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BUILDING SERVICES - III
climatology
B A h 3rd
S
CLIMATE AND IT’s ELEMENTS
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Global climatic
factors
INDEX
•Global climatic
factors
•Elements ofclimates
•Classification of
tropical climates
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PAGE CLIMATOLOGY
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• ELEVATION OR ALTITUDE EFFECT CLIMATE
Climate changes with increase or decrese in altitude.
• PREVAILING GLOBAL WIND PATTERNS
There are 3 major wind patterns found in the Northern Hemisphere and
also 3 in the Southern Hemisphere. These are average conditions and
do not essentially reveal conditions on a particular day. As seasons
change, the wind patterns shift north or south. So does the intertropical
convergence zone, which moves back and forth across the Equator.
Sailors called this zone the doldrums because its winds are normally
weak.
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c l i m a t o l o g y
INDEX
•Global climatic
factors
•Elements ofclimates
•Classification of
tropical climates
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• LATITUDE AND ANGLES OF THE SUNS RAYS
As the Earth circles the sun, the tilt of its axis causes changes in the
angle of which sun’s rays contact the earth and hence changes the
daylight hours at different latitudes. Polar regions experience thegreatest variation, with long periods of limited or no sunlight in winter
and up to 24 hours of daylight in the summer.
• TOPOGRAPHY
The Topography of an area can greatly influence our climate. Mountainranges are natural barriers to air movement. In costal areas, winds off
the ocean carry moisture-laden air toward the coast. The Coastal Range
allows for some condensation and light precipitation. Inland, the taller
Plateau range rings more significant precipitation in the air. On the
eastern slopes of the Deccan plateau, sinking air warms fromcompression, clouds evaporate, and dry conditions prevail.
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• EFFECTS OF GEOGRAPHY
The position of a town, city or place and its distance from mountains
and substantial areas of water help determine its prevailing wind
patterns and what types of air masses affect it. Coastal areas may enjoyrefreshing breezes in summer, when cooler ocean air moves ashore.
In spring and summer, people in eastern ghats watch for thunderstorms,
these storms are caused where three types of air masses frequently
converge: cold and dry from the north, warm and dry from thesouthwest, and warm and moist from the Bay of Bengal- these colliding
air masses often generate storms.
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• SURFACE OF THE EARTH
Just look at any globe or a world map showing land cover, and you will
see another important factor which has a influence on climate: the
surface of the Earth. The amount of sunlight that is absorbed orreflected by the surface determines how much atmospheric heating
occurs. Darker areas, such as heavily vegetated regions, tend to be
good absorbers; lighter areas, such as snow and ice-covered regions,
tend to be good reflectors. The ocean absorbs and loses heat more
slowly than land. Its waters gradually release heat into the atmosphere,which then distributes heat around the globe.
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• Climate change over time
Cold and warm periods punctuate Earth’s long history. Some were fairly
short; others spanned hundreds of thousands of years. In some cold
periods, glaciers grew and spread over large regions. In subsequentwarm periods, the ice retreated. Each period profoundly affected plant
and animal life.
Since the turn of the 20th century, temperatures have been rising
steadily throughout the world. But it is not yet clear how much of thisglobal warming is due to natural causes and how much derives from
human activities, such as the burning of fossil fuels and the clearing of
forests.
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Elements of climates
INDEX
•Global climatic
factors
•Elements ofclimates
•Classification of
tropical climates
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PAGE
CLIMATOLOGY
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• ELEMENT OF CLIMATE:
- TEMPERATURE
- HUMIDITY
- VAPOUR PRESSURE
- PRECIPITATION
- SKY CONDITION
- SOLAR RADIATIONS- WIND FLOW & WIND INTENSITY
- VEGITATION
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• TEMPERATURE :
• Measured in ° C (degree Celsius)
• Measured for dry air with the means of simple Mercury thermometer.
DRY BULB/ TRUE TEMPERATURE:
The dry-bulb temperature is the temperature of air measured by
a thermometer freely exposed to the air but shieldedfrom radiation and moisture.
Dry bulb temperature is the temperature that is usually thought of as air
temperature, and it is the true thermodynamic temperature. It is the
temperature measured by a regular thermometer exposed to the
airstream. Unlike wet bulb temperature, dry bulb temperature does notindicate the amount of moisture in the air. In construction, it is an
important consideration when designing a building for a certain climate.
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WET BULB/ TRUE TEMPERATURE:
The wet-bulb temperature is a type of temperature measurement that
reflects the physical properties of a system with a mixture of a gas and
a vapour, usually air and water vapour. Wet bulb temperature is thelowest temperature that can be reached by the evaporation of water
only. It is the temperature one feels when one's skin is wet and is
exposed to moving air. Unlike dry bulb temperature, wet bulb
temperature is an indication of the amount of moisture in the air.
Wet-bulb temperature can have several technical meanings:• Thermodynamic wet-bulb temperature: the temperature a volume of air
would have if cooled adiabatically to saturation at constant pressure by
evaporation of water into it, all latent heat being supplied by the volume
of air.
• The temperature read from a wet bulb thermometer
• Adiabatic wet-bulb temperature: the temperature a volume of air would
have if cooled adiabatically to saturation and then compressed
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HUMIDITY:
• Humidity is a term for the amount of water vapour in the air, and can
refer to any one of several measurements of humidity. Formally, humid
air is not "moist air" but a mixture of water vapour and other constituentsof air, and humidity is defined in terms of the water content of this
mixture, called the Absolute humidity. In everyday usage, it commonly
refers to relative humidity, expressed as a percent in weather forecasts
and on household humidity status; it is so called because it measures
the current absolute humidity relative to the maximum. Specifichumidity is a ratio of the water vapour content of the mixture to the dry
air content (on a mass basis). The water vapour content of the mixture
can be measured either as mass per volume or as a partial pressure,
depending on the usage.
• Measured in g/Kg or g/m3 (degree Celsius)• Measured by instrument called DRY & WET BULB HYGROMETRE
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A hygrometer is a device used for
measuring the humidity of the air
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RALATIVE HUMIDITY:
• It is the ratio of actual amount of moisture present, to that amount of
moisture the air can hold at a given temperature and is expressed in %.
RH=AH / SH X 100(%)
RH= RELATIVE HUMIDITY
AH= ABSOLUTE HUMIDITY
SH= SATURATION POINT HUMIDITY
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ABSOLUTE HUMIDITY:
• Absolute humidity on a heat basis is the mass of water in the unit
volume of air. The most common units are grams per cubic meter,
although any mass unit and any volume unit could be used.
• If all the water vapour in one cubic meter of air were condensed into a
container, the mass of the water in the container could be measured
with a scale to determine absolute humidity. The amount of water
vapour in that cube of air is the absolute humidity of that cubic meter ofair. More technically, absolute humidity on a volume basis is the mass of
dissolved water vapour, mw, per cubic meter of total moist air, Vnet:
AH= m (by volume) / V (net)
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• Absolute humidity ranges from 0 grams per cubic meter in dry air to
30 grams per cubic meter (0.03 ounce per cubic foot) when the vapour
is saturated at 30 °C.
• The absolute humidity changes as air pressure changes. This is very
inconvenient for architectural engineering calculations, e.g. For rooms,
where temperature can vary considerably.
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SATURATION POINT HUMIDITY / SPECIFIC HUMIDITY:
• Specific humidity is the ratio of water vapour to dry air in a particular
mass, and is sometimes referred to as absolute humidity or humidity
ratio. Specific humidity ratio is expressed as a ratio of kilograms of watervapour, mv , per kilogram of dry air ma .
• That ratio is defined as:
SH=m
(by
volume)
/ m (mass of air)
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EFFECT OF HUMIDTY:
• HUMAN COMFORT:
Humans control their body temperature mainly
by sweating and shivering. At high humidity, sweating is less effective,and we feel hotter. At low humidity, the risk for nosebleeds increases,
especially during cold winter seasons.
• Some people experience difficulty breathing in high humidity
environments. Some cases may possibly be related to respiratory
conditions such as asthma, while others may be the product of anxiety.
- Air conditioning works by reducing humidity in summer.
- In winter, heating cold outdoor air can decrease relative humidity
levels indoor to below 30%, leading to discomfort such as dry skin
and excessive thirst.
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EFFECT OF HUMIDTY:
• BUILDING CONSTRUCTION:
Traditional building designs typically had weak insulation, and it allowed
air moisture to flow freely between the interior and exterior. The energy-efficient, heavily-sealed architecture introduced in the 20th century also
sealed off the movement of moisture, and this has resulted in a
secondary problem of condensation forming in and around walls, which
encourages the development of mold and mildew.
Additionally, buildings with foundations not properly sealed will allow
water to flow through the walls due to capillary action of pores found in
masonry products. Solutions for energy-efficient buildings that avoid
condensation are a current topic of architecture.
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VAPOUR PRESSURE:
It is the partial pressure of water vapour present in the air.
The atmospheric pressure (P) is the sum of the partial pressure of dry
air (Pa) & the partial vapour pressure (Pv) :
P= Pa+Pv
• The air is saturated when the P v = the pressure of saturated vapour
presure of the same temperature (Pvs) .
• Relative humidity can also be expressed as the ratio of actual vapour
pressure to the ’ SATURATION POINT VAPOUR PRESSURE ’.
P= AH /SH X100 = Pv / Pvs X 100(%)• VP is measured in pressure unit : N/ m
2
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• Humidity data is basically used to state the futur eclimatic condition of a
specific location , as it is used to predict the :
- Monsoon trend
- Amount of precipitation
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SKY CONDITIONS:
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It is the PRESENCE/ ABSENCE of clouds in the sky.
On an average two observations are made per day:• DURING DAY
• DURING NIGHT
It is expressed in % of the sky hemisphere is covered by the clouds.
IMPORTANCE: Sky luminance values are needed if daylighting in
buildings is to be predicted.
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SOLAR RADIATIONS:
A simple sunshine recorded during the day time which is expressed in
NUMBER OF HOURS/ DAY , as an average for each month.
Average Daily Amounts of Solar Radiations for each month of year
would give a fair indication of climatic conditions, including seasonal
variations.
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WIND FLOW AND WIND INTENSITY:
Air movement is affected by the following:
• Differences in air pressure as air moves from areas of high pressure toareas of low pressure.
• Differences in temperature
o Thermal buoyancy- describes the action of air as it is heated. Because
heated air is less dense it rises, moving from a cool, high-density area toward a warm, low-density area.
o Convection currents- The movement of cooler air moving in to replace
rising warm air will establish convection currents any place in the building in
which temperature differentials exist, with main areas of concern being theliving space and attics. Supply and return registers are key points of
temperature differentials and also key points of pressure differences caused
by heating and cooling system hair handlers.