Global climate can be understood with the help of following parameters :
(a) Heat
The earth receives almost all its energy from the sun in the form of radiation and thus the sun has the dominant influence on climate. This radiant energy is transmitted to earth as ultra-violet radiation (short waves) and infra-red radiation (long waves). Most of this energy is perceived as heat and only an insignificant part is visible as light.
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| Radiation Spectrum |
The earth moves around the sun in a slightly elliptical orbit. One revolution is completed in 365 days, 5 hours and 48 minutes. The solar distance varies from 152 million kilometers to 147 million kilometers. The earth rotates around its own axis, every 24 hours, and the axis is tilted to the plane of the elliptical orbit at an angle of 23.5' w.r.t. the vertical. Due to this tilted position, the area receiving the maximum radiant intensity falls alternately to north and south between the tropic of Cancer (Latitude 23.5 N) and the tropic of Capricorn (latitude 23 S). This is the main cause of seasonal climatic changes.
The earth has an atmosphere which revolves along with it. As the earth rotates the length of the suns rays passing through the atmosphere changes and hence, the intensity of radiation, incident on earth surface, from the sun also changes.
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| Earth Revolving Around the Sun |
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| Length of Sun's Rays Traversing Atmosphere |
Fallowing figure illustrates the incoming radiation while below figure shows how the earths surface releases heat and thus, achieving a heat balance, each year. Buildings and the surrounding vegetation affect the heat balance and hence, the micro-climate of the site. Unless thermal balance is maintained, the temperature of the earth and its atmosphere.
Toht radiation amving (solar constant =100% )
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| Incoming Radiation thorough Atmosphere |
would keep increasing and would soon cease to be favorable to most form of life. In fact we are concerned about global warming and ozone depletion these days. There are alarming indications of this imbalance which have arisen due to uncontrolled developmental activities and use of dangerous chemicals.
Temperature is measured in degrees Celsius by a maximum-minimum thermometer. A simple sunshine recorder will register the duration of sunshine, expressed in number of hours per day. The temperature which is reported usually is called dry bulb temperature (DBT).
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| Heat Release from Ground Atmosphere |
(b) Winds
Winds are basically convection currents in the atmosphere, tending to even out the differential heating of various zones. The pattern of movements is modified by the earth's rotation which has a speed of 27 km/min at the equator. At the maximum heating zone, which is somewhere between the tropics of Cancer and Capricorn, air is heated'by the hot surface of the earth, it expands and its pressure is reduced, and thus, it becomes lighter.
As a result, this air rises vertically and flows at a high level towards colder regions, parts of this air having cooled down at the higher level; and it descends to the surface in the sub-tropic regions from where the cooler, heavier air is drawn in towards the equator from both the Earth and south regions.
The area where the air rises, and these northerly and southerly winds meet and where the tropical calm ,is formed is called as the inter-tropical convergence zone. This area experiences either completely calm conditions or light breezes.
The atmospheric envelope rotates along with the earth. As it is light in weight and behaves like a fluid, it has a tendency to lag behind the earth at the equator, where the tangential speed of earth's surface is the largest. Thus, there is a slippage at the boundary layer between the earth and its atmosphere; and this is experienced as a wind blowing in a direction opposite to that of the earth's rotation, which gives rise to the north easterly and south easterly winds. These are known as trade winds (Koenigsbergar, et al, 1975).
Around 30' N and S, there are two bands of continuously high barometric pressure zones (descending air). Winds in these zones are typically light and variable. Between 30' N and 50' N and similarly in the south. strong westerly winds prevail, blowing in the same direction as the earth's rotation. The origin of these winds have been explained by the law of conservation of angular momentum of the earth-atmosphere system.
Further, down the poles form 60' N and S the air flow pattern is similar to that of the equator. Hence, the north easterly and south easterly polar winds.
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| Global Pressure Belts and Prevailing Winds |
The prevailing wind direction is a major factor in selecting a site for a populated place and the allocation of functional zones. Account should be taken of the fact that the local topography has a significant influence; for example, in the daytime, the earth is heated faster than water and hence the air mass near the earth becomes less dense, creating a zone of low pressure. This gives rise to a wind that blows from the sea towards the shore.
At night, the temperature of water is higher than that of the air and this condition reverses the direction of the wind. Wind velocity is measured by a cup-type propeller known as anemometer, and its direction is measured by a wind-vane.
(c) Humidity
The air in the lowly layers of the atmosphere always contains a certain amount of water vapor resulting from evaporation of water on and in the earth's surface. The rate of evaporation depends primarily on temperature and wind. It has been estimated that the surface of the ocean, in the tropics, evaporates in a year a layer of water up to 3 meters in depth. This must be replenished by rainfall and run-off from the rivers. The ability of air to absorb and retain the maximum amount of water vapor is directly related to temperature.
Absolute Humidity is defied as the amount of water vapor in moist air at any given time and is expressed in gm/m3 of air.
Relative Humidity (RH) is defied as the ratio of water vapor in moist air to water vapor in saturated air, expressed in percentage. For example, if the absolute humidity of air, for any given temperature is equal to 7gm/m3, and saturated air (at the same temperature) contains 10 gm/m3, the ratio 7 : 10, expressed as a percentage will be the I relative humidity.
RH= 7/10 x100=70%
Humidity is usually measured by wet- and dry-bulb hygrometer.
(d) Precipitation (Rainfall)
Precipitation is the collective term used for rain, snow, hail, dew and frost, i.e. all forms of water precipiated from the atmosphere. It is measured by rain gauges and snow gauges and is expressed in millimeters per day or month.
(e) Climate Data
It is important to note that climate at a global/regional level is best described by measured data, over a period of time, such as sunshine (hours per day), radiant heat (watts per meter square per day), wind velocity (meter/sec) and wind direction (N, NE, E, SE, S, SW, W and NW), rainfall (mm per month), relative humidity (percentage) and temperature (maximum and minimum in degrees Celsius).
It is not easy to understand the nature of a particular climate by merely looking at the vast i amount of data collected by the nearest meteorological station. It is necessary to sort, summarize and simplify the available data with reference to the objectives of site-selection exercise. This is the best accomplished by adopting a standardized method of graphical representation. Fallowing figure illustrates a graphical method that was developed for describing the climate of any location.
Below figure shows how wind speeds and directions are recorded for a specified period. Based on such climatic data certain zones and belts of approximately uniform climates, have been identified and described as under (Koenigsberger, et al, 1975).
(f) Clssification of Climates
Climates can be classified, broadly, as given below :
(i) Warm-Humid Climate
Warm-humid climates are found in a belt near the equator extending to 15' N and 15's. There is very little seasonal variation except for the occurrence periods of more or less rain.
Air temperature is 27' to 35OC during the day and 21' to 27' C in the night. Relative humidity remains high at about 75%; but it may vary from 55 to almost 100%. Rainfall is high throughout the year. Annual rainfall can vary from 2000 mm to 5000 mm and monthly rainfall may exceed 500 mm. During severe storms the down pour may be also as high as 100 mm/hr.
Wind velocities are low, calm periods are frequent but strong winds exceeding 30 m/s can occur during rain squalls. There are usually one or two dominant directions of wind.
Special characteristics of this region are that the high humidity accelerates mould and algae growth, rusting and rotting. Organic building materials teild to decay rapidly. Mosquitoes and other insects abound. The thunder-storms are accompanied by frequent air-to-air electric discharges. The subsoil water table is usually high and the ground may be water-logged.
(ii) Hot Dry Climate
This type of climate occurs in two belts at latitudes between 15' and 30' north and south of the equator. Two marked seasons occur, a hot and a somewhat cooler period.
Air temperature during day time may reach a maximum of 43' to 49' C in the summer. The night temperature will range from 27' to 32'C during summer. During the cool season the day time temperature
reaches 27' to 32" C. Night time temperature may vary from.O' to 18' C. The relative humidity varies from 10 to 55 percent. Rainfall is slight and variable throughout the year from 50 mm to 150 mm per annum. Flash storms may occur over limited areas with 50 mm rain in a few hours.
Winds are usually local and whirlwinds are often created. Winds are hot, carrying dust and sand, and often develop into dust-storms.
The soil is usually dusty and very dry. The sub-soil water table is very low. The high day time temperatures and rapid cooling at night may cause materials to crack and break up since the daily temperature variation reaches 17' to 22'C.
(iii) Monsoon Climate
Monsoon climate occurs over large land masses near the tropics of Cancer and Capricorn, which are sufficiently far from the equator to experience marked seasonal changes in solar radiation and wind directions. Our capital, New Delhi is subject to such a climate.
Approximately one-third to two-thirds of the year is hot and dry and the other third may be warm and humid or cool and dry as the latitude increases from the tropics; day-time temperatures in the hot and dry seasons will vary from 32' to 43OC. Night time temperatures may be 21" to 27'C. During the cool and
dry seasons the day time temperature may dip down to 4' to 10°C. Relative humidity during dry period
rises from 55 to 95%. Rainfall varies from 500 to 1300 mm per year with 200 to 250 mm in the wettest month; there is little or no rain during the dry season.
Winds are hot and dusty during the dry period. Wind changes direction in the beginning of the warm-humid seasons, but monsoon winds are fairly strong and steady.
There is a risk of soil erosion during monsoons. In the dry season. strong ground glare is experienced. Seasonal changes in relative humidity cause rapid weakening of building materials. Dust and sand storms may occur. Termites are common.
(iv) Other Climates
Mountain regions and plateaus, more than 900 to 1200 meters above sea level experience special climates as also the coastal regions and islands. Site selection and orientation of buildings, and the selection of building designs will have to be suitably adjusted to suit such climates.
(c) Humidity
The air in the lowly layers of the atmosphere always contains a certain amount of water vapor resulting from evaporation of water on and in the earth's surface. The rate of evaporation depends primarily on temperature and wind. It has been estimated that the surface of the ocean, in the tropics, evaporates in a year a layer of water up to 3 meters in depth. This must be replenished by rainfall and run-off from the rivers. The ability of air to absorb and retain the maximum amount of water vapor is directly related to temperature.
Absolute Humidity is defied as the amount of water vapor in moist air at any given time and is expressed in gm/m3 of air.
Relative Humidity (RH) is defied as the ratio of water vapor in moist air to water vapor in saturated air, expressed in percentage. For example, if the absolute humidity of air, for any given temperature is equal to 7gm/m3, and saturated air (at the same temperature) contains 10 gm/m3, the ratio 7 : 10, expressed as a percentage will be the I relative humidity.
RH= 7/10 x100=70%
Humidity is usually measured by wet- and dry-bulb hygrometer.
(d) Precipitation (Rainfall)
Precipitation is the collective term used for rain, snow, hail, dew and frost, i.e. all forms of water precipiated from the atmosphere. It is measured by rain gauges and snow gauges and is expressed in millimeters per day or month.
(e) Climate Data
It is important to note that climate at a global/regional level is best described by measured data, over a period of time, such as sunshine (hours per day), radiant heat (watts per meter square per day), wind velocity (meter/sec) and wind direction (N, NE, E, SE, S, SW, W and NW), rainfall (mm per month), relative humidity (percentage) and temperature (maximum and minimum in degrees Celsius).
It is not easy to understand the nature of a particular climate by merely looking at the vast i amount of data collected by the nearest meteorological station. It is necessary to sort, summarize and simplify the available data with reference to the objectives of site-selection exercise. This is the best accomplished by adopting a standardized method of graphical representation. Fallowing figure illustrates a graphical method that was developed for describing the climate of any location.
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| Graphical Representation of Meteorological Data |
(f) Clssification of Climates
Climates can be classified, broadly, as given below :
(i) Warm-Humid Climate
Warm-humid climates are found in a belt near the equator extending to 15' N and 15's. There is very little seasonal variation except for the occurrence periods of more or less rain.
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| Monthly Wind Frequency Graph |
Air temperature is 27' to 35OC during the day and 21' to 27' C in the night. Relative humidity remains high at about 75%; but it may vary from 55 to almost 100%. Rainfall is high throughout the year. Annual rainfall can vary from 2000 mm to 5000 mm and monthly rainfall may exceed 500 mm. During severe storms the down pour may be also as high as 100 mm/hr.
Wind velocities are low, calm periods are frequent but strong winds exceeding 30 m/s can occur during rain squalls. There are usually one or two dominant directions of wind.
Special characteristics of this region are that the high humidity accelerates mould and algae growth, rusting and rotting. Organic building materials teild to decay rapidly. Mosquitoes and other insects abound. The thunder-storms are accompanied by frequent air-to-air electric discharges. The subsoil water table is usually high and the ground may be water-logged.
(ii) Hot Dry Climate
This type of climate occurs in two belts at latitudes between 15' and 30' north and south of the equator. Two marked seasons occur, a hot and a somewhat cooler period.
Air temperature during day time may reach a maximum of 43' to 49' C in the summer. The night temperature will range from 27' to 32'C during summer. During the cool season the day time temperature
reaches 27' to 32" C. Night time temperature may vary from.O' to 18' C. The relative humidity varies from 10 to 55 percent. Rainfall is slight and variable throughout the year from 50 mm to 150 mm per annum. Flash storms may occur over limited areas with 50 mm rain in a few hours.
Winds are usually local and whirlwinds are often created. Winds are hot, carrying dust and sand, and often develop into dust-storms.
The soil is usually dusty and very dry. The sub-soil water table is very low. The high day time temperatures and rapid cooling at night may cause materials to crack and break up since the daily temperature variation reaches 17' to 22'C.
(iii) Monsoon Climate
Monsoon climate occurs over large land masses near the tropics of Cancer and Capricorn, which are sufficiently far from the equator to experience marked seasonal changes in solar radiation and wind directions. Our capital, New Delhi is subject to such a climate.
Approximately one-third to two-thirds of the year is hot and dry and the other third may be warm and humid or cool and dry as the latitude increases from the tropics; day-time temperatures in the hot and dry seasons will vary from 32' to 43OC. Night time temperatures may be 21" to 27'C. During the cool and
dry seasons the day time temperature may dip down to 4' to 10°C. Relative humidity during dry period
rises from 55 to 95%. Rainfall varies from 500 to 1300 mm per year with 200 to 250 mm in the wettest month; there is little or no rain during the dry season.
Winds are hot and dusty during the dry period. Wind changes direction in the beginning of the warm-humid seasons, but monsoon winds are fairly strong and steady.
There is a risk of soil erosion during monsoons. In the dry season. strong ground glare is experienced. Seasonal changes in relative humidity cause rapid weakening of building materials. Dust and sand storms may occur. Termites are common.
(iv) Other Climates
Mountain regions and plateaus, more than 900 to 1200 meters above sea level experience special climates as also the coastal regions and islands. Site selection and orientation of buildings, and the selection of building designs will have to be suitably adjusted to suit such climates.
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