Natural and Man Made Para-28-34

ARCHITECTURE - Time Space & People January 200828

It has been observed that there are

micro level changes in the climatic

conditions when we move from one

part of an urban area to other. These

small scale patterns of climate are

termed as microclimate. With the

beginning of a new century; we are

faced with discussions and arguments

about the future of our cities. It is being

argued that cities have undergone a

major transformation process. Due

to rapid urbanization in developing

countries, these issues have gained

increased attention. As a consequence,

the interest in the microclimate around

buildings in urban areas has increased.

So the challenge lies in systematic

study of variations in climatic condi-

tions w.r.t the various urban parameters

influencing the microclimate.

Although Climatic variations are

observed when we move from one city to

city, there are considerable variations in

climatic conditions within cities also. In

Ar. Sheeba Valsson and Dr. Alka Bharat

ENVIRONMENT

An adequate

understanding of

microclimatic features

can help us shape

environmentally

sustainable habitats.

Natural and Man-madeParameter induced Spatial Variation in Microclimate

January 2008 ARCHITECTURE - Time Space & People 29

some patch of area it is raining, while in

other it is not, some area is colder than

the other and so on. It is very important

and interesting to understand these vari-

ations in temperature, humidity, wind

speed and direction etc. and investigate

the cause.

Modification of the earth's surface by

different urban parameters has a dramat-

ic impact on local climate resulting in

variety of microclimates.

Understanding the effects of natural

and human activities and their interven-

tions on climate will help in linking explic-

itly the urbanization process and the

microclimate and thus proper steps can

be taken to attain the objective of sus-

tainable settlement.

CLIMATE VS MICROCLIMATE

Climate is the average of the atmospheric

conditions over an extended period of

time. Climate is commonly defined as the

weather averaged over a long period of

time. For practical uses the standard aver-

aging period is 30 years, but other peri-

ods may be used depending on the pur-

pose (World Meteorological Organization

- WMO). These quantities are most often

surface variables such as temperature,

precipitation, and wind.

Microclimate refers to the climate of

a site or location. Small scale patterns of

climate resulting from the influence

of topography, urban forms, water bod-

ies, vegetation, etc. are known as

Microclimates. It implies to any local devi-

ation from the Climate of a large region or

zone. Microclimate is the distinctive cli-

mate of a small-scale area, such as a gar-

den, park, valley or part of a city. The

weather variables in a microclimate, such

as temperature, rainfall, wind or humidity,

may be subtly different from the condi-

tions prevailing over the area as a whole

and from those that might be reasonably

expected under certain types of pressure

or cloud cover. Indeed, it is the amalgam

of many, slightly different local microcli-

mates that actually makes up the micro-

climate for a town, city or wood. (Met

Office: Microclimates).

CLIMATIC VARIABLES AND

THEIR CORRELATION

The air temperature variation brings

about a change in water evaporation and

air saturation, leading to the changed air

humidity. Furthermore, the air tempera-

ture differences between different loca-

tions will also cause air pressure differ-

ences, which in turn would produce air

movement, thereby wind (direction and

speed). This variation in humidity and

wind speed and direction affects rainfall.

Thus, all weather variables on the Earth

will more or less be affected by the each

other (Table 1).

Basically the variations in these climatic

parameters is felt and observed when we

move from one part of a city to the other

and it is these subtle differences and

exceptions to the rule that make microcli-

mates so fascinating to study.

VARIATIONS IN

MICROCLIMATE

INFLUENCE OF DENSITY, ASPECT

RATIO AND SKY VIEW FACTOR

To understand the variation in tempera-

ture, a research conducted by Erik

Johanson (1) is studied. The selected sites

are in city of Fez in Morocco. The climate

is characterized by hot and dry summer

and cold winters. One site is in the old city

which is extremely compact with narrow

streets, minimum open spaces and hav-

ing 3-4 storied structures and thus having

deep canyon (H/W=9.7). The second site

is in the new city having wide road pave-

ments, only 2-3 storeyed structures with

both front and backyards, thus having

shallow canyons(H/W=0.6).

Site map from the studied area in the old city(the Seffarine district).

Table 1 - Climatic variables and their correlation

The measured air temperatures in

the two examples are shown along with

the temperature at the ariport. From

the graph it is clearly seen that during

the afternoon, which is the warmest

part of the day, the deep canyon was

markedly cooler than the shallow ones.

The difference was observed all year

round , but was especially pronounced

during summer.

Explanation to the Variation

Because of the high height to width ratio

(aspect ratio), the lower part is in com-

plete shade and consequently, surfaces

remain cool and the air is not warmed up.

By night the cooling of the surfaces at the

canyon bottom is very weak. The deep

canyon has higher temperature because

of the much lower Sky view factor of the

deep canyon.

Inference

As the Height to width ratio increases

the minimum temperature increases

and the maximum tempera-

ture decreases.

INFLUENCE OF GROUND

CHARACTER

Ground sur face whether

natural or man-made, its char-

acteristics of reflectance, per-

meability and soil temperature

influence the microclimate.

Depending on the ground sur-

face, incident radiation can


A narrow alley in the residential part of the oldneighbourhood-deep canyon

Site map from the studied area in the new city(the Adarissa district).

A street in the new neighbourhood - shallowcanyon

Summer

Winter

Average air temperatures


be absorbed, reflected or stored and re-

radiated later.

■ The color and texture of a material

surface determines its reflectivity.

The lighter the color and smoother

the surface, more the reflectivity of

the material. The darker the surface

and rougher it is, the lower the

reflectivity. Such materials would

store more heat during the day and

reradiate it at night. This re-radia-

tion mostly takes place at night

when the surroundings are at a

lower temperature. Vegetation,

namely, trees, shrubs, plants and

grass utilize sunlight for photosyn-

thesis. They absorb and consume

the radiation. In this case the heat is

neither reflected nor reradiated.

Black surfaces in the sun can

become up to 40°C hotter than the

most reflective white surfaces.

■ Thermal properties of materials that

may increase storage of sensible

heat in the fabric of the city during

the day time and release their

stored heat into the urban atmos-

phere after sunset.

■ The replacement of natural soil or

vegetation by materials like con-

crete or asphalt reduces the ability

to decrease ambient temperature

through evaporation and plant

transpiration because the ground

paved and Water disappears quickly

in the canalisation and the cooling

effect is low.

In cities however, the ground is

sealed. Water disappears quickly, and this

increased rapid runoff of precipitation

leads to low cooling effect. Thus the

replacement of natural soil or vegetation

by materials used in cities like concrete or

asphalt reduces the ability to decrease

ambient temperature through evapora-

tion and plant transpiration.

INFLUENCE OF PARKS/VEGETATION

Plants , shrubs and t rees absorb

radiation in the process of photosyn-

thesis. As a result, they actually cool

the environment.

Places with plants and vegetation

have different thermal properties as

compared to built-up and hard-

surfaced unplanted areas. The main

differences are:

■ Plants have lower heat capacity and

thermal conductivity than building

materials and hard surfaces.

■ Solar radiation is mostly absorbed in

the leaves, so that the reflected radia-

tion is very small.

■ Rain water is absorbed in the soil.

Water is later evaporated from the

soil and mainly from the leaves. The

evaporation rate is much higher in

green areas than in unplanted, hard

covered areas.

■ Trees have great potential to cool

cities by shading and by "evapotran-

spiration." Evapotranspiration occurs

when plants secrete or "transpire"

water through pores in their leaves,in

a way, plants sweat like people do.

Different ground materials reflect, store and absorb heat to different degrees

Table 1: Reflectivity values of various surfaces

Surface Details Albedo

Soil Dark & wet versus 0.05 -Light & Dry 0.40

Sand 0.15 - 0.45

Grass Long versus 0.16 -short 0.26

Agricultural crops 0.18 - 0.25

Tundra 0.18 - 0.25

Forests Deciduous 0.15 - 0.20Coniferous 0.05 - 0.15

Water Small zenith angle versus 0.03 - 0.10Large zenith angle 0.10 - 1.0

Snow Old 0.40 -Fresh 0.95

Ice Sea 0.30 - 0.45Glacier 0.20 - 0.40

Clouds Thick 0.60 - 0.90Thin 0.30 - 0.50

Sources: Oke, 1998; Ahrens, 2001


The water draws heat as it evaporates,

cooling the air in the process.

Trees and hedges also affect airflow.

Thick vegetation effectively cut it off. On

the other hand, careful placement of

trees and hedges can direct and increase

air speeds. This is achieved by planting

trees and hedges so as to make a narrow-

ing 'path' for the air. This reduction of area

increases air speed. The placement of

trees and hedges cause minor pressure

differences which marginally changes the

air path. This is easy to understand. The

leeward side/ wind shadow area is a low

pressure zone. Air tends to shift towards

this. In the case of hedges, the low pres-

sure shifts the path downwards. Airflow

below the canopy of a tree is similarly

shifted upwards.

Vegetation causes pressure differ-

ences which shifts the air path

■ Plants reduce the wind speed and its

fluctuations near the ground.

■ Different types of vegetation and

planting patterns can act as wind

catchers and wind screens. A single

tree with high trunk and wide canopy

can concentrate the wind near the

ground, thus increasing the wind

speed and a group of trees can

reduce significantly the wind speed in

the area.

■ Trees besides controlling the precipi-

tation also control the seasonal and

annual temperature variations.

As a result, the micro-climate within

and near to green areas differs from

unplanted, built-up areas. The main dif-

ferences are in the temperature, wind

velocity and turbulence, air and radiant

temperatures, humidity, and air cleanli-

ness. The leaves of plants absorb most

of the solar radiation which strikes

them. They transform a very small part

of the radiant energy by photosynthesis

into chemical energy, and in this way

reduce the rate of heating of the urban

space slightly.

INFLUENCE OF PROXIMITY TO WATER

BODIES

Water has a relatively high latent heat of

evaporation as well as specific heat. In

other words, water uses up a compara-

tively large amount of heat in evaporat-

ing. It also absorbs or releases a compara-

tively large amount of heat for a unit rise

or fall of temperature. So, when water

evaporates by the movement of air, it

cools the air. This is evaporative cooling.

The proximity of a site to the sea or

other large water bodies also affects the

climatic conditions in and around the site.

Wind movement from the water body dur-

ing the day, and towards it at night, is

caused by temperature differences of the

air close to the surfaces of the soil and

water. The relative humidity of air is also

affected, since the air coming from a water

body is more humid. Such phenomena are

stronger close to water bodies, but may

also affect the regional climate by creating

strong air movement reaching large dis-

tances. This is mainly affected by the phys-

ical characteristics of the region, such as

topography and vegetation. Usually, the

effect that a water body may have on the

local climatic conditions of a site diminish-

es not only with distance from the water

body, but with elevation as well.

Water has a relatively high latent heat

of evaporation as well as specific heat. In

other words, water uses up a compara-

tively large amount of heat in evaporat-

ing. It also absorbs or releases a compara-

tively large amount of heat for a unit rise

or fall of temperature. So, when water

Vegetation increasing, decreasing anddirecting airflow

Vegetation causes pressure differences whichshifts the air path

Water bodies absorb much heat during the dayand reradiate it at night


evaporates by the movement of air, it

cools the air. This is evaporative cooling.

During the day time the land is heat-

ed more than water. The air from the land

is warmer, it rises up and cool air from

water blows to take its place. At night the

land cools faster than water, so, the water

is warmer than the land. The warm air

above water rises up and the cool air over

land rushes to take its place.

Presence of water bodies plays a

very important role in characterizing

a microclimate

■ Water absorbs relatively large amount

of radiation.

■ They also allow evaporative cooling.

As a result, during the daytime areas

around water bodies are generally

cooler. At night, however, water bod-

ies release relatively large amounts of

heat to the surroundings.

Water bodies influences air tempera-

ture of the surrounding areas and the

drop in temperature has a direct impact

on the wind velocity

PONDS AND FOUNTAINS

Evaporation in a pond is proportional to

the air-water contact area. Such areas

increase considerably with the incorpora-

tion of fountains and sprayers, thus pro-

ducing an extra decrease in water temper-

ature. The smaller drops are, the greater

the air-water contact surface is, increasing

evaporation. A single water drop moving

through still air experiences two process-

es: Heat flows from the air to the drop (if

the air is hotter than the drop). Water evap-

orates from the drop to the surrounding

air. The hotter the drop is, the more water

will be evaporated. Inward heat transfer

will warm it up but evaporation will cool it

down. As a result of these two opposite

tendencies, an equilibrium drop tempera-

ture is reached (the wet bulb temperature

of the air). Once the drop has reached the

wet bulb temperature, the extra energy

needed to evaporate more water has to

come from the surrounding air. Cooling by

fountains is achieved simultaneously by

two different ways, by cooling directly the

air of the surrounding space and by cool-

ing the water of the pond.

INFLUENCE OF LAND FORM AND

TOPOGRAPHY

The landform or topography of a site and

surrounding could either be flat, sloping

or undulating (mounds etc.). If the land is

flat, similar conditions would prevail over

the entire site. Topography or modula-

tions of earth either in natural undis-

turbed or manmade conditions has the

ability to modify, ameliorate or accentu-

ate climatic variations in different ways.

Mountain ranges are diverters of air

masses. They affect the flow of moisture-

laden air and cause rain shadows for the

areas on the leeward side. Temperature

decreases with the rise in height and cold

air flows downhill and settles in valleys.

As a result, the air temperature is lower in

such areas. Also air speed increases up

the windward slope. Air speed is maxi-

mum at the crest and minimum on the

leeward side.

Cool air has a higher density than

hot air. As a result cool air is heavier and

tends to settle down in depressions

while hot air rises. On the other hand, air

movement is also affected by pressure

difference. Airflow normally takes place

from high-pressure zones to low pres-

sure zones. Obstacles in the path of air-

flow cause an air-buildup and therefore

a high pressure area on the windward

side. Similarly, direction of the airflow

would now depend on the shape of the

obstacle and the magnitude of the pres-

sure difference.

Sun facing slopes (south facing in the

northern hemisphere) enable the expo-

sure of buildings and open spaces to solar

radiation, during winter when the solar

altitude is low. Windward slopes experi-

ence higher wind speeds, and in some

cases higher relative humidity, than the

leeward ones. Low areas, such as valleys

surrounded by mountains may usually

suffer from poor ventilation and thermal

inversion or, conversely, may become

reservoirs of cool air. These, in turn, may

affect air quality by trapping smoke and

gas emissions from vehicles, industry and

heating systems. Long and narrow valleys

may experience strong winds parallel to

their axis. Such areas also experience air

movement parallel to the slope-rising, or

anabatic breezes during the day, and

descending, or katabatic winds at night.

These movements are the result of the

heating and cooling of air close to the

ground. Such adiabatic processes result

in a change of about 0.8 deg C in the air

temperature for every 100 m in altitude.


L a n d fo r m v a r i a t i o n s a n d t h e

microclimate.

■ Flat site experience little variation.

■ Air speed increases up the slope and

decrease down it.

■ Depression valleys experience lower

air temperatures.

■ Elevated places experience low pres-

sure, air expands and cools when

pressure decreases, thus low temper-

ature and high relative humidity.

INFLUENCE OF ANTHROPOGENIC HEAT

Anthropogenic heat release is the release

of heat due to combustion of fuels. It is a

heat source for the city not found in unin-

habited surroundings. Its magnitude

varies greatly between cities, according to

per capita energy use and population den-

sity, and depends on the climate (due to

the demand for space heating or cooling),

the degree and type of industrial activity

and the type of urban transport system.

The emission of hygroscopic pollu-

tants from cars and heavy industry act as

condensation nuclei, leading to the for-

mation of cloud and smog, which can

trap radiation. In some cases, a pollution

dome can also build up;

CONCLUSION

The study shows that all features/parame-

ters, i.e. natural as well as man-made, influ-

ence the microclimate. Proper under-

standing of the influence of these parame-

ters states explicitly the variations in

microclimate in various patches of an

urban area. The important conclusions are:

● As the aspect ratio in a settlement

increases the minimum temperature

increases and the maximum tempera-

ture decreases.

● Lower the Sky View factor, higher

the temperature.

● Higher the building density, higher

the temperature.

● Temperature increases with the

increase in paved area.

● Presence of vegetation lowers the

temperature.

● Presence or nearness to water bodies

cools the air.

● Wind movement is from the water

body during the day and towards it

at night.

● Elevated terrains have higher wind

speed.

● Elevated places experience low pres-

sure, low temperature and high rela-

tive humidity

● Heat Island formed by anthropogenic

heat increases the temperature.

REFERENCES

1. Johansson E., 2006: Influence of

urban geometry on outdoor thermal

comfort in a hot dry climate: A study

in Fez, Morocco, Building and

Environment 41 (2006) 1326-1338.

2. Upmanis H., Eliasson I.1999:Influence

of parks on local climate,

International Journal of Climatology

18(6), 681-700.

3. Emmanuel R. (2001), An Urban

approach to climate-sensitive design,

Strategies to the tropics, Spon Press,

Taylor and Francis Group, London.

4. Givoni B. (1976), Man, Climate and

Architecture, Van Nostrand Reinhold

Company, London.

5. G. Mills (2006), Progress towards sus-

tainable settlements:a role of urban

climatology, Theoritical and applied

climatology 84, 69-76.

6. Oke T. (1988) Street design and

Urban canopy layer Climate, Energy

and Buildings 11, 103-113.

7. Oke,T.R.(1987) Boundary layer

Climates, Taylor and Francis Group,

London.

8. Ochoa. J.M. and Roset J.(2000),

Influence of Vegetation on the ener-

getic balance of Urban outdoor

spaces, Proceedings, ISES Millenium

Solar Forum, Mexico, Sept.2000.

9. Gulyas A. (2005), Differences in

human comfort conditions within a

complex urban environment. A case

study. ACTA CLIMATOLOGICA ET

CHRONOLOGICA, 71-84.

10. WHO/UNEP (1992) Urban AIR pollu-

tion in Megacities of the World.

World Health organization, United

Nations Environmental Program,

Oxford, Blackwell. ■

Ar. S. Valsson, Lecturer, Smt MM

Deptt of Architecture, LAD College,

Nagpur and Dr. Alka Bharat,

Professor, Deptt of Architecture &

Planning, MANIT Bhopal.

Photographs: Courtesy the Author.

Emission of anthropogenic heat - coolingtowersSource: www.freefoto.com

Depression valleys have little air movementunless they lie in the direction of airflow

Emission of anthropogenic heat - carsphoto:Sebastian Wypych

Documents

Natural and Man Made Para-28-34