by - Smt. Kalpana Palkhiwala, Deputy Director(M & C), PIB, New Delhi
Climate in a narrow sense is usually defined as the “average weather”, or more rigorously, as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years. The classical period is 30 years, as defined by the World Meteorological Organization (WMO). These quantities are most often surface variables such as temperature, precipitation and wind. Climate in a wider sense is the state, including a statistical description, of the climate system.
The difference between climate and weather is usefully summarized by the popular phrase “Climate is what you expect, weather is what you get”. Over historical time spans there are a number of static variables that determine climate, including latitude, altitude, proportion of land to water, proximity to oceans and their currents, mountains, persistent ice or snow cover, humidity, rainfall, atmospheric particle count, the density and type of vegetation coverage affecting solar heat absorption, water retention, and rainfall on a regional level.
Alterations in the quantity of atmospheric greenhouse gases determines the amount of solar energy retained by the planet, leading to global warming or global cooling. Modern climate classification methods also focus on the relative frequency of different air mass types or locations within synoptic weather disturbances, plant hardiness, evapotranspiration, air mass origin and certain biomes. Regions having similar characteristics features of climate are grouped under one climatic zone based on the climatic factors. The country can be divided into a number of climatic zones. India can be divided into six climatic zones, namely, hot and dry, warm and humid, moderate, cold and cloudy, cold and sunny and composite.
Particularly mean monthly temperatures-minimum and maximum and relative humidity are considered here. A place is assigned to one of the first five climatic zones only when the defined conditions prevail there for more than six months. In cases where none of the defined categories can be identified for six months or longer, the climatic zone is called composite. According to recent code of Bureau of Indian Standards, the country may be divided into five major climatic zones. It is seen that the recent classification is not very different from the earlier one except that the cold and cloudy, and cold and sunny have been groupedtogether as cold climate; the moderate climate is renamed as temperate climate. However, a small variation is noticed as far as the land area of the country corresponding to different zones is concerned.
It may be mentioned that each climatic zone does not experience the same climate for the whole year. It has a particular season for more than six months and may experience other seasons for the remaining period.
Hot and Dry
The hot and dry zone lies in the western and the central part of India, Jaisalmer, Jodhpur and Sholapur are some of the towns that experience this type of climate. The mean monthly temperature remains 30 degree celsius and relative humidity 55 %.A typical hot and dry region is usually flat with sandy or rocky ground conditions, and sparse vegetation comprising cacti, thorny trees and bushes.
There are few sources of water on the surface, and the underground water level is also very low. Due to intense solar radiation (values as high as 800-950 W/m2), the ground and the surroundings of this region are heated up very quickly during day time. In summer, the maximum ambient temperatures are as high as 40-45 oC during the day and 20-30 oC at night. In winter, the values are between 5 and 25 oC during the day and 0 to 10 oC at night. It may be noted that the diurnal variation in temperature is quite high, that is, more than 10 oC.
The climate is described as dry because the relative humidity is generally very low, ranging from 25 to 40% due to low vegetation and surface water bodies. Moreover, the hot and dry regions receive less rainfall – the annual precipitation being less than 500 mm. Hot winds blow during the day in summers and sand storms are also experienced. The night is usually cool and pleasant.
A generally clear sky, with high solar radiation causing an uncomfortable glare, is typical of this zone. As the sky is clear at night, the heat absorbed by the ground during the day is quickly dissipated to the atmosphere. Hence, the air is much cooler at night than during the day. In such a climate, it is imperative to control solar radiation and movement of hot winds. The design criteria should therefore aim at resisting heat gain by providing shading, reducing exposed area, controlling and scheduling ventilation, and increasing thermal capacity.
The presence of “water bodies” is desirable as they can help increase the humidity, lot of heat in the afternoons and evenings. As far as possible, this heat should be avoided by appropriate design features.
Warm and Humid
The warm and humid zone covers the coastal parts of the country. Some cities that fall under this zone are Mumbai, Chennai and Kolkata. The high humidity encourages abundant vegetation in these regions. The diffuse fraction of solar radiation is quite high due to cloud cover, and the radiation can be intense on clear days. The dissipation of the accumulated heat from the earth to the night sky is generally marginal due to the presence of clouds. Hence, the diurnal variation in temperature is quite low. In summer, temperatures can reach as high as 30 – 35 o C during the day and 25-30 o C at night. In winter, the maximum temperature is between 25 to 30 o C during the day and 20 to 25 o C at night. Although the temperatures are not excessive, the high humidity causes discomfort. An important characteristic of this region is the relative humidity, which is generally very high, about 70-90% throughout the year. Precipitation is also high, being about 1200 mm per year, or even more. Hence, the provision for quick drainage of water is essential in this zone. The wind is generally from one or two prevailing directions with speed ranging from extremely low to very high. Wind is desirable in this climate, as it can cause sensible cooling of the body. The main design criteria in the warm and humid region are to reduce heat gain by providing shading, and promote heat loss by maximizing cross ventilation. Dissipation of humidity is also essential to reduce discomfort.
Pune and Banglore are examples of cities that fall under this climatic zone. Areas having a moderate climate are generally located on hilly or high-plateau regions with fairly abundant vegetation. The solar radiation in this region is more or less the same throughout the year. Being located at relatively higher elevations, these places experience lower temperatures than hot and dry regions. The temperatures are neither too hot nor too cold. In summers, the temperature reaches 30 – 34 o C during the day and 17 – 24 o C at night. In winter, the maximum temperature is between 27 to 33 o C during the day and 16 to 18 o C at night. The design criteria in the moderate zone are to reduce heat gain by providing shading, and to promote heat loss by ventilation.
The composite zone covers the central part of India. Some cities that experience this type of climate are New Delhi, Kanpur and Allahabad. A variable landscape and seasonal vegetation characterize this zone. The intensity of solar radiation is very high in summer with diffuse radiation amounting to a small fraction of the total. In monsoons, the intensity is low with predominantly diffuse radiation. The maximum daytime temperature in summers is in the range of 32 – 43 o C, and night time values are from 27 to 32 o C. In winter, the values are between 10 to 25 o C during the day and 4 to 10 o C at night. The relative humidity is about 20 – 25 % in dry periods and 55 – 95 % in wet periods. The presence of high humidity during monsoon months is one of the reasons why places like New Delhi and Nagpur are grouped under the composite and not hot and dry climate. Precipitation in this zone varies between 500 – 1300 mm per year. This region receives strong winds during monsoons from the south-east and dry cold winds from the north-east. In summer, the winds are hot and dusty. The sky is overcast and dull in the monsoon, clear in winter and frequently hazy in summer. Generally, composite regions experience higher humidity levels during monsoons than hot and dry zones. Otherwise most of their characteristics are similar to the latter. Thus, the design criteria are more or less the same as for hot and dry climate except that maximizing cross ventilation is desirable in the monsoon period.
Cold and Cloudy
Generally, the northern part of India experiences this type of climate. Most cold and cloudy regions are situated at high altitudes. Ootacamund, Shimla, Shillong, Srinagar and Mahabaleshwar are examples of places belonging to this climatic zone. These are generally highland regions having abundant vegetation in summer. The intensity of solar radiation is low in winter with a high percentage of diffuse radiation. Hence, winters are extremely cold. In summer, the maximum ambient temperatures is in the range of 20 – 30 o C during the day and 17 – 27 o C at night, making summers quite pleasant. In winter, the values range between 4 and 8 o C during the day and -3 to 4 o C at night, making it quite chilly. The relative humidity is generally high and ranges from 70 – 80 %. Annual total precipitation is about 1000 mm and is disturbed evenly throughout the year. This region experiences cold winds in the winter season. Hence, protection from winds is essential in this type of climate. The sky is overcast for most part of the year except during the brief summer. Conditions in summer are usually clear and pleasant, but owing to cold winters, the main criteria for design in the cold and cloudy region aim at resisting heat loss by insulation and infiltration, and promoting heat gain by directly admitting and trapping solar radiation within the living space.
Cold and Sunny
The cold and sunny type of climate is experienced in Leh (Ladakh). The region is mountainous, has little vegetation, and is considered to be a cold desert. The solar radiation is generally intense with a very low percentage of diffuse radiation. In summer, the temperature reaches 17 – 24 o C during the day and 4 – 11 o C at night. In winter, the values range from -7 to 8 o C during the day and -14 to 0 o C at night. Winters thus, are extremely cold.
The relative humidity is consistently low ranging from about 10 – 50 % and precipitation is generally less than 200 mm per year. Winds are occasionally intense. The sky is fairly clear throughout the year with a cloud cover of less than 50%.As this region experiences cold desert climatic conditions, the design criteria are to resist heat loss by insulation and controlling infiltration. Simultaneously, heat gain needs to be promoted by admitting and trapping solar radiation within the living space.