William A. Gough, Atmospheric Environment, Volume 36, Issue 14, May 2002, Pages 2319-2325, doi:10.1016/S1352-2310(02)00184-X.

Spectral analysis of weekday–weekend differences in ambient ozone, nitrogen oxide, and non-methane hydrocarbon time series in California – Marr & Harley (2002) “We describe the history and spatial distribution of day-of-week differences in ambient ozone, NO_x, and VOC concentrations through the analysis of two decades of measurements from sites located throughout California. Spectral analysis of the concentration time series shows that weekly patterns in ozone concentrations, typically with higher values of ozone on weekends, have become more widespread in California between 1980 and 1999. In contrast, a strong weekly pattern in NO_x concentrations has been present throughout the entire period, and weekly patterns in VOC concentrations, though not as evident, have also been present during the entire 20-yr period. We examine 8-h average ozone concentrations, which appear to be a more sensitive measure of day-of-week differences in ozone than are 1-h averages. At sites with significant weekly cycles, fluctuations in pollutant concentrations that occur on a weekly time scale account for 6.6±3.5%, 3.0±1.7%, and 2.1±0.9% of the total variance in NO_x, NMHC, and ozone concentrations, respectively. Concentrations of all three pollutants have been declining in most locations over the past 20 yr. Our results support the hypothesis that the weekend ozone effect is due to a combination of VOC-sensitivity and reduced NOx emissions on weekends. The spread of the weekend ozone effect may be due to a shift in ozone formation towards VOC-sensitivity, as control programs have reduced emissions of VOC more than NO_x.” Linsey C. Marr and Robert A. Harley, Atmospheric Environment, Volume 36, Issue 14, May 2002, Pages 2327-2335, doi:10.1016/S1352-2310(02)00188-7.

Comparison between weekend and weekday ozone concentration in large cities in France – Pont & Fontan (2001) “This paper examines ozone data from five large French cities (Marseilles, Lyon, Paris, Strasbourg and Toulouse) in spring and summer over a three-year period to study the possible influence of local primary pollutant emissions. In these cities the level of traffic emission varies according to the day of the week. There is a decrease of about 25% in traffic emissions between non-consecutive Tuesdays and Sundays. Traffic emissions on Fridays are about 40% more than on non-consecutive Sundays whereas they seem to be similar for non-consecutive Tuesdays and Thursdays. Despite this variation in traffic emissions between Fridays and Sundays, 85% of daily ozone maxima are identical for all days compared; in 15% of cases, percentiles of daily ozone maxima vary by about 20% at the most. This difference is observed for the highest values of daily ozone maxima that we can find both in rural and urban sites. Marseilles is the most pollution-sensitive city; every site of this area is concerned, which gives a regional origin to ozone variability. In the less-populated Toulouse area, differences between ozone on Fridays and Sundays are less significant. Our results show the importance of advection phenomena of ozone. It calls into question strategies of local reductions in traffic during ozone episodes.” Véronique Pont and Jacques Fontan, Atmospheric Environment, Volume 35, Issue 8, 2001, Pages 1527-1535, doi:10.1016/S1352-2310(00)00308-3.

Weekly Precipitation Cycles along the Northeast Corridor? – DeLisi et al. (2001) “Twenty years of precipitation data from seven cities along or near the east coast of the United States from the northern mid-Atlantic region to northern New England have been analyzed to determine if there are any weekly cycles in either daily precipitation frequency or intensity. Any such weekly cycle could be considered evidence of anthropogenic influence on the climate of that region. Data were examined for each individual site and for all sites combined. The data were subjected to various statistical procedures, including one-way analysis of variance, Student’s t-test, and the chi-square goodness-of-fit test. Overall, results were not significant at the 95% confidence level. Thus, this study is unable to detect any weekly cycle in daily precipitation intensity or frequency.” DeLisi, Mark P., Alan M. Cope, Jason K. Franklin, 2001, Wea. Forecasting, 16, 343–353. [full text]

Spectral analysis of air pollutants. Part 1: elemental carbon time series – Hies et al. (2000) “An effective method to analyse different air pollution sources in an elemental carbon time series is presented. As a second feature, this technique allows a fast and efficient classification of monitoring sites. Time series of daily elemental carbon measurements at various urban locations have been evaluated with the corresponding power spectra. Typical and well-known periodicities caused by anthropogenic and meteorological influences have been identified using coherence and phase spectra. It will be shown that domestic heating by coal combustion appears as a 365 day periodicity, traffic contributes 3.5, 4.6 and 7 day peaks in the spectrum and elevated long range elemental carbon can be identified as characteristic peaks with periodicities in the range from 13 to 42 days. As the relative amplitudes of the various influences vary depending on the location of the measurement site in the urban area, the use of estimated power spectra helps to find the influence of traffic, domestic coal-heating and long range transport on the elemental carbon concentration.” Thomas Hies, Renate Treffeisen, Ludwig Sebald and Eberhard Reimer, Atmospheric Environment, Volume 34, Issue 21, 2000, Pages 3495-3502, doi:10.1016/S1352-2310(00)00146-1.

Comparisons of weekday–weekend ozone: importance of biogenic volatile organic compound emissions in the semi-arid southwest USA – Diem (2000) “This paper examines differences between daily maximum weekday and weekend ambient ozone concentrations in the Tucson, AZ metropolitan area. Temporal variations in the Weekend Effect (i.e. weekend ozone concentrations are larger than weekday concentrations) are not explained entirely by changes in anthropogenic emissions of ozone precursor chemicals (i.e. nitrogen oxides and volatile organic compounds). A dramatic change from the Weekend Effect in June to the Weekday Effect (i.e. weekday ozone concentrations are larger than weekend concentrations) in July is associated with the onset of the North American Monsoon. A transition from a relatively dry atmosphere during the arid foresummer months of May and June to a relatively moist atmosphere during the monsoon months of July and August seems to explain the changes in ozone concentrations. Moist conditions are associated with increases in biogenic volatile organic compound (BVOC) emissions in the urban forest and surrounding desert areas. BVOC emissions appear to be an important source of VOCs, especially during the monsoon months. Therefore, an increase in ambient BVOC concentrations from June to July presumably reverses the sensitivity of ozone production in the Tucson area from VOC- to NO_x-sensitive.” Jeremy E. Diem, Atmospheric Environment, Volume 34, Issue 20, 2000, Pages 3445-3451, doi:10.1016/S1352-2310(99)00511-7.

Weekly cycles of air pollutants, precipitation and tropical cyclones in the coastal NW Atlantic region – Cerveny & Balling (1998) “Direct human influences on climate have been detected at local scales, such as urban temperature increases and precipitation enhancement, and at global scales. A possible indication of an anthropogenic effect on regional climate is by identification of equivalent weekly cycles in climate and pollution variables. Weekly cycles have been observed in both global surface temperature and local pollution data sets. Here we describe statistical analyses that reveal weekly cycles in three independent regional-scale coastal Atlantic data sets: lower-troposphere pollution, precipitation and tropical cyclones. Three atmospheric monitoring stations record minimum concentrations of ozone and carbon monoxide early in the week, while highest concentrations are observed later in the week. This air-pollution cycle corresponds to observed weekly variability in regional rainfall and tropical cyclones. Specifically, satellite-based precipitation estimates indicate that near-coastal ocean areas receive significantly more precipitation at weekends than on weekdays. Near-coastal tropical cyclones have, on average, significantly weaker surface winds, higher surface pressure and higher frequency at weekends. Although our statistical findings limit the identification of cause–effect relationships, we advance the hypothesis that the thermal influence of pollution-derived aerosols on storms may drive these weekly climate cycles.” Randall S. Cerveny & Robert C. Balling, Jr., Nature 394, 561-563 (6 August 1998) | doi:10.1038/29043. [full text]

Weekly cycle of meteorological variations in Melbourne and the role of pollution and anthropogenic heat release – Simmonds & Keay (1997) “An aspect of anthropogenic impacts on climate have been assessed by examining the day-of-the-week variation (DOWV) of important meteorological elements. The data used were those of daily maximum and minimum temperature and rainfall for Melbourne for the period 1856–1990. This long series has been broken up into five 27-yr subperiods to expose how any such variation has changed over the record. We find there to be no DOWV in the summer “half” of the year for any of the subperiods. The only statistically significant variations to have physical meaning occur in the winter of the most recent subperiod (1964–1990). In that time maximum temperature exhibits a significant (10% confidence level) DOWV and weekdays are 0.29°C warmer than weekends (5%). Minimum temperatures and rainfall amounts were also found to be greater (10% level) on weekdays by 0.24°C and 0.20 mm d−1, respectively. We have considered the possible impacts of day-of-the-week variation of atmospheric pollution loading and of the local generation of heat. We hypothesise that the magnitude of the contrast between weekday-weekend anthropogenic heat emissions is sufficient to explain the temperature differences and these in turn are consistent with the weekday excess of rainfall. This perspective is concordant with the results of many recent studies which stress the importance of anthropogenic heating.” Ian Simmonds and Kevin Keay, Atmospheric Environment, Volume 31, Issue 11, June 1997, Pages 1589-1603, doi:10.1016/S1352-2310(96)00344-5. [full text]

Weekend-weekday differences of near-surface ozone concentrations in Switzerland for different meteorological conditions – Brönnimann & Neu (1997) “Mean weekly cycles of daily ozone peak levels are extracted out of 8 yr data sets under selected meteorological conditions. In the region under study, the emissions of precursor substances are considerably lower on weekends than on weekdays, as can be derived from the Swiss emission inventory or from traffic frequencies. Chemical production as well as destruction of ozone are affected differently by sudden changes in emissions, depending on meteorology and on the structure of the emissions. Therefore, in Switzerland, several distinct patterns of the weekly cycle of mean daily ozone peak concentrations can be detected. When meteorology is not favourable to ozone production, weekends show generally higher ozone peaks than weekdays. Favourable meteorology (i.e. high solar radiation, high temperatures, low wind speeds) produces an inverse pattern, the mean ozone peaks being 10–15% lower on Sundays than on Thursdays or Fridays. Differences in emission structures slightly modify the patterns and can delay the effects. Threshold values to separate favourable conditions can be estimated for radiation and temperature. In Switzerland, “favourable” meteorology is achieved on about 30–50 days per year, corresponding well with “summer smog days”.” Stefan Brönnimann and Urs Neu, Atmospheric Environment, Volume 31, Issue 8, April 1997, Pages 1127-1135, doi:10.1016/S1352-2310(96)00311-1.

C₂—C₆ hydrocarbon measurements at four rural locations across Canada – Bottenheim & Shepherd (1995) “Observations of low molecular weight hydrocarbons at four rural locations in Canada are reported. The measurements cover a period of one year (1991), and the seasonal trends are discussed. It is deduced that most variation is due to photochemically driven processes (OH chemistry). Although at least two of the sites were well removed from large urban source regions, the observations show a clear anthropogenic influence on the rural hydrocarbon levels at all sites. Air mass back-trajectories have been used to investigate the origin of the observed hydrocarbons. Weekday/weekend effects are distinguishable at two of the sites, and correlation with a limited set of CO observations at one of the sites is found to be very good for those compounds that are known to originate from transportation related processes. Comparison of the data with published observations suggests that the background distribution of hydrocarbons over the North American continent is quite homogeneous. Isoprene is the only hydrocarbon of biogenic origin that was measured in this study, and its importance relative to the other measured compounds with respect to photochemical processes is indicated.” Jan W. Bottenheim and Marjorie F. Shepherd, Atmospheric Environment, Volume 29, Issue 6, 1995, Pages 647-664, doi:10.1016/1352-2310(94)00318-F.

Weekdays warmer than weekends? – Gordon (1994) No abstract available, but is described in Bäumer et al. (2008): “Gordon (1994) found a significant but very small weekly temperature cycle for the northern hemisphere for the period 1979–1992.” A. H. Gordon, Nature 367, 325 – 326 (27 January 1994); doi:10.1038/367325b0.

Photochemistry of the “Sunday Effect” – Graedel et al. (1977) No abstract available, but apparently finds a weekly cycle from pollution data sets. Thomas E. Graedel, Leonilda A. Farrow, Thomas A. Weber, Environ. Sci. Technol., 1977, 11 (7), pp 690–694, DOI: 10.1021/es60130a005.

A comparison of weekend-weekday ozone and hydrocarbon concentrations in the Baltimore-Washington metropolitan area – Lebron (1975) “A “smog index”, related to dosage (ppm-h) of ozone, was derived. Indices were calculated using data from the Baltimore-Washington metropolitan area for the months of June–September of 1972 and 1973. On the average, weekends had higher indices than weekdays, although this difference may not be statistically significant. Further analysis of the data indicates that 06:00–09:00 h average concentrations of non-methane hydrocarbons are significantly higher during weekdays than during weekends and no relationship exists between these morning hydrocarbon levels and the afternoon peak ozone concentration. These results place some doubt on the effectiveness of early morning hydrocarbon emission control alone in an abatement program for photochemical oxidants.” Felipe Lebron, Atmospheric Environment, Volume 9, Issue 9, September 1975, Pages 861-863, doi:10.1016/0004-6981(75)90046-3.

Sunday and Workday Variations in Photochemical Air Pollutants in New Jersey and New York – Cleveland et al. (1974) “Concentration distributions of air contaminants and meteorological variables in New Jersey and New York for workdays (Mondays through Fridays, omitting holidays) and Sundays are compared by means of quantile-quantile plots. The ozone distributions are slightly higher on Sundays, and the primary pollutant distributions are lower. These results raise serious questions about the validity of current concepts underlying ozone reduction in urban atmospheres.” W. S. Cleveland, T. E. Graedel, B. Kleiner and J. L. Warner, Science 13 December 1974, Vol. 186 no. 4168 pp. 1037-1038, DOI: 10.1126/science.186.4168.1037.

Note on the effect of the weekly cycle of air pollution on solar radiation at Toronto – Mateer (1961) Int J Air Water Pollut. 1961 Jun;4:52-4.