1. Introduction
A considerable amount of human and financial commitment by local governments is dedicated to cleaning public spaces in order to manage litter and maintain satisfactory aesthetic and sanitary conditions. Spanish municipalities spend approximately between 10% and 20% of their budgets on cleaning (Ramos 2007), and, despite it being one of a municipality’s largest expenses, citizens are generally not satisfied with the results, as the social perception of urban cleanliness tends to score low in surveys.
The quality of street cleaning stands as a primary demand made by citizens to their local governments, and it hinges on various factors such as the adopted cleaning system, cleaning frequency, citizen usage of the streets, citizen civility levels, and the financial investment that municipal budgets allocate to street cleaning.
Urban surfaces amass waste from both natural and human origins (Bris et al. 1999; Slater et al. 2008; Amato et al. 2010; Bovea et al. 2010; Hong et al. 2010). Human activities in the city generally produce street waste through inappropriate activities or lack of cleaning, although waste can also result from atmospheric transport. A notable amount of street garbage accumulates due to deficiencies in the garbage collection infrastructure, especially in the case of large cities.
The type of waste deposited on the streets normally consists of soil sediment, small pieces of pavement, and various types of garbage (Jang et al. 2009). This creates a negative visual impact, has indirect repercussions on the city’s economy (Riccio et al. 1988), and contributes to water and air pollution (Gertler et al. 2006; Irvine et al. 2009). Several authors demonstrate a positive connection between cleanliness and tourism in a municipality (Bel 2006; Benito-López et al. 2011), as well as with economic activity (De Borger et al. 1994). In addition to a clean city attracting business and tourism, citizens also place a high value on its cleanliness (Bel 2006; Benito-López et al. 2011; De Borger et al. 1994). Conversely, a dirty city accumulates more litter, thereby encouraging people to litter even more (Arafat et al. 2007; Al-Khatib 2009).
The relationship between street cleaning and weather is not new. As early as February 1898, a letter to the editor of the Medical Record titled “Street Cleaning: Is the Weather or the Dirt?” (Gibson 1898) referred to some criticism and sparked a social debate in several New York newspapers. Deputy commissioner F. W. Gibson asserted in this letter that “the streets of the city are and have been in just as good conditions since Commissioner McCartney took office as the condition of the weather would permit, and any criticism to the contrary is unjust and by no means in accord with the facts.”
Pioneering studies on street dirt accumulation conclude that its removal can result from natural processes such as wind and rain or from the efforts of urban cleaning teams (Pitt 1987). Sartor and Boyd (1972) constructed several curves illustrating street dirt accumulation, by which they found that it is substantially influenced by the number of days since the street’s last municipal cleaning or the interval between two rainy days. Additionally, they observed that a significant rainfall event, typically around 10 mm or higher, removes at least 90% of the available street dirt wash-off load.
Donigian and Crawford (1976) estimated the percentage of existing garbage particulates on the streets that would wash off at various rain intensities, finding that the change in street dirt loading per unit time is directly proportional to rain intensity and street dirt loading. Novotny and Chesters (1981) determined that a particulate residue factor varies with a power of 1.1 of rain intensity. A reduction of 10 in the dirt factor can be observed from a rain intensity of 1–18 mm h−1.
Pitt (1987) conducted several wash-off experimental tests in the city of Toronto, Canada, analyzing rain intensity over approximately 2 h (total precipitation ranging between 5 and 25 mm), street texture, and street dirt loading. They observed that higher precipitation intensities of between 11 and 12.2 mm h−1 produced runoff that removed dirt from the streets, while intensities of between 3.9 and 3.2 mm h−1 generated insufficient runoff to eliminate the dirt load. Pitt also observed that the concentration of solid particles in the streets decreased with increasing rainfall, leading to better wash-off. Nearly complete removal of available dirt loads from the streets occurred during rain accumulation of about 25 mm, and complete removal was achieved with larger values of close to 100 mm. Although such precipitation amounts far exceed the scope of typical washout tests, they periodically occur in some regions of the world. An exponential solid washout model may be applicable for rainfall in the range of approximately 3–30 mm, which represents the most common rainfall events.
Studies examining the relationship between weather and the perception of cleanliness are notably absent in the literature, specifically any investigations exploring how urban cleanliness perception is influenced by precipitation patterns, average temperature, or aridity. This article seeks to fill the research gap by undertaking an analysis of this question across the 47 most populated cities in Spain, situated in the country’s three distinct climatic zones: oceanic, continental, and Mediterranean. Despite the diverse geographic distribution, we assume a uniform cultural criterion regarding cleanliness perception.
Urban cleanliness and climate change
Cities contribute substantially to global warming (Mills 2007; Deng et al. 2023) while also experiencing higher temperatures compared to their surrounding areas. This phenomenon stems from various factors like limited vegetation cover and the absence of water bodies, leading to a scarcity of natural cooling mechanisms. These rising temperatures in urban areas, along with potential shifts in precipitation patterns, may be contributing to heightened perceptions that urban environments are unhealthy and unclean.
According to the sixth report of the Intergovernmental Panel on Climate Change (Ali et al. 2022), the Mediterranean area of Spain is projected to experience increased droughts by midcentury (2041–60), accompanied by warming levels of 2°C or above (high confidence) compared to the period 1995–2014. Additional midcentury projections under these conditions indicate a combination of climatic impact–driver changes over the Mediterranean and the central Iberian Peninsula, including warming, temperature extremes, increased droughts and aridity, and a decrease in precipitation.
Focusing on the Iberian Peninsula, climatic numerical simulations conducted by Lorenzo and Alvarez (2020) reveal a general decrease in seasonal and annual precipitation, as well as a reduction in the number of rainy days across Spain for the period 2021–50 compared to 1971–2001. Projections indicate an overall increase in dry days, particularly during spring, summer, and autumn.
Aridity in Spain has increased over recent decades (Moral et al. 2017; Paniagua et al. 2019). Andrade et al. (2021) calculated the De Martonne aridity index (IDM; De Martonne 1925) for the Iberian Peninsula, initially for the baseline period 1961–90, and subsequently projected it for the periods 2011–40 (short range) and 2041–70 (medium range). Two representative concentration pathways (RCPs) were analyzed: an intermediate anthropogenic radiative forcing scenario (RCP4.5) and a fossil-intensive emission scenario (RCP8.5). Based on the IDM and other precipitation indexes, their findings indicate a strengthening of aridity and dry conditions in the central and southern Iberian Peninsula until 2070, particularly under RCP8.5, but also for RCP4.5. The prevalence of years with arid conditions (exceeding 70% for 2041–70 under both RCPs) is projected to have major impacts in certain regions, including southern Portugal and the central, southern, and Mediterranean regions of Spain. Similarly, Moral et al. (2023), utilizing IDM, obtain comparable results by analyzing the spatial distribution of aridity in southwestern Spain. They considered the reference period 1971–2005 and three time periods: 2006–35 (near future), 2036–65 (midcentury), and 2066–95 (end of the century). Their analysis reveals a progressive strengthening of aridity conditions over southwestern Spain until the end of the century, primarily under the RCP8.5 scenario.
Based on the aforementioned research, the underlying hypothesis of this investigation suggests that a higher number of rainy days and greater total annual precipitation would correlate with elevated cleanliness perception ratings from citizens. This hypothesis is grounded in the observation that continuous rainfall contributes to street runoff, which effectively sweeps away litter particles, minimizes unpleasant odors, and enhances the cleanliness of urban spaces, thereby providing a more aesthetically pleasing environment compared to drier cities. Our hypothesis additionally posits that a more favorable perception of cleanliness by citizens should be observed in colder cities, while worse perceptions would likely manifest in warmer ones. This is attributed to the tendency of warmer cities to attract more social activities on the streets than colder ones, potentially leading to increased litter dispersion.
The structure of this work is as follows. Section 2 explains the methodology used. Sections 3 and 4 describe the influence of precipitation and temperature on urban cleanliness perception, respectively. Section 5 analyzes the relationship between aridity (evaluated through the De Martonne index) and urban cleanliness perception (UCP). Finally, section 6 is dedicated to the discussion and conclusions.
2. Methodology
The Consumers and Users Organization (OCU, www.ocu.org), established in 1975, stands as the largest consumer organization in Spain. As a private, independent, nonprofit entity, its mission is to champion consumer rights, furnish information and assistance to consumers, and safeguard their interests and rights, ultimately fostering a transparent and equitable consumer society.
OCU conducts a cleanliness perception survey every 4 years in Spain’s largest cities. This survey covers various cleanliness aspects such as streets, sidewalks, parks and gardens, excrement in the streets, graffiti, damage to urban furniture, areas around dumpsters, pollution, and areas outside the downtown area. The average rating derived from these questions contributes to the overall satisfaction score on a 100-point scale, which is utilized in this investigation. Data from surveys in 2015, 2019, and 2023 serve as primary sources. OCU’s two-part methodology involves first distributing approximately 60 000 electronic surveys to its members, yielding 5825 responses in 2015, 5260 in 2019, and 6863 in 2023. Simultaneously, a questionnaire was sent to the councils of the 60 most populated cities, gathering technical and economic data on their cleanliness systems. Of these 60 cities, we analyze only the 47 that meet the criterion of having at least one automatic weather station from the Spanish Meteorological Agency (AEMET) installed within their municipality. These 47 cities represent Spain’s most populous areas, collectively accounting for nearly 15 million people, or approximately 32% of the population. While the total number of survey responses may seem modest compared to the overall population, OCU’s survey stands as the broadest and most comprehensive of its kind in Spain.
Classes of aridity based on the De Martonne aridity index values (Middleton et al. 1992; Baltas 2007).
Figure 1 shows the locations of the 47 analyzed cities, all of which are major cities in Spain equipped with an official weather station. Two colors distinguish cities according to their cleanliness ratings obtained from the OCU’s survey: above (green) or below (red) 51 points. The average rating is calculated based on the average score across seven citizen perceptions of cleanliness: streets and sidewalks, parks and gardens, excrement in walking areas, graffiti, around urban waste bins and dumpsters, air quality, and areas distant from downtown. Each question is scored on a scale of 100 points.
Location of analyzed Spanish cities in the Iberian Peninsula. Red dots indicate an average score below 51 (fairly and poorly clean) in the OCU’s 2015, 2019, and 2023 surveys, while green dots indicate an average score above 51 (very clean and clean). Each score is indicated beside each dot.
Citation: Weather, Climate, and Society 16, 3; 10.1175/WCAS-D-23-0145.1
A distinctive regional distribution is evident: Green dots prevail in the north and northwestern parts of Spain (characterized by oceanic and continental climates). The cities with red dots within these regions scored below but nevertheless close to the threshold of 51. In contrast, the southern plateau and Mediterranean basin (Mediterranean climate) are covered with red dots (indicating lower average scores).
Regarding the weather variables, we utilized precipitation and temperature data recorded by AEMET automatic weather stations over the period 1981–2022. Specifically, we analyzed annual total precipitation, number of rainy days, average annual temperature, and average annual maximum and minimum temperatures. The IDM was computed from these data.
3. The influence of precipitation on urban cleanliness perception
Figure 2 illustrates the average UCP values for each analyzed city across the 2015, 2019, and 2023 surveys. These values are plotted as a function of the respective cities’ average annual precipitation for the years 2014, 2018, and 2022, which precede the corresponding surveys conducted in the first trimester of 2015, 2019, and 2023, respectively. A notable dispersion of UCP values is observed for cities with similar average annual precipitation levels, particularly for low and medium annual precipitation values. For instance, at approximately 500 mm of precipitation, UCP values vary between 40 and 70.
Average UCP obtained from the 2015, 2019, and 2023 surveys as a function of annual precipitation averaged between the years 2014, 2018, and 2022. Each dot represents one of the studied cities, with the black line indicating the linear adjustment.
Citation: Weather, Climate, and Society 16, 3; 10.1175/WCAS-D-23-0145.1
Urban cleanliness is clearly influenced by numerous social factors beyond average annual precipitation, including civic behavior, sanitation infrastructure, and investment in cleaning resources. The significant variation in cleanliness perception values suggests that, statistically, no direct mathematical relationship exists between the perception of urban cleanliness and average annual precipitation. A Pearson chi-square test was conducted, indicating a weak statistical significance between the two variables (chi-square correlation coefficient of 0.753 and linear correlation coefficient of 0.417). Nevertheless, a trend can be observed: Higher annual precipitation tends to correlate with higher perceived urban cleanliness values.
This correlation becomes more evident when examining the relationship between UCP and the annual number of rainy days averaged over the three survey years (Fig. 3). Cities with a higher frequency of rainy days tend to score higher in urban cleanliness perception. These cities are typically located in the northern regions and feature an Atlantic climate, characterized by more frequent rainfall compared to their southern and eastern counterparts, where prolonged dry spells are prevalent. This divergence in precipitation patterns may impact runoff, with regular rainfall prompting more frequent washing and sweeping, thus leading to the removal of debris from the streets.
The UCP as a function of the annual number of rainy days averaged across 2014, 2019, and 2022. Each dot represents a studied city, and the black line represents the linear adjustment.
Citation: Weather, Climate, and Society 16, 3; 10.1175/WCAS-D-23-0145.1
4. The influence of temperature on urban cleanliness perception
The three panels in Fig. 4 illustrate the average urban cleanliness perception from the 2015, 2019, and 2023 surveys, comparing the mean annual temperature (Fig. 4, upper panel), mean annual maximum temperature (Fig. 4, middle panel), and the mean annual minimum temperature (Fig. 4, lower panel) averaged across the years 2014, 2018, and 2022. Similar to precipitation, a notable dispersion can be observed among these variables. However, no statistical significance was observed based on the Pearson chi-square test conducted. Nevertheless, a discernible negative correlation emerges: Cities with higher mean annual, maximum, and minimum temperatures tend to exhibit lower values in urban cleanliness perception, and vice versa, that is, lower temperatures point toward higher UCP. The warmest cities, situated mainly in the south and especially the east of Spain, where a Mediterranean climate prevails, experience mild temperatures, both annual and minimum, which fosters increased social use of public spaces and potentially greater accumulation of dirt. Consequently, maintaining cleanliness in these areas may require greater efforts and pose increased challenges. Additionally, fewer rainy days contribute to less natural cleansing of dirt through runoff, further contributing to the observed downward trend in the perception of urban cleanliness in warmer cities.
The UCP as a function of (upper) mean annual, (middle) mean maximum, and (lower) mean minimum temperatures averaged across the years 2014, 2018, and 2022. Each dot represents one of the studied cities, and the black line indicates the linear adjustment.
Citation: Weather, Climate, and Society 16, 3; 10.1175/WCAS-D-23-0145.1
5. Aridity: The combined influence of precipitation and temperature on urban cleanliness perception
Our analysis indicates that both precipitation and temperatures collectively impact the perception of urban cleanliness.
The IDM was calculated using the annual mean temperature and precipitation averaged across the years 2014, 2018, and 2022 for the studied cities. These cities were subsequently categorized into five clusters based on their IDM values, representing varying degrees of aridity, as outlined in Table 1. A single averaged UCP value was determined for all cities within each cluster. Figure 5 depicts the relationship between this average UCP and the corresponding IDM averaged for each cluster.
Averaged UCP as a function of averaged IDM for each aridity class. The black line represents the Gompertz distribution function.
Citation: Weather, Climate, and Society 16, 3; 10.1175/WCAS-D-23-0145.1
The relationship between IDM and UCP becomes evident at lower index values, which correspond to cities with arid or Mediterranean and subhumid climates. UCP exhibits a linear increase as the climate transitions from arid to Mediterranean and then to subhumid. In contrast, cities with a humid or superhumid climate show no significant change in urban cleanliness perception relative to the IDM. Unlike the pronounced increase observed from arid to Mediterranean or from Mediterranean to subhumid climates, the shift in urban cleanliness perception between cities in humid and superhumid climates is not as evident. The relationship between UCP and IDM displays a nonlinear growth pattern.
6. Discussion and conclusions
Climate projections suggest that Spanish cities will be warmer and drier by midcentury, experiencing prolonged periods without precipitation and an anticipated decrease in average annual precipitation ranging from 4% to 22%, depending on various IPCC scenarios, particularly in the Mediterranean region (Ali et al. 2022). This heightened aridity is likely to contribute to a potential deterioration in urban cleanliness perception. To maintain the same level of urban cleanliness perception observed during the period analyzed in this study (2015, 2019, and 2023) under scenarios of heightened aridity, municipalities will likely need to increase their economic investments. These investments should extend beyond improving urban cleaning systems and methods, and they should also include initiatives targeting the reduction of street-level dirt through the promotion of greater civility and awareness regarding the impacts of climate change. Such awareness is crucial, particularly in the context of water scarcity, which greatly impacts water usage for cleaning urban environments.
Cleaning the streets not only addresses issues of perception and hygiene but also concerns health and air pollution. Part of urban street dust comprises particles deposited on sidewalks and city streets stemming from traffic, vehicle exhaust, and brake and tire wear (Norman and Johansson 2006; Amato et al. 2009, 2017). This constitutes a critical source due to the high content of heavy metals and organic compounds, which are prone to being reincorporated into the atmosphere via turbulence or wind generated by traffic and vehicle passage. However, street sweeping evidently does not immediately improve air quality, as studies find no evidence that sweeping procedures lead to a reduction in levels of suspended particulate matter (PM10) (Fitz 1998; Norman and Johansson 2006; Chow et al. 1990).
This does not imply that street sweeping fails to improve air quality in the long run, however. The removal of dirt or coarse sediment undoubtedly benefits residents and passers-by. Studies indicate that water washing is more effective in reducing the dust load deposited on street surfaces, similar to the effect of rain, and it therefore represents a potentially effective measure for mitigating dust resuspension (John et al. 2006; Norman and Johansson 2006; Keuken et al. 2010; Karanasiou et al. 2011; Kantamaneni et al. 1996). In a scenario of decreased annual precipitation and longer periods without rain in Mediterranean climates (Ali et al. 2022), reducing atmospheric pollution from fine particles originating from streets and sidewalks will also reduce the resuspension of fine and ultrafine particles. This can be achieved by decreasing traffic volume (especially from more polluting vehicles), reducing speed on the main streets, and modifying street cleaning practices, particularly in Mediterranean environments. With the increase in dry periods limiting the use of water for street washing, this has already become a reality in some Mediterranean cities not only in Spain, but also in others with Mediterranean and arid climates. Consequently, traffic and speed regulation may serve as adaptations to the availability of water for frequent street washing.
Little research exists on adapting street cleaning practices to climate change. Based on the findings of this study, it is imperative that urban climate adaptation and mitigation plans adapt street cleaning to more arid scenarios in order to mitigate declining perceptions of good cleanliness. These plans should likely incorporate strategies for reducing water usage on streets in cities facing water scarcity. Additionally, these strategies should include measures that reduce dirt deposits on streets, not only through public awareness campaigns and initiatives promoting civility but also through exploration of pavement modifications that create a perception of cleanliness and facilitate easier sweeping. Furthermore, increasing the number of litter bins and promoting waste reduction at the source should be considered, among many other measures that are contingent on the specific characteristics of each city.
The perception of street cleanliness is a political concern, particularly in the face of increased aridity due to falling precipitation levels and rising temperatures, which in turn will reduce citizens’ perceptions of clean streets.
The Gompertz model satisfactorily adjusts the relationship between UCP and IDM, yielding a correlation coefficient R2 = 0.984. Initially, cleanliness perception values increase with IDM, but this growth diminishes as IDM increases. In other words, at low IDM values, an increase leads to a rapid rise in cleanliness perception, but at high IDM values, UCP experiences minimal increases in parallel with IDM. Essentially, as the aridity index increases, cleanliness perception rises rapidly in arid climates but varies only slightly in humid and superhumid climates. This equation can be valuable for public and political decision-making, extending beyond the geographical focus of the Iberian Peninsula examined in this study, and be employed to estimate urban cleanliness perception in the face of future aridity scenarios in other cities. It can aid in making critical timely decisions to mitigate potential declines in cleaning satisfaction based on the evolving climate reality.
In summary, the anticipated escalation of both the intensity and persistence of arid conditions in many regions of the Iberian Peninsula due to climate change will not only exacerbate the exposure and vulnerability of this region to climate change, but it will also negatively impact the perception of urban cleanliness unless appropriate adaptation measures are enacted. Although this study primarily focuses on the Iberian Peninsula, the findings may be relevant to cities with street cleaning standards akin to those of cities in the Iberian Peninsula.
Acknowledgments.
We thank the national Consumers and Users Organization (OCU) of Spain for the survey data provided, as well as the Spanish Meteorological Agency (AEMET) for the meteorological data they provided.
Data availability statement.
Datasets analyzed during the current study are available in the AEMET database (www.aemet.es) and in the OCU database (www.ocu.es).
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