Urban Climate

INTRODUCTION

Human beings have a lways been in constant interaction with the nature. The nature, in turn, bears many visible imprints of this interaction. On such interaction i s in the form of settlements. Any kind of human settlement has a resultant environmental imprint. Humans along with adapting to their environment/ c l imate, generate certain modifications in i t . These c l imates came to be identified with their corresponding settlements and display peculiar characteristics. Consequently, there are two types of c l imates associated with human settlements – Rural Climate and Urban Climate.

The rural and the urban cl imates vary on a variety of indices. The atmospheric composition, amount of radiation received, wind speed, humidity levels, incidence of cloudiness and rainfall are not the same between rural and urban areas. ( Table 17 . 1 ) .

Table 17 . 1 Generalized urban features compared with rural environs. ( After Landsberg, 1960 ) Remake the table

Element	Comparison with rural environs
Contaminants: Dust particles Sulphur dioxide Carbon dioxide Carbon monoxide	10 times more 5 times more 10 times more 25 times more
Radiation: Total on horizontal surface Ultraviolet, winter Ultraviolet, summer	15 to 20% less 30% less 5% less
Cloudiness: Clouds Fog, winter	5 to 10% more 100% more

Fog, summer                                                                          .	30% more                                   .
Precipitation: Amounts Days with 5 mm	5 to 10% more 10% more
Temperature: Annual mean Winter minima	0.5 to 0.8 °C more 1 to 1.5 °C more
Relative humidity: Annual mean Winter Summer	6% less 2% less 8% less
Wind speed : Annual mean Extreme gusts Calms	20 to 30% less 10 to 20% less 5 to 20% , more

The Urban Climate

Urban c l imate i s the c l imate modified by the resulting effects of the development of urban areas v iz. towns, cities, metropolitan et al.

Figure 17 . 1 : Urban Climate at Meso, Local and Microscale.

Urban Climate i s the study of atmospheric phenomena attributable to the development of human settlements. I t encompasses work on the process involved ( physical, chemical and biological), the resulting cl imate effects, and the application of this knowledge to the planning and operation of urban areas. I t i s one of the best examples of man’ s role in cl imate modification.

The Urban areas are made of concrete houses, asphalt roads, factory shades and various other structures that are heat absorbant.

The construction of every house, road or factory destroys existing microclimates and creates new ones of great complexity depending on the design, density and function of the building.

17 . 2 : ( A) The urban heat dome

( B) Difference between urban and rural climate

Urban expansion and growth disrupts the cl imatic properties of the surface and the atmosphere. These, in turn, a l ter the exchanges and budgets of heat, mass and momentum which underlie the climate of any s i te. Every land clearance, drainage, paving and building project leads to the creation of a new microclimate in i ts v ic inity, and the collection of these diverse, human- affected microclimates i s what constitutes the urban c l imate in the a i r layer below roof level ( henceforward called the urban canopy layer, or UCL. These very localized effects tend to be merged by turbulence above roof level where they form the urban boundary layer ( UBL), ( Fig. 17 . 2 ) which appears l ike a g iant urban plume over and downwind of the c i ty.

The range of influences of urban structures upon the climatic conditions therein can be summed up under the following three heading:

1 . Modification of atmospheric composition. 2 . Modification of the heat budget.

3 . Other effects of modification of surface roughness and composition.

Despite the wide- ranging nature of the factors that determine the urban atmospheric environment, a lot of local modification i s dependent on three main climatic factors: the heat balance, the composition of the a i r , and surface roughness conditions. These factors influence more than one c l imatic element. For instance, when changes in the heat balance have repercussions on urban visibility and rainfall in addition to thermal conditions. Thus, there are three major controls of the cl imatic elements of urban areas.

Figure 17 . 3 : Schematic depiction of the main components of the Urban Atmosphere.

Modification of Atmospheric Composition

Urban cl imates are dominated by the geometry and composition of built- up surfaces and by the effects of human urban activities. The composition of the urban atmosphere i s modified by the addition of aerosols, producing smoke, pollution and fogs, by industrial gases such as sulphur dioxide, and by a chain of chemical reactions, in i t iated by automobile exhaust fumes, which causes smog and inhibits both incoming and outgoing radiation. Pollution domes ( F ig. 17 . 2 ) and plumes are produced around c i t ies under appropriate conditions of vertical temperature structure and wind velocity.

The atmospheric pollution i s most severe in urban areas as a result of the concentration of the combustion process for residential, commercial, and industrial requirements. The most important solid pollutants are smoke particles, which are caused mainly by the incomplete combustion of solid fuels in domestic appliances, while sulphur dioxide remains the greatest source of general gaseous pollution and arises f rom the burning of the sulphur content which most fossil fuels have. On an average, the nature and intensity of urban a i r pollution can be related to the character and density of the built- up area.

The pollutants produced in the urban areas belong to two categories.

• Aerosols are suspended particulate matter ( measured in mg m^{– 3} or µg m^{– 3} ) chiefly or carbon, lead and aluminium compounds and s i l i ca.
• Gases. The production of gases ( measured in parts per million ( ppm) can be viewed either f rom the t raditional standpoint with i ts concentration on industrial and domestic coal burning and the production of such gases as sulphur dioxide ( SO 2 ) , or from the newer standpoint of petrol and oil combustion and the production of carbon monoxide ( CO), hydrocarbons ( Hc), nitrogen oxides ( NOx ) , ozone ( O 3 ) and the l ike.

The thermal balance of the globe i s s ignificantly affected by the natural production of aerosols which are deflated f rom deserts, erupted from volcanoes, produced by f i res and so on. The overall thermal effect of low- level particulate aerosols i s probably one of warming, due to increased absorption, and this may augment the warming associated with increasing amounts of carbon dioxide and certaint race gases.

The proportion of atmospheric dust directly or indirectly attributable to human activity has been estimated at 30 pr cent. I t i s interesting that the North African battle tanks of the Second World War disturbed the desert surface to such an extent that the material subsequently deflated was visible in c louds over the Caribbean.

The most direct effect of particulate pollution i s to reduce incoming radiation and sunshine. The abundance of condensation nuclei in the c i ty atmospheres, particularly those s i tuated on low- lying land adjacent to large r ivers, explains the abundance of city fogs. Occasionally very s table atmospheric conditions combine with excessive pollution production to give dense smog, pollution, plus the associated fogs ( termed smog) of a lethal character.

Gases

Along with the pollution by smoke and other particulate matter produced by the traditional urban and industrial activities involving the combustion of coal and coke, urban areas has been associated the generation of pollutant gases.

Urban complexes are being affected by a newer less obvious, but nevertheless equally serious, form of pollution resulting from the combustion of petrol and oil by cars, multi- axle trucks and a i rcraft, as well as from petro- chemical industries. Los Angeles, l ying in a topographically constricted basin and often subject to temperature inversions ( see Chapter- 5 , Temperature ) i s the prime example of such pollution, a l though this affects al l modern c i t ies to some extent.

Smog involves at least four main components: carbon soot, particulate organic matter ( POM), sulphate ( SO 4 – 2 ) and nitrate. Half of the aerosol mass i s typically POM and sulphate. The production of the Los Angles smog which, unlike traditional city smogs, occurs characteristically during the daytime in summer and autumn i s the result of a very complex chain of chemical reactions termed the disrupted photolytic cycle. Ultraviolet radiation ( 0 . 37 – 0 . 42 µm) dissociates natural NO 2 into NO and O. Monoatomic oxygen ( O) may then combine with natural oxygen ( O 2 ) to produce ozone ( O 3 ) . The ozone in turn reacts with the artificial NO to produce NO 2 ( which goes back into the photochemical cycle forming a dangerous positive feedback loop) and oxygen. The hydrocarbons produced by the combustion of petrol combine with oxygen atoms to produce the hydrocarbon- free radical and these react with the products of the O 3 – reaction to generate oxygen and photochemical smog.

Polluted atmospheres commonly assume well- marked physical configurations around urban areas, which are very dependent upon environmental lapse rates, particularly the presence of temperature inversions and on wind speed. A pollution dome forms as a result of the collection of pollution below an inversion forming the urban boundary layer.

Sunshine and Visibility

Like incoming radiation, the duration of bright sunshine shows s ignificant reductions over urban areas, and in large cities a progressive decline can be traced towards the city centre.

Such effects have been largely attributed to urban smoke pollution which, during recent years, has been progressively reduced over many c i t ies as a result of statutory controls on smoke emission.

In addition to reduced sunshine duration, most urban areas are a l so associated with poor v i s ib i l i ty as a result of the local pollution.

Modification of the Heat Budget

Urban influences combine to g ive generally higher temperatures than in the surrounding countryside, mostly because of the growing importance of heat production by human activities. These factors give r i se to the urban heat i s land which may be 6 ° – 8 °C warmer than surrounding areas in the early hours of calm, clear nights when heat s tored by urban surfaces i s being released.

The urban- rural temperature difference under calm conditions i s statistically related to the c i ty population s ize; the urban canyon geometry and sky view factor are major controlling factors. The heat i s land may be a few hundred metres deep, depending on the building configuration. Urban wind speeds are generally less than for rural areas by day, but the wind f low i s extremely complex, depending on the geometry of c i ty- built forms.

Some of the causes of distinctive urban c l imates are as follows:

1. . Replacement of the natural surface by buildings, often tall and densely assembled, causes increased roughness. This can reduce surface wind speeds.
2. . Replacement of natural soil by impermeable roads and roofs, combined with drainage systems, reduces evaporation and humidity and leads to faster run- off.
3. . Roads and building materials have physical constants substantially different from natural soil. Generally, many have lower albedos and greater heat conductivity and capacity. This alters the radiation balance and has consequences for air temperature. In most instances, i t leads to heat storage from the solar radiation received. Also, in effect a new primary radiative surface i s centred in densely built- up areas at roof level, and this leads to considerable alteration of the lapse rates in the lowest layer.

Heat i s added by human activities, and this can be a substantial part of the local energy balance. This, together with the effects described above leads to an increase in temperature above that of the surroundings in turn, this can lead to convective r i s ing of air causing cloudiness and promoting precipitation.

Addition of foreign substances, such as water vapour, fumes and gases from combustion and industrial processes has obvious affects. Because of the large numbers of hygroscopic nuclei released, v i s ibilities are reduced, radiation i s intercepted, fogs are formed, and precipitation i s probably increased.

The Urban Heat Is land occurs due to following reasons –

1. . Increased counter radiation due to absorption of outgoing long wave radiation and re- emission by polluted atmosphere.
2. . Decrease in net long- wave radiation f rom tall buildings, narrow s idewalks, as these buildings reduce the sky v iew factor.
3. . Greater day t ime heat storage due to thermal properties of urban material ( concrete, asphalt) and i ts release during night t ime.
4. . Anthropogenic heat f rom building s ides.
5. . Decreased evaporation due to non- vegetated surface.
6. . Decreased loss of sensible heat due to reduction of wind speed in the canopy.

On an annual basis the canopy layer of a large city ( 10 million inhabitants) i s typically 1 – 3 °C warmer than i ts surrounding countryside. This may seem small, but the magnitude of the heat i s land varies diurnally ( largest near midnight, the smallest in the afternoon) and in response to weather ( largest with calm and no cloud). The difference i s also related to c i ty s i ze ( measured by i ts population or better by the geometry of i ts central street canyons,). On the most favourable nights in a large city, difference of 10 ” C and more have been recorded. Spatial variation of temperature within the UCL bears a strong relation to land use and building density and there i s a sharp gradient at the urban/ rural boundary. The city’ s warmth extends down into the underlying ground and upward into the UBL above. At night the heat i s land maintains a weak mixed layer above the city ( tens to hundreds of metres deep) when rural areas are s table.

URBAN HEAT ISLAND CIRCULATION

The average annual temperature contrasts of a city i s typically s l ightly higher than that of the surrounding countryside and on some days the contrast can be as much as 10 ° C. This c l imatic effect i s known as urban heat i s land. High concentration of heat i s the chief factor contributing to the urban heat i s land. Urban areas generate their own heat. The source of heat i s f rom motorcars, industry, a i r conditioner and furnaces. The c i ty i s composed of a great variety of structures: houses, blocks of f lats and offices, f lat and s loping roofs, streets, and gardens. This presents a maze of reflecting and absorbant surfaces to insolation, so that much less i s reflected than in the countryside. There are more surfaces, which absorb the heat, and the turbulence caused by the winds, being s lowed down by the buildings or speeded up through the canyon l ike streets, distributes this heat as i t i s reradiated from the warm bricks or concrete. All this heat reaches the atmosphere eventually, so that the air receives large amount of waste heat input. The thermal properties of the urban building materials also facilitates daytime absorption of solar radiation and emission of heat into the c i ty a i r ( Fig. 17 b. 1 ) . Concrete asphalt and brick, in which the c i ty abounds, conduct heat more readily than soil and vegetative cover so that more heat i s stored during the day for release at night. Thus, the night t ime long wave radiation and consequent cooling i s offset to a large extent by release of heat f rom the building and streets and by the obstruction of sky by tall buildings.

Figure 17 b. 1 : Sky view factor and absorption of radiation.

The contrast, between the city and countryside, i s further accentuated by low evapotranspiration rate of the city. Urban sewers quickly remove most run off from rain and snowmelt. This means that less solar energy i s involved in processes l ike evaporation or melting of snow in the urban areas and i s , thus, available for heating the a i r directly.

 

Figure 17 b. 2 : Heat island over a large urban area and dust dome.

When the regional winds are weak, the relative warmth of a large c i ty compared with i ts surrounding, promotes convective circulation of the air. The r i s ing warm air of the c i ty i s replaced by cooler denser air f lowing in f rom countryside, while the r i s ing a i r columns gather aerosols into a dust dome over the c i ty ( F ig. 17 b. 2 ) . I f regional winds become stronger the dust dome elongates downwind in the form of dust plume and spreads city pollutants over the countryside.

WHY THE URBAN HEAT ISLAND?

The Urban Heat Island occurs due to following reasons—

1. . Increased counter radiation due to absorption of outgoing long wave radiation and re- emission by polluted atmosphere.
2. . Decrease in net long- wave radiation f rom tall buildings, narrow sidewalks, as these buildings reduce the sky view factor.
3. . Greater day t ime heat storage due to thermal properties of urban material ( concrete, asphalt) and i ts release during night t ime.
4. . Anthropogenic heat from building sides.
5. . Decreased evaporation due to non- vegetated surface.
6. . Decreased loss of sensible heat due to reduction of wind speed in the canopy.

Modification of City Characteristics

Wind circulation

A city exerts both roughness and thermal influences on winds. When synoptic winds are s trong the greater roughness produces greater turbulence ( by 10 – 20 per cent), increased fr ictional drag, s lower winds ( by about 25 per cent), cyclonic turning, and a general tendency towards uplift. In the downwind rural area, the near surface f low recovers i ts original characteristics but urban effects are detectable in the elevated UBL plume for tens of ki lometers. The drag may even retard the passage of weather f ronts. In windy conditions, f low in the UCL i s extremely variable. While some areas are sheltered, others may be experiencing strong across- street vortices, gustiness or jets ( especially near tall buildings). The city may generate i ts own thermal c i rculation, analogous to sea/ land breeze, with ‘ country’ breezes converging on the city centre, r i s ing and diverging a loft to form a counter f low. Urban thermal effects can a l so lead to acceleration near the surface both as a result of the heat pressure f ield and because thermal turbulence helps transport momentum downwards.

Figure 17 . 4 : Typical airflow patterns around a slab and an adjacent low building based on wind- tunnel experiments.

Airflow on the average, city wind speeds are lower than those recorded in the surrounding open country owing to the sheltering effect of the buildings, and central average wind speeds are usually at least 5 per cent less than those of the suburbs.

However, the urban effect on air motion varies greatly depending on the t ime of day and season. During the day, city wind speeds are considerably less than those surrounding rural areas, but during the night the greater mechanical turbulence over the city means that the higher wind speed aloft are t ransferred to the air at lower levels by turbulent mixing.

The major modification of a i r f low i s created by the surface morphology of towns, as the i rregular skyline presents a rough surface which, in turn, acts as a mechanical brake. This reduces mean wind speeds within the city complex.

Urban structures have considerable effects on the movement of air both by producing turbulence as a result of their roughening the surface and by the channeling effects of the urban canyons.

Structures play a major role in the diffusion of pollution within the urban canopy; for example, narrow streets often cannot be f lushed by vortices. The formation of high- velocity streams and eddies in the usually dry and dusty urban atmosphere, where there i s ample debris supply, leads to general urban airflows of only 5 m s ^{– 1} ( 11 mph) being annoying, and those of more than 20 m s ^{– 1} ( 45 mph), being dangerous.

Urban wind speeds are also s ignificant in the above, certain threshold values, airflow can lead to either the dislocation or the complete elimination of heat- i s lands through the combined effects of upward turbulence and lateral advection in the lower atmosphere.

Moisture Conditions

Cities naturally tend to be less humid than rural areas but their topography, roughness and thermal qualities tend to intensify the effects of summer convective activity over and downwind of the urban areas, giving more thunderstorms and heavier storm rainfall. The exchange of moisture between the surface and the air i s a l tered by changes in the availability of water and energy, and in the perturbed airflow. Normally values of atmospheric moisture in the daytime UCL are lower than in the country s ide ( on account of less evapotranspiration and greater mixing), but the reverse holds at night ( because of decreased dewfall and the release of water vapour from combustion). The effects seen in the UCL are a l so evident in the UBL plume. An exception i s provided by high latitude c i t ies in winter where evaporation from frozen surfaces i s very small so that humidity i s largely governed by vapour from combustion, with the result that the city i s more humid by both day and night. At temperatures below  30 °C ice fog i s a common, and unpleasant, fact of urban l i fe. Above freezing, point, urban effects on fogs are complex: extra warmth may decrease their frequency but extra condensation nuclei may increase their density and severity, Aerosol i s a l so responsible for a general increase in daytime haze in the subcloud layer of the UBL, and a deterioration of visibility.

Precipitation

Urban modification of Precipitation i s a subject that has received considerable re, search study. There seems to be a consensus v iew that c i t ies enhance precipitation in their downwind areas. These effects seem to be most marked in relation to summer convective rainfall, especially heavy rain, and severe weather ( thunder- and hail- s torms) rather than f rontal precipitation. Annual increases of up to 10 per cent are commonly reported, but the exact role of urban versus non- urban influences i s often hard to determine. There i s a l so difficulty in i solating the most important of the causes. I t i s possible that the microphysics of urban clouds i s a l tered ( e. g. c loud droplet s izes and numbers) and/ or that c loud dynamics are changed by the UBL ( e. g. strength of uplift, height of mixed layer) leading to more favourable precipitation conditions.

Snow i s a particular type of precipitation which might be expected to occur less frequently in towns than in comparable areas of the countryside, s ince heat- i s land influences are l i kely to reduce both the frequency of snowfall and the subsequent period during which the snow l ies.

Tropical Urban Climates

Most tropical landscapes differ f rom urban areas in higher latitudes in that i t i s commonly composed of high- density, s ingle- storey buildings with few open spaces and poor drainage. The composition of roofs i s relatively more important than that of walls in terms of thermal energy exchanges. Also the production of anthropogenic heat i s more uniformly distributed spatially and i s of lower intensity than what i s to be observed in European and North American c i t ies. In the dry tropics such as, arid and semi arid regions, buildings have a relatively high thermal mass to delay heat penetration and this, combined with the low soil moisture in the surrounding bare rural areas, makes the ratio of urban/ rural thermal absorption greater than in the temperate regions. However, i t i s not always possible to generalize regarding the thermal effects of cities in the dry tropics due to the differing ‘ oasis’ effects of urban vegetation. Building construction in the humid t ropics i s characteristically l ightweight to promote the much- needed ventilation. Tropical heat i s lands are naturally most marked at night during the dry season, a l though topographic effects are important.

Despite a marked lack of data, there appears to be some urban precipitation enhancement in tropical regions, which i s maintained for a larger part of the year than that associated with the short- term high- sum convective period in temperate latitudes.

Conclusion

Urban c l imate indeed i s a very distinctive cl imate having distinctive temperature, wind c i rculation patterns, moisture level, humidity level and precipitation. The distinctiveness has been summarized in the f igure below: