Formation and Dynamics of Night-Time Cold Air Pools in Peri-Urban Topographic Basins: A Case Study of Coimbra, Portugal

Abstract

This study investigates the formation of cold air pools during calm, anticyclonic winter nights in a topographic basin bounded by a medium-sized mountain to the east and near-flat terrain elsewhere. The main objective is to understand how local topography drives unique topoclimatic conditions—specifically cold air lakes and an inversion layer at approximately 100/120 m altitude—in a peri-urban depression where a major cement factory and several residential areas are located. To achieve this, the research design combined surface measurements (collected at 10:00 p.m., 3:00 a.m., 7:00 a.m., and 3:00 p.m.) using a motorized vehicle, with vertical measurements (at 7:00 a.m.) collected via two unmanned aerial vehicles (UAVs), with the three vehicles equipped with Tinytag data loggers. The Empirical Bayesian Kriging tool in ArcGIS Pro was employed to generate the surface temperature cartograms. The results show that shortly after sunset, a cold air layer of approximately 100–120 m thickness forms, with nocturnal air temperature variations of up to 8 °C on the night measurements. An inversion layer was detected at around 120–130 m, while near-zero wind speeds in the basin’s core facilitate the retention of cold air. Surface spatialization confirms earlier findings of a cold air lake and thermal belts on the basin’s perimeter, forming in the early evening and dissipating by late morning. A 3D visualization underscores the influence of the mountain in directing cold air downslope, leading to stabilization and stratification within the lower atmospheric layers. These findings carry significant health implications: air pollutants released by the cement plant tend to accumulate within the cold air pool and beneath the inversion layer, posing potential risks to nearby populations.

1. Introduction

The formation of a cold air pool, a phenomenon tied to topographic conditions, is one of the unique features of topoclimatology and is observed primarily on calm winter anticyclonic nights, characterized by thermal inversion at the ground level gaining strong expression [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]. Cold air pooling results from the accumulation of cooler air in valleys and topographic basins, forming as the surface cools at night or due to cold air advection from nearby slopes and valleys [12,15].

Research in urban climatology has established that factors like morphology, vegetation, building density, soil sealing, and water bodies influence thermal and hygrometric contrasts, impacting wind fields directly. Although urban climate studies often center on “urban heat islands” (UHIs), other stable boundary layer phenomena, like “cold air pooling”, significantly affect urban life quality, especially when pollution is involved. The distinction in research focus is underscored by T. Oke et al. [12], who categorize “cold air pooling” under “geographical controls” while dedicating a full chapter to UHIs in urban climate.

Given the morphological characteristics of Portuguese cities and the disordered planning in peri-urban areas, cold air pooling is an essential consideration for urban planning. Research has shown that cold air pooling is relevant not only to urban planning and design [15,16] but also for managing thermal inversion and pollutant dispersion, particularly near polluting sources situated downstream from prevailing winds [5,14,17]. Moreover, the studies on cold air “cushions” are limited and primarily focused on rural settings.

Cold air accumulation within the stable boundary layer (SBL) during anticyclonic nights of high atmospheric stability remains a complex, partially understood phenomenon [18,19]. It involves warmer air at altitude, leading to a thermal inversion [1,3,18]. This inversion hampers air dispersion, fostering pollutant accumulation and resulting in public health issues [5,9,10,13,20,21,22].

Initially, cold air pooling mechanisms were attributed mainly to katabatic flows or cold air drainage [18,23]. However, recent studies suggest a “double feeding” process, with cold air pooling resulting both from irradiation and from colder air flowing down slopes and valleys [19]-in extreme circumstances, such as those that occur in tectonic vents with downstream bottlenecks, the temperature difference between the bottom of the valley and the top of the thermal belts can exceed ten degrees Celsius [4,15,24,25,26,27,28,29]. Drainage flows occur as radiative heat loss cools the slope, pushing cold air into the surface layer and lowering temperatures in valley bottoms [19]. However, as Whiteman et al. [25,26] noted, this explanation may not apply universally.

In Portugal, cold air pooling differs between the regions of Lisbon [2,23] and Coimbra, where peri-urban valley areas have evolved into urban boundaries [4,15,24], although morphology remains the primary factor in both locations. A paper on cold air pools in the Douro Valley (northern Portugal) was recently published in relation to rural territories, although here directly related to their inter-ference in wine production [30]. This paper is part of a broader project examining cold air pooling in the municipality of Coimbra, located on Portugal’s central coast. It builds on previous research conducted in a narrow urban valley, the Coselhas Valley [15], while focusing specifically on a larger, tectonic basin situated approximately 5 km north of that valley.

Understanding areas prone to cold air accumulation in urban and peri-urban spaces is essential for effective territorial planning. Identifying such areas allows for better recognition of zones with heightened bioclimatic discomfort during cold waves, as well as regions susceptible to frost, ice, and fog formation [1]. This is particularly relevant in managing atmospheric pollution close to the ground, a significant issue already observed in the study area [31]. Pollution-related problems are intensified in areas that retain pollutants more readily, as seen here with emissions from the nearby large factory [5,32].

2. Geographical Context

The study area lies north of Coimbra on Portugal’s central coastline. This location features distinct climatic and morphological characteristics typical of a transitional region (Figure 1).

Climatically, the area is situated west of the reliefs that oceanic air masses encounter as they move eastward, imparting a strong Mediterranean influence while maintaining an intense maritime impact [33,34]. Winters are rainy, with an average annual rainfall just under 1000 mm, and cold air masses occasionally affect the region. Summers, by contrast, are typically dry and hot.

Morphologically, the northern part of the municipality displays notable contrasts shaped by its structural genesis. Small valleys along the right bank of the Mondego River, running north–south along the western slope of the Coimbra Marginal Massif, accentuate the area’s rigidity. These valleys include the Souselas/Fornos depression, the Eiras Valley, and the Coselhas Valley (Figure 1).

The elongated Fornos/Souselas depression, oriented northeast–southwest, has a relatively flat floor, with elevations in the inner basin ranging from 20 to 50 m. The lowest points lie along the banks of the Resmungão River and Botão Brook, while the highest elevations are found in the depression’s northern sector (Figure 1). The boundaries of the depression are strongly influenced by surrounding structural features [34,35,36]: to the west lie the coastal platform levels, at approximately 100–120 m (Sta. Luzia sea levels); to the north lie the Larçã and Pampilhosa do Botão levels, slightly lower at around 110–112 m; and to the south lie the Logo de Deus levels, at about 125 m. The Brasfemes levels mark the foreground to the east, and the eastern boundary of the basin is defined by a mid-sized mountain—the “Maciço Marginal de Coimbra”—which rises over 500 m, with slopes reaching gradients above 25° as illustrated in Figure 1.

Accounting for these specific climatic and morphological contexts—within a region north of a medium-sized city on the central coast of Portugal—this study aims to evaluate the role of topography in the formation and thickness of cold air pooling within a broad topographic basin, which also contains a cement plant established in the late 1960s.

3. Objectives and Methodologies

This analysis aims to provide a three-dimensional understanding of cold air accumulation in a tectonic depression by gathering real temperature values for the 200 m closest to the surface through diverse data collection and representation methods. The need for data on cold air pooling, one of the least studied climatological phenomena in peri-urban and urban areas, has driven this investigation, particularly within the context of urban planning and broader climate change adaptation projects.

The primary objectives of this study are to analyze variations in the cold air mass at the surface and to observe the development of the cold air “cushion” at higher altitudes, particularly in the early morning. Another key aim is to define and delimit the inversion layer, examining its potential oscillations in thickness and shape across different sectors of the depression and its relationship to the outlets of valleys draining from the Marginal Massif.

The methodology builds on previous strategies [15,24,37] and employs mobile data loggers (calibrated by the manufacturer and validated in our the laboratory) attached to motorcycles and two unmanned aerial vehicles (UAVs) to record data at both surface level (1.5 m) and altitude. Tinytag Plus 2 data loggers (TGP-4020, Chichester, West Sussex, UK) recorded thermal data during two days of a cold wave at four different times—03:00, 07:00, 15:00, and 22:00—for up to 80 min, with a 90 s pause before recording at each predefined point. Recording points were established to create a network capable of representing the vast depres-sion, allowing for a survey duration of about 60–70 min (Figure 2). The Empirical Bayesian Kriging tool in ArcGIS Pro [38] was then used to generate georeferenced temperature maps.

To define the vertical thermal profile of the lower atmosphere, altitude temperature surveys were carried out with two DJI Mavic Enterprise UAVs, in different sectors of the basin—surrounding flattened levels and the bottom of the depression in a 3-axis sequence. This approach, adapted from previous studies [15,37,39], involved connecting Tinytag Plus 2 data loggers to the UAVs, which descended at 1 m/s, with data recorded every second, thus allowing vertical temperature profiles up to altitudes of 200–250 m. To minimize interference from the rotor, the UAVs and sensors were moved more than 10 m apart in the descent trajectory during data collection, relative to their ascent. Wind speed and direction were also recorded in the UAV to assess the stability of the stable boundary layer in the basin.

4. Cold Air Accumulation in the Context of a Topographic Depression

The presence of a major manufacturing plant (a cement factory, ranked as the sixth largest polluter in the country in 2022 and 2023 [40]), the growth of buildings in the lower sectors on the flattened levels that delimit the depression, and previous knowledge of cold air pooling within this topographic basin during cold, anticyclonic dawns necessitated a revised approach.

Two types of data collection were implemented: the protocol from previous campaigns was repeated with an increased number of routes, and data loggers were attached to drones to capture temperature readings at sequential profiles up to 200 m in the stable boundary layer. This approach yielded 11 thermal profiles over approximately 60–70 min, recorded in the absence of direct solar radiation. These profiles enabled a preliminary three-dimensional visualization, providing new perspectives to analyze the surface and vertical structure of the cold air pool in the Souselas/Fornos basin.

4.1. Surface Temperature Variability as a Function of Topography

Between 10 p.m. and 11 p.m., the thermal profile reveals the formation of a “cold air pool” across the depression, with the highest temperatures recorded at elevated points—namely, Santa Luzia, Larçã, and Logo de Deus. Minor temperature increases were observed in areas with concentrated buildings, likely due to residual warmth as night-time began. The temperature range along the route was approximately 8 °C, with the highest temperatures in the western and southern sectors, where the terrain received direct radiation later in the day. The orientation of the “cold air pool” aligns closely with the basin’s morphology, with the coldest points occurring in sheltered areas (Figure 3A).

At 3:00 a.m., the thermal dynamics remained similar, though temperatures within the depression showed a slight decrease, attributable to prolonged nocturnal cooling typical of anticyclonic nights. The varied land use and basin floor morphology contributed to a more intensified temperature drop in certain sectors. The coldest points were now located south of the plant, while the warmest temperatures remained at the Santa Luzia levels (Figure 3B).

The third survey, conducted at dawn (7:00 a.m.) during the transition from night to day, recorded the lowest temperatures along the route, with a difference of approximately 8.1 °C. Lower temperatures intensified south and west of the cement plant, where the Resmungão River valley enters the basin from the mountain. Beyond variations in soil irradiation, the basin’s morphology and the gravitational flow of cold air from the Marginal Massif seem to reinforce the “cold air pool” effect in this area (Figure 3C).

The only survey conducted during the afternoon, at 3:00 p.m., showed warmer temperatures and higher solar radiation levels compared to colder sectors of the basin, influenced by airflow around the Santa Luzia levels. The thermal profile at this time reflected a dissipation of the “cold air pool”, with more uniform temperatures and a reduced thermal variation of around 4 °C. Here, temperature variations are predominantly influenced by solar exposure at each recorded site, demonstrating the terrain’s morphology (Figure 3D).

4.2. Vertical Structure of the Lower Atmosphere

To understand how the Souselas cold air pool forms, it was essential to examine the distribution of cold air and the positioning of the surface and inversion layers during cold, anticyclonic nights. This aspect is of particular concern due to pollution issues associated with a major factory in the depression, ranked as the sixth most polluting in Portugal according to the Portuguese Environment Agency [40]. On 13 January 2021, a set of 11 thermal profiles was sampled (simultaneously with the 07:00 a.m. surface survey), covering both the depression floor [6] and the bordering flattened levels [5]. A joint analysis of these profiles revealed an accumulation of cold air near the surface, along with a variable inversion layer thickness ranging between 25 and 50 m, depending on its location within the basin (Figure 4 and Figure 5A).

The profile analysis shows that temperatures within the basin are relatively uniform, with surface and lower sector temperatures averaging around −2 °C (P1, P5, and P9). At slightly higher altitudes (50–55 m), profiles P3 and P7 show slightly higher but still negative temperatures near the ground (around −0.5 °C), reflecting similar conditions observed along the surface route. Further up, at approximately 75 m, profile P4 shows a temperature of −0.58 °C, close to 0.5 °C. Across the different altitudes sampled within the basin’s interior, these profiles nearly overlap, indicating a homogeneous air mass up to an altitude of 100–120 m.

This uniformity extends to the thermal profiles obtained from the different levels surrounding the Souselas/Fornos depression, although these levels display different inversion layer characteristics. Surface temperatures at the 100/120 m level range between 1.28 °C and 2.44 °C (P10, P11, and P13), while temperatures increase to 5.28 °C and 6.71 °C at 200 m on the slopes of the Marginal Massif (profiles P6 and P2).

The differences in the profiles are more noticeable at higher altitudes, with higher temperatures at the highest levels and the highest inversion surface in the depression profiles at around 150/175 m. These values rise to 200/225 m in the surrounding levels. Therefore, it can be observed that the thickness of the cold air pool varies between 75 and 100 m, bearing in mind that it is greater at the bottom of the depression. This can be seen in Figure 4 and Figure 5 below.

Regarding the stability of the lower atmospheric layer, an analysis of local wind direction and speed, based on UAV data, aligns with expected atmospheric dynamics in cold air pooling. With regional drainage from the eastern quadrants, the lower layers—especially within the basin’s interior—show wind speeds rarely exceeding 5–6 km/h. This pattern also applies to the central profiles, where recorded values were between 0 km/h and 2 km/h (Figure 6). This low wind speed is consistent across all profiles at the basin’s bottom (P1, P3, P4, P5, P9, and P12) and at lower levels—between 75 and 90 m—in transitional sectors (P2, P6, P7, and P11), as observed in the Santa Luzia level profiles. Concerning wind direction, even at minimal speeds, the flow predominantly comes from the south within the first 30–60 m, with exceptions in profiles P4 (northwest), P9, and P12 (northeast).

At lower levels of the Marginal Massif, different dynamics are observed, with wind speeds reaching over 13–15 km/h and predominantly directed from the eastern quadrants, consistent with regional patterns. At the highest elevations, wind speeds can exceed 20 km/h, even at relatively low altitudes.

In this context, the wind data collected from each profile provide valuable insight into the atmospheric dynamics within the depression on this anticyclonic morning of 13 January 2021. On this day, conditions within the inner basin remained almost calm, especially in the segment where the cold air pool was most intense and coldest. Significant wind speeds were only recorded in the upper levels of certain profiles, particularly in the profiles from mountain levels east of the basin.

5. Discussion

The attempt to understand the morphological role in forming cold air pools in northern Coimbra is linked to ongoing reflections on planning and land use in urban and peri-urban areas, including the siting of factories. An initial approach focused on a narrow valley [15], but an analysis of a larger cold air pool approximately 5 km north of Coimbra’s urban center and the Coselhas Valley—marked by distinct topography—revealed similarities at the surface yet significant differences at altitude. This cold air pool forms within a broad, low-sloped tectonic depression with elevations between 27 and 36 m. The area has hosted a significant cement plant for decades, and its boundaries are defined by the Marginal Massif to the west and a series of flattened levels around 120–130 m, enclosing it to the west, south, and north.

In this context, a data collection campaign was conducted on 12, 13, and 14 January 2021, grounded in four key premises: (1) car and motorcycle routes recorded temperatures at 1.5 m from the surface at different times, with digital cartography using Empirical Bayesian kriging (EBK) to spatialize temperatures across the depression; (2) a true three-dimensional analysis was implemented by attaching data loggers to UAVs, allowing 11 sequential profiles to be collected on 13 January, prior to surface solar radiation, at 07:00 a.m.; (3) the study aimed to better understand the vertical structure of the stable boundary layer in the predominantly northeast–southwest-oriented depression, with particular focus on the inversion layer’s positioning and characteristics; and (4) this analysis considered the role of topoclimatology in territorial planning for equipment and infrastructure, particularly regarding regional wind dynamics and local climate effects on industrial particle dispersion.

If the motorized surveys offer the spatialization of temperature at the surface and show that they will present very interesting similarities with what was observed in the other study in a tight valley, those carried out at altitude show a somewhat different reality.

While motorized surveys provided spatialized surface temperatures that aligned closely with previous observations in narrow valleys, the data collected at altitude showed different dynamics. Observations from the January 2021 campaign revealed the following: (1) as documented in the literature, cold air forms nightly after sunset, intensifying throughout the night, dissipating by morning and early afternoon, and re-forming as sunlight wanes; (2) the cold air pool at Souselas remained consistent in spatialization and intensity from 03:00 a.m. to 07:00 a.m. (with only a 0.2 °C temperature decrease by dawn); (3) the coldest temperatures were recorded near the Marginal Massif, especially at the valley ends of the Resmungão River and Botão Brook, likely due to the movement of dense, cold air in valleys still shielded from early sunlight and wind; and (4) the flattened levels that surround the depression to the north, west, and south demonstrate thermal belt behavior, verifying the pooling effect of cold air.

The thermal profiles taken at 7:00 a.m. across 11 locations in the basin and adjacent levels indicated the following: (1) Basin profiles (P1, P5, P3, P9, P7, P10, and P11) revealed consistent temperature and profile shapes, distinct from those observed in the Coselhas Valley [14]. This suggests a stable cold air pool that aligns with surface cartograms. (2) The inversion layer was located around 100–125 m at the depression floor, with the cold air pool’s average thickness at about 100–125 m. (3) The thickest and most intense cold air pool sections were observed near the western slope of the Marginal Massif, particularly near the Resmungão River outlet (P11) and close to Souselas, indicative of katabatic dynamics. (4) Wind speeds in the basin were minimal (2–3 km/h), with notable increases only above 150 m—ranging from 15 to 35 km/h at northern points near the Marginal Massif slopes. (5) On ridge profiles (P2, P4, P6, and P10), temperatures showed a pause in increase, indicating an inversion absence at higher levels. (6) Peripheral levels displayed slightly higher temperatures than basin profiles at the same altitude. (7) At these levels, wind speeds reached 15–20 km/h at altitudes between 250 and 300 m, predominantly from the north near the Marginal Massif slope.

These findings confirm the presence of the cold air pool, observed forming shortly after sunset and intensifying with colder temperatures through the mid- and late-night surveys. The night-time surveys revealed a thermal variability of approximately 8 °C between the lower sectors of the depression and the surrounding thermal belts. By early afternoon, the cold air pool dissipated, reducing the temperature range to about 4 °C, with higher temperatures observed across the basin. As evening set in, the cold air pool re-formed.

At the surface, colder air (often below freezing) was consistently observed at the bottom of the depression, with significant accumulation in the terminal sectors of valleys draining from the Marginal Massif, particularly near the Resmungão River and Ribeira do Botão.

The interpretation of these temperature patterns was further reinforced by wind data, which highlighted that radiative cooling is the primary driver of cold air lake formation, particularly in calm air sectors. Additionally, katabatic drainage of cold air was evident, especially in terminal sectors of mountain valleys where breezes were weak (<5 km/h), further reinforcing the accumulation of cold air.

Winds with higher speeds were observed above 100–120 m, consistently originating from the east, in line with general atmospheric circulation during these conditions. An exception was observed at P12, located at the Resmungão River entrance to the depression, where winds were consistently from the east but reached speeds of 12–19 km/h above 30 m and 20–30 km/h above 75 m.

The thermal profiles at altitude showed a high degree of similarity across most locations, with the exception of profiles from the Marginal Massif (P2 and P6). These profiles exhibited a continuous rise in temperatures over a layer approximately 100 m thick. This pattern was also observed in the profiles from surrounding levels, although temperatures at these levels were slightly higher. The inversion layer, located roughly between 100 and 125 m, reflects one of the study’s main objectives—delineating this layer—and its connection to pollution issues stemming from the factory in the northern sector of the depression, identified as the sixth most polluting facility in Portugal in recent years. On days when the cold air lake forms, pollutant concentrations remain trapped for part of the day under the inversion layer (around 100–125 m), posing significant health risks to local residents, particularly in terms of respiratory health [41,42].

Similarly to the Coselhas Valley to the south, cold air pooling in this basin forms shortly after sunset, dissipates by late morning, and re-establishes in the evening [15]. However, at altitude, the thermal profiles in this basin revealed significantly greater stabilization, in contrast to the more dynamic longitudinal profiles typically observed in narrow valleys.

These data demonstrate the cold air lake’s presence both at the surface and in vertical spatialization on anticyclonic mornings in this wide basin with gentle slopes, with notable cold air flows originating from Marginal Massif watercourses and the surrounding levels. These data demonstrate the cold air lake’s presence both at the surface and in vertical spatialization on anticyclonic mornings in this wide basin with gentle slopes, with notable cold air flows originating from Marginal Massif watercourses and the surrounding levels (Figure 7).

Similarly to observations in the Coselhas Valley to the south, cold air pooling here forms shortly after sunset, dissipates by late morning, and re-establishes in the evening [15]. However, at altitude, the profiles in this basin showed much greater stabilization, contrasting with the longitudinal profiles typical in narrow valleys.

The stability observed in the thermal profiles at the depression’s bottom and beyond suggests that pollution emitted from the cement plant accumulates, posing health risks, particularly for respiratory conditions among residents [42]. The thermal inversion layer, positioned around 100–125 m, significantly hampers pollutant dispersion on these anticyclonic winter nights, raising public health concerns.

6. Concluding Remarks

The topoclimatological approach to territorial planning, rooted in a holistic understanding of various physical factors, emphasizes the critical relationship between topography and pollution. This connection is a core element of a broader project focused on adaptation and resilience to urban and peri-urban climate change, where the role of topography and vegetation is crucial in forming cold air pools in depressions and cold air cells associated with urban green spaces [37,39]. In central Portugal, the presence of cool air cushions within urban and peri-urban valleys and depressions in a medium-sized city has long been recognized [4,5,15,24]. Cold air pooling is believed to result from both radiative cooling and katabatic flows, with cold air accumulating at the valley floors due to topography and gravitational drainage.

This study’s primary objective is to analyze the structure of cold air pooling within a broad depression, contrasting it with observations from a nearby narrow valley [15]. Using today’s advanced technologies, such as UAVs and data loggers, the study attempts a detailed three-dimensional visualization of the cold air pool. This approach enabled researchers to examine variations in thickness, observe how the inversion layer adapts to surface morphology, track its dissipation throughout the day, and note its reformation at night. These observations also highlight how slope orientation, mountain borders, and associated valleys contribute to increasing cold air concentration in the lower atmosphere.

As resilience to climate change becomes a priority for urban planning, aligning with the United Nations 2030 Agenda for Sustainable Development and the European Union’s Sustainable Cities Agenda, this research sheds light on these dynamics in Mediterranean-climate territories. Furthermore, it contributes to the discourse on the role of an inversion layer—positioned at approximately 100–125 m—in trapping pollutants from an 80 m industrial chimney, impacting air quality and public health within the depression.

These findings underscore the importance of recognizing cold air concentration in peri-urban valleys, not only for improving energy efficiency and quality of life but also for protecting residents’ health, especially in areas where industrial pollution sources exist near residential neighborhoods. This observation should serve as a foundation for differentiated approaches to climate, sustainable development, and urban planning.