City Ventilation Research Articles

Flow patterns and heat transfer of an idealized square city in non-uniform heat flux and different background wind conditions

The city scale buoyancy-driven flow is crucial for the wind and thermal environment in urban areas, exerting a significant impact on the city ventilation, pollutants dispersion and heat removal. The intra-urban variations of hot spots and heat flux are drawing more attention on beating the urban heat. This study aims at analyzing the influences of non-uniform heat flux and background wind on the city-scale heat removal ability and underlying mechanisms. The large eddy simulation model was used and verified with water tank experiments. Results show that positive heat flux and negative heat flux in the rural areas will enhance and depress the strength of the urban heat dome flow respectively compared to the adiabatic condition. The high-speed regions are wider and stronger when the rural area has positive heat flux condition. Under positive rural heat flux condition, the time-average mixed layer height in urban areas across the city center plane is nearly twice that under negative rural heat flux condition. Besides, the city-scale heat transfer coefficients in positive rural heat flux condition are higher than those under negative rural heat flux and adiabatic conditions. For cases of non-uniform heat flux in urban areas, the differences of time-average mixed layer height and heat transfer coefficients are not obvious. The heat transfer coefficients decrease initially and then increase with increasing background wind speed indicating the non-linear interaction between approaching wind and urban heat dome flow. Moreover, the growth rate of the heat transfer coefficient decreases when the non-dimensional wind speed exceeds 1.19.

Study on transformations in land use in the functional context of extensive city ventilation routes using the example of Bródnowski Ventilation Corridor in Warsaw, Poland

Abstract Ventilation corridors are critical for regulating the appropriate flow of urban air streams. Details on how they function, especially in a specific location, can be found in local eco-physiographic studies. Such analyses are designed during conceptual work on the conditions and directions of the development of a studied area. Such documentation includes advanced research on the impact of natural conditions on the area's growth and evolution. In Warsaw, Poland, such a study makes it possible to define various natural areas that are essential for the appropriate functioning of the city and increase its inhabitants’ well-being. This provides an initial basis for further analysis and motivates researchers to investigate the distribution of natural sites. It inspired the authors to assess the land-use structure in the context of the location of ventilation corridors. Thus, land use and land cover (LULC) modifications were examined in the context of their tendency to aid in the ventilation of the city. The object of the study is the Bródnowski Ventilation Corridor (BVC) – one of several designated for Warsaw in the spatial planning concepts of the 1980s and 1990s. In the research encompassing the years 1982–2020, the authors analysed the land use and roughness determined by the development characteristics and vegetation cover. References were made to current and archival planning documents, aerial photographs, satellite images, and the Topographic Objects Database BDOT10k. The research was prompted by the municipality's intention to urbanize the study area, which will undoubtedly eliminate the ventilation function of the southern part of the corridor and significantly reduce its remaining territory. Analysis of changes in the land cover of the south part of the corridor confirms its marginal importance for the city's ventilation. In this area, ongoing modifications proceed slowly, and urbanization – mainly low-rise and less intensive buildings – is limited. This zone has largely retained and is likely to maintain its favourable features from the point of view of ventilation and air regeneration.

Diode effects on street canyon ventilation in valley city: Temperature inversion and calm geostrophic wind

In urban areas, especially in valleys, challenges from heat and pollution are common. The situation worsens due to temperature inversions, where warm air traps pollutants. A new study employed multi-scale computational fluid dynamics (CFD) to assess ventilation in valley cities, specifically during temperature inversions and calm winds. Parameters like air change rate (ACH), local mean age of air (LMAA), and purging flow rate (PFR) were used to gauge street canyon ventilation efficiency. Factors such as atmospheric stability, city-slope positioning, and surface materials are altered to analyze impacts on urban heat island (UHI) intensity and air quality.Results indicated that katabatic winds had varied effects on pollution in different city regions, influenced by atmospheric conditions. Higher temperature lapse rates led to enhanced pollutant removal near slopes due to vortex interactions, but distant regions experienced thicker inversion layers, limiting pollutant dispersion. Conversely, anabatic winds improved ventilation with distance. Under inversion conditions, slope-city distance and surface materials played roles.For katabatic winds, city positioning significantly reduced UHI intensity, lowering LMAA by around 440s. Anabatic winds' UHI impacts ranked as atmospheric conditions, city location, and surface materials. A neutral atmosphere reduced UHI by 4.5K, but increased distance between city and slopes strengthened UHI by 4.04K. Ventilation efficiency was mainly tied to atmospheric conditions, while city location impacted pollution mitigation.This research aids in understanding air quality and airflow patterns during extreme weather events, aiding urban canyon design in valleys, especially with recurring heatwaves.

Weakened Sea‐Land Breeze in a Coastal Megacity Driven by Urbanization and Ocean Warming

AbstractThe sea‐land breeze (SLB) occurs regularly in coastal areas and influences the local climate and atmospheric environment. Fifteen‐year observations (2005–2019) from urban, coastal, and marine stations were collected to examine the trend in the number of SLB days (SLBD) in Tianjin, a coastal megacity in China. The number of SLBD in Tianjin generally showed a decreasing trend with an annual average decline rate of 3.89%. The average land breeze speed decreased by 0.20 m·s−1 and sea breeze speed decreased by 0.69 m·s−1 in the examined 15 years. Simultaneously, the distance of influence of SLB in summer was reduced by ∼30 km. The Weather Research and Forecasting model developed with high‐resolution land use data and variations in the sea surface temperature (SST) was applied to investigate the mechanisms leading to the weakening of SLB. The increasing SST led to a reduction in the temperature difference between sea and land, which was found to be the dominant factor driving the weakening of SLB in summer. The changes in the density and height of buildings due to urbanization altered the wind direction in the coastal areas, which in turn drive the reduction of the sea breeze during fall, while increased surface temperature at night in urban areas led to the reduction of the land breeze. With projections indicating ocean warming and increasing urbanization, the SLB will become weaker and occur less frequently in the future, probably leading to more frequent air pollution due to poor ventilation in coastal cities.

Evaluation of the ventilation and pollutant exposure risk level inside 3D street canyon with void deck under different wind directions

With continuous global warming, growing urban population density, and increasing compactness of urban buildings, VD (void deck) street design has become increasingly popular in city planning, especially in tropical countries. However, understanding on traffic pollutant dispersion inside the street canyons with VDs is still at early stage. This paper evaluates quantitatively the effects of VD location and wind direction on the ventilation and traffic pollutant exposure inside the street canyon with VDs. The results show that under seven wind directions (0°, 15°, 30°, 45°, 60°, 75°, and 90°), the VD provides higher ACH than that of the regular canyon, especially at high α (angle between the approaching wind and the canyon axis). Also, mean K (dimensionless pollutant concentration) values of the canyon wall and pedestrian respiration plane on one side where VD is located are significantly reduced compared to the regular canyon. Therefore, when VDs are at both buildings, both pedestrian respiration planes and walls have the lowest K values, thus providing the best living environment for pedestrians and near-road residents. In addition, as α increases, the K values on both respiration planes significantly decrease except for the leeward respiration plane of the canyon with the windward VD. These findings can help to design urban street canyons for mitigating traffic pollution risk and improving ventilation in tropical cities with frequently changing wind directions.

Evaluation on ventilation and traffic pollutant dispersion in asymmetric street canyons with void decks.

With continuous global warming, growing urban population density, and increasing compactness of urban buildings, the "void deck" street canyon design has become increasingly popular in city planning, especially for urban streets located in tropical areas. Nevertheless, research on traffic pollutant dispersion in street canyons with void decks (VDs) is still at its early stage. This study quantitatively evaluates the effects of void deck height and location on the canyon ventilation and pollutant dispersion in asymmetric street canyons with void decks, and the pollutant exposure risk level for pedestrians and street dwellers. Void decks introduce more fresh air, thereby greatly improving the ventilation properties of the asymmetric canyon. The air exchange rate (ACH: 147.9%, 270.9%) and net escape velocity (NEV*: 416.7%, 915.8%) of the step-up and step-down canyons with VDs (3m high at full scale) at both buildings are higher than those of regular asymmetric canyons. Moreover, the mean dimensionless pollutant concentration (K) on the building wall and pedestrian respiration plane in which VDs are located stands at a low level, because pollutants are removed by the airflow entering or exiting through the void decks. Increased VD height (4.5m at full scale) enhances the strength of airflow flowing into and out of the canyon, significantly increasing ACH (177.3%, 380.9%) and NEV* (595.2%, 1268.4%) and decreasing the mean K on both pedestrian respiration planes and canyon walls. In particular, the K values on both pedestrian respiration planes and both walls are almost zero for the canyons with VDs at both buildings. Therefore, among the three VD locations, both VDs provide the best living environment for pedestrians and near-road residents. These findings can help to design urban street canyons for mitigating traffic pollution risk and improving ventilation in tropical cities.

Proper orthogonal decomposition of large-eddy simulation data over real urban morphology

Proper orthogonal decomposition (POD) is implemented to the atmospheric surface layer (ASL) over real urban morphology based on the data of large-eddy simulation (LES) with explicitly resolved buildings. Turbulence structures with different scales in the inertial (ISLs) and roughness (RSLs) sublayers are unveiled by decomposing the fluctuating velocity components into a range of spatial eigenmodes ψm(x) and time coefficients am(t). POD spatial modes, which are ranked according to their turbulence kinetic energy (TKE), are visualized to contrast their sizes. These dominant structures illustrate the footprints of most energetic eddies at the ground level which could modify the dynamics locally. Moreover, their temporal behaviors are examined by energy spectrum and Lissajous curve, elucidating the existence of large-scale traveling structures. It is also shown that around 80 and 100 modes (out of 1000) are able to retrace 90% of the original TKE and momentum flux, respectively. The aforementioned remarkable data reduction demonstrates the capability of POD for flow characteristic extraction and city-scale, reduced-order model (ROM) development. Besides, dominant modes transport more efficiently especially in ISLs. With proper urban planning, such as building height and orientation, these energy-carrying eddies would sweep into street canyons, improving pedestrian-level air quality as well as city ventilation. (200 words)

Ventilation of a Mid-Size City under Stable Boundary Layer Conditions: A Simulation Using the LES Model PALM

City centers have to cope with an increasing amount of air pollution. The supply of fresh air is crucial yet difficult to ensure, especially under stable conditions of the atmospheric boundary layer. This case study used the PArallelized Large eddy simulation (LES) Model PALM to investigate the wind field over an urban lake that had once been built as a designated fresh air corridor for the city center of Münster, northwest, Germany. The model initialization was performed using the main wind direction and stable boundary layer conditions as input. The initial wind and temperature profiles included a weak nocturnal low-level jet. By emitting a passive scalar at one point on top of a bridge, the dispersion of fresh air could be traced over the lake’s surface, within street canyons leading to the city center and within the urban boundary layer above. The concept of city ventilation was confirmed in principle, but the air took a direct route from the shore of the lake to the city center above a former river bed and its adjoining streets rather than through the street canyons. According to the dispersion of the passive scalar, half of the city center was supplied with fresh air originating from the lake. PALM proved to be a useful tool to study fresh air corridors under stable boundary layer conditions.

Quantitative city ventilation evaluation for urban canopy under heat island circulation without geostrophic winds: Multi-scale CFD model and parametric investigations

Urban breathability is fundamentally important for improving urban air environment. Nowadays, air pollution and extreme heat waves heavily threaten modern cities and residents. In this paper, a methodology combined with an idealized High-Reynolds-Number Porous-Media city model and a multi-scale Computational Fluid Dynamics (CFD) method will be firstly proposed to quantify the urban ventilation breathability under no geostrophic winds. Breathability of city was then evaluated in terms of air change rate per hour (ACH) and age of air. The contribution of five factors to urban heat island and air quality is analyzed qualitatively and quantitatively. Numerical results further demonstrate that, 1) For heat island intensity, the dominant importance of influencing factors is: heat flux > temperature lap rate > urban scale > building height > urban density. Urban heat flux and temperature lapse rate have great influences on the urban heat island intensity and air quality. Comparing with neutral atmosphere, ACH in urban canopy decreased by more than a half under inversion condition, whereas ACH in pedestrian layer decreased by nearly 100%. 2) For air quality, the importance of the influencing factors could be listed as, building height > heat flux > temperature lapse rate > urban scale > urban density. Building height has the greatest impact on air quality. With the increase of urban scale, average age of air for a city decreases first and then increases. There exists an optimal city size to minimize the age of urban air. Urban building density (regardless of heat emission) on the buoyancy driven flow was very weak. Present model and research results could have theoretical significance for the council and urban planners to alleviate urban heat island and urban air pollution through reasonable planning of urban building layouts.

Planning and Implementation of Ventilation Corridor in Coastal Cities Based on the Overall Urban Design

Zhang, R.-P. and Xie, J.-H., 2020. Planning and implementation of ventilation corridor in coastal cities based on the overall urban design. In: Qiu, Y.; Zhu, H., and Fang, X. (eds.), Current Advancements in Marine and Coastal Research for Technological and Sociological Applications. Journal of Coastal Research, Special Issue No. 107, pp. 105-108. Coconut Creek (Florida), ISSN 0749-0208.With the continuous development of marine economy, the construction of coastal cities is also accelerating. However, coastal cities also face many environmental problems in the process of rapid development, such as heat island effect, air quality decline and so on. Based on this, this paper first analyzes the theoretical basis of the coastal city ventilation corridor design, then studies the theoretical basis of the coastal city ventilation corridor design, and finally gives the coastal city ventilation corridor construction design and planning implementation strategy.

The urban moisture island phenomenon and its mechanisms in a high‐rise high‐density city

AbstractPrevious studies tend to report the effect of rapid urbanization on temperature, but not on humidity. In this study, we aimed to address this research gap by examining diurnal profiles of temperature and humidity in Hong Kong. Our analysis revealed a more significant rising trend in humidity. In addition, our analysis of equivalent temperature revealed that latent temperature (i.e., moisture) contributes more to the aggravation of urban heat stress than does sensible temperature. The average urban air moisture concentration exceeds that in rural areas for both day and night, confirming the existence of an urban moisture island. A new lumped moisture model is developed to predict the hourly urban air humidity using weather data from the operational suburban weather stations, to better understand and quantify the urban moisture environment. The resulting coupled thermal‐moisture model can capture the main characteristics of diurnal profiles of urban air temperature and humidity in Hong Kong. The modelling results demonstrate that weakened city ventilation tends to lead to an increase in urban air moisture. Furthermore, natural surfaces (vegetation and bare soil) reduce urban heat stress by evapotranspiration. It is noteworthy that although the increase in fractions of natural surfaces can lead to more potential moisture sources, the absolute humidity will start to decrease when the fraction of natural surfaces reaches a threshold. This decreasing moisture trend may be attributable to reduced evaporation owing to the decreased surface temperature, reduced anthropogenic moisture and increased ventilation. Besides, the large wall areas in the compact high‐rise areas of Hong Kong function as both sinks and storage sites, contributing to a noticeable diurnal trend in the urban air humidity profile. In brief, the significant urban moisture island phenomenon in Hong Kong may result from enhanced evapotranspiration, restrained moisture sinks, intensive human activities and a weakened wind environment.

Plume or bubble? Mixed-convection flow regimes and city-scale circulations

Large-scale circulations around a city are co-modulated by the urban heat island and by regional wind patterns. Depending on these variables, the circulations fall into different regimes ranging from advection-dominated (plume regime) to convection-driven (bubble regime). Using dimensional analysis and large-eddy simulations, this study investigates how these different circulations scale with urban and rural heat fluxes, as well as upstream wind speed. Two dimensionless parameters are shown to control the dynamics of the flow: (1) the ratio of rural to urban thermal convective velocities that contrasts their respective buoyancy fluxes and (2) the ratio of bulk inflow velocity to the convection velocity in the rural area. Finally, the vertical flow velocities transecting the rural to urban transitions are used to develop a criterion for categorizing different large-scale circulations into plume, bubble or transitional regimes. The findings have implications for city ventilation since bubble regimes are expected to trap pollutants, as well as for scaling analysis in canonical mixed-convection flows.

Conditions for transition from a plume to a dome above a heated horizontal area

Natural convection flow above a heated horizontal area plays a major role in energy utilisation, indoor/outdoor thermal environment, city ventilation and pollutant dispersion. Such natural convection can present different flow patterns, and identifying the type of flow pattern is crucial for determining the bulk heat transfer coefficient. Water tank experiments were carried out to study two different forms of convection flow above a large area with different levels of background inversion. Two major flow patterns, i.e., plume-like and dome-like, were observed. The critical Froude number (Frc = 0.08) for the transition from plume-like to dome-like was identified. Plume flow developed when Fr > Frc and dome flow developed when Fr < Frc. The heat transfer coefficients in a stable environment were significantly lower than those in a neutral environment. The Nusselt number, which is a function of both the Rayleigh number Ra and buoyancy frequency N in a stable environment, was calculated and analysed. Natural convection develops over the built-up area due to the urban heat island effect. The diameter Dc of the critical built-up area was found to be 2.3 km under typical atmospheric background conditions for the transition from the plume-shape natural convective flow to the dome-shape. When atmospheric conditions are met, villages and small towns with diameters less than 2.3 km will probably present plume-like structures, whereas cities and megacities with larger diameters are most likely to form heat domes. Our findings may also have implications for city- and regional-scale urban planning.

Effects of envelope features and upstream buildings on ventilation performance of naturally-ventilated building

In order to study the influence of building facade components (sunshade, wing wall and balcony, etc.) on indoor and outdoor air distribution and indoor natural ventilation in high-density cities being hot in summer and warm in winter, 18 different cases are established to stimulate the effect of natural ventilation frequency of different position in the room by using the computational fluid dynamics (CFD) numerical simulation method through changing the incoming wind direction (wind incidence angle <italic>α</italic>=0° and <italic>α</italic>=45°), the height width ratio of upstream turbulence buildings and vertical and horizontal facade components. The results show the target building generally has a better natural ventilation performance when the wind obliquely incidents on the building (45°). In addition, the effect on ventilation performance of wing walls is highly dependent on the environmental factors including the approaching wind directions, the orientation of the target building and height of the upstream building. The interaction of these parameters affects the flow field characteristics, and changes the effect of facade components on the natural ventilation efficiency of the target building. This study can provide reference for improving the indoor natural ventilation efficiency and air quality by the facade optimization design of the building.

Data-driven model for cross ventilation potential in high-density cities based on coupled CFD simulation and machine learning

Effective urban ventilation through decent urban planning and building design can alleviate the deterioration of the urban built environment. However, natural ventilation requirements and guidelines in current building codes and standards are either qualitative or quantitative but subject to an absolute indoor airspeed threshold without considering the outdoor wind environment. To fill this gap, this paper develops an urban-scale coupled indoor and outdoor computational fluid dynamics (CFD) model and defines a novel ventilation index to assess natural ventilation potential. The index considers wind environments of both indoor and outdoor spaces. First, the coupled CFD model is developed to study wind-driven cross ventilation in high-density cities. A 3D isothermal CFD simulation is solved using the RNG k-ε turbulent model. The simulation results are compared with wind tunnel experiment data from literature. Second, six key design variables are used to generate 3840 parametric design variations for natural ventilation assessment. Third, a novel integrated index CIOIv (coupled indoor and outdoor interaction) is proposed to evaluate the wind speed ratio between the indoor area and outdoor reference area. For demonstration, CIOIv,F1 is used to represent CIOIv on the ground level. Data-driven CIOIv,F1 models are developed to predict indoor building ventilation potential for quick early design support. Compared with multivariate linear regression model, the Gradient Boosting non-linear model displays much higher prediction accuracy (mean absolute percentage error = 0.16 with R2 = 0.8). In early design stage, designers and engineers can skip the computational expensive CFD simulations and use this data-drive model to quickly check the building natural ventilation potentials of different design options in an urban environment.

Prediction of pollution dispersion under urban heat island circulation for different atmospheric stratification

The background atmospheric stratification with inversion layers in association with urban heat island phenomena affects the city ventilation and pollution transport. In this study, the effects of different stratification including ground inversion, isotherm, sub-adiabatic and super-adiabatic profiles with an inversion layer on pattern, intensity and extend of urban heat island induced circulation (UHIC) are investigated. The effect of the UHIC pattern on pollutant transport for two different scenarios are studied. The study is conducted using a modified CFD model that is adopted to include temperature background layers, compressibility and anelasticity of the atmosphere. The modified CFD model and results of simulation are validated against the available data in literature for UHIC pattern and pollution transport scenarios. The results indicate that background temperature profile controls the convergence and divergence of flow, and it affects the mixing height of flow inside of urban area, significantly. The flow speed at the urban border for inversion, isotherm, and sub-adiabatic stratification is 55%, 52%, and 40% less than super-adiabatic stratification, respectively. It is observed that the existence of inversion layer is essential for convergence of flow in case of super-adiabatic stratification. In case of ground inversion, pollutants that are released inside urban area are transferred to higher levels in rural area. The induced flow due to ground inversion transfers thepollutant in rural area into the urban dome. The pollutant concentration for inversion, isothermal and super-adiabatic profiles is 350%, 200%, and 150% more than sub-adiabatic stratification, respectively.

Interacting urban heat island circulations as affected by weak background wind

This paper presents a numerical study of two interacting urban heat island circulations (UHICs) in the presence of weak background wind. The Weather Research and Forecasting (WRF) model was used with ideal settings without the Earth's rotation effect. UHICs were simulated over circular cities of the same size arranged in tandem or side by side with respect to the background wind direction, at different spacing distances. The study analyzed the flow structures and city ventilation arising from thermally driven UHICs and background wind. The arrangements of the cities have a significant modifying effect on the interaction between UHICs and winds. If side-by-side UHICs are sufficiently close (spacing distance less than half of the city diameter), they have a blockage effect similar to that of a single larger UHIC, and only one counter-rotating vortex pair structure exists around the cities. As the spacing distance increases, the city ventilation improves slightly. In the tandem arrangement, a chain flow is formed between UHICs at a spacing distance of between 0.5 and 1.5 times the city diameter. An upstream UHIC blocks the incoming wind flow to the downstream city, which results in a ventilation deficiency. Due to lateral entrainment, which partly compensates for the deficiency in the incoming flow, the ventilation of the downstream city is slightly improved by an increase in spacing distance.

Investigation of Ventilation Energy Recovery with Polymer Membrane Material-Based Counter-Flow Energy Exchanger for Nearly Zero-Energy Buildings

The usage of energy recovery ventilation units was extended in European countries. Air-to-air heat and energy recovery is an effective procedure to reduce energy consumption of the ventilation air. However, the material of the core significantly influences the performance of the exchangers, which is becoming an extremely important aspect to meet the energy requirements of nearly zero-energy buildings. In this study, the performance of two counter-flow heat/enthalpy energy exchangers are experimentally tested under different operating conditions, and the values of the sensible, latent, and total effectiveness are presented. Moreover, the effects of the material of two exchangers (polystyrene for the sensible heat exchanger and polymer membrane for the energy exchanger) on the energy consumption of ventilation in European cities with three different climates (in Reykjavík in Iceland as a cold climate, in Budapest in Hungary as a temperate climate, and in Rome in Italy as a warm climate) are evaluated. The results show that the energy recovery of ventilation air with a polymer membrane material-based counter-flow energy exchanger performs better than using a polystyrene sensible heat recovery unit.

Natural ventilation in cities: the implications of fluid mechanics

ABSTRACTResearch under the Managing Air for Green Inner Cities (MAGIC) project uses measurements and modelling to investigate the connections between external and internal conditions: the impact of urban airflow on the natural ventilation of a building. The test site was chosen so that under different environmental conditions the levels of external pollutants entering the building, from either a polluted road or a relatively clean courtyard, would be significantly different. Measurements included temperature, relative humidity, local wind and solar radiation, together with levels of carbon monoxide (CO) and carbon dioxide (CO2) both inside and outside the building to assess the indoor–outdoor exchange flows. Building ventilation took place through windows on two sides, allowing for single-sided and crosswind-driven ventilation, and also stack-driven ventilation in low wind conditions. The external flow around the test site was modelled in an urban boundary layer in a wind tunnel. The wind tunnel results were incorporated in a large-eddy-simulation model, Fluidity, and the results compared with monitoring data taken both within the building and from the surrounding area. In particular, the effects of street layout and associated street canyons, of roof geometry and the wakes of nearby tall buildings were examined.

Effects of Unstable Stratification on Ventilation in Hong Kong

Ventilation in cities is crucial for the well being of their inhabitants. Therefore, local governments require air ventilation assessments (AVAs) prior to the construction of new buildings. In a standard AVA, however, only neutral stratification is considered, although diabatic and particularly unstable conditions may be observed more frequently in nature. The results presented here indicate significant changes in ventilation within most of the area of Kowloon City, Hong Kong, included in the study. A new definition for calculating ventilation was introduced, and used to compare the influence of buildings on ventilation under conditions of neutral and unstable stratification. The overall ventilation increased due to enhanced vertical mixing. In the vicinity of exposed buildings, however, ventilation was weaker for unstable stratification than for neutral stratification. The influence on ventilation by building parameters, such as the plan area index, was altered when unstable stratification was considered. Consequently, differences in stratification were shown to have marked effects on ventilation estimates, which should be taken into consideration in future AVAs.