Downloaded from www.studiestoday.com CBSE Class 9 Social Science Climate Exam Notes WEATHER AND CLIMATE Weather Climate 1. It refers to short run atmospheric conditions 1. Climate is the aggregate of day to day weather that exist for a given time in a specific area. conditions over a long period of time. 2. It refers to a particular station. 2. Climate refers to a large area. 3. It may change at a very short interval of time. 3. Climate remains more or less unchanged year after year. 4. It is influenced by any one of its predominant 4. It is the collective effect of all its elements. elements i.e. temperature, humidity. CLIMATE OF INDIA The climate of India is described as of monsoon type. Derived from an Arabic word ‘mausim’, monsoon refers to the seasonal reversal in the wind direction through the year. This type of climate is found mainly in the south and the south east Asia. Despite an overall unity and commonality in the general pattern, there are perceptible regional variations in climatic conditions within the country. The temperature touches as high as 50ºC in the western desert during the summer season, whereas it drops down as low as –40ºC in Leh during the winter. Similarly, variations are noticeable not only in the type of precipitation but also in its amount. CLIMATIC CONTROLS There are six major controls of climate of any place. They are - 1. latitude 2. altitude 3. pressure and wind system 4. distance from the sea 5. ocean current 6. relief features. Latitude : Due to the curvature of the earth, the amount of solar energy received varies according to the latitude. Altitude : Air temperature decreases from the equator towards the poles. As one goes from the surface of the earth to higher altitudes. The atmosphere becomes less dense and temperature decreases. The hills are therefore cooler during summer. The Pressure and Winds : The system of any area depends, on the latitude and altitude of the place. Thus it influences the temperature and rain fall pattern. Distance from the sea : The sea exerts a moderating influence on climate. As the distance from the sea increases, its moderating influence decreases. Ocean current : Ocean current along with on shore winds affects the climate of the coastal areas. Relief : It plays a major role in determining the climate of a place. High mountains act as barriers for cold or hot winds. They may also cause precipitation. Downloaded from www.studiestoday.com FACTORS AFFECTING THE CLIMATE OF INDIA Latitude : The tropic of cancer passes through the middle of the country from the Rann of Kuchchh in the west to Mizoram in the east. Almost half of the country lying south of the tropic of cancer belongs to the tropical area. All the remaining area, north of the Tropic, lies in the subtropics. Therefore, India’s climate has tropical & subtropical climate. Altitude : India has mountains, to the north, which have an average height of about 6000 metres. India has a vast coastal area where the maximum elevation is about 30 metres. The Himalayas prevent the cold winds from Central Asia from entering the subcontinent. Because of these mountains, experiences comparatively milder winter as compared to Central Asia. Pressure and winds : The climate and associated weather conditions in India are governed by the following atmospheric condition. (A) Pressure and surface winds. (B) Upper air circulation. (C) Western cyclonic disturbances and tropical cyclone. The pressure conditions exerted by the surrounding countries like East Africa, Iran, Central Asia also influence the climate condition of India. The fury of monsoons as well as long dry spell in India are mainly due to the pressure conditions in the above mentioned countries. Winds especially Typhoons, Air current and monsoon also have a great bearing on the climate of India. Typhoons originating in the China sea also have a great bearing on the weather conditions in India. Distance from the sea : The location of seas on the three sides of India has exerted a moderate influence on much of the Indian sub continent. As the distance from the sea increases its moderating influence decreases. Physiography : It plays an important role in determining the climate of a place. The location of the Himalayas in the north has acted as a great barrier to the cold winds of the north. But for the Himalayas, India would have become a cold country. THE INDIAN MONSOON Definition : The word monsoon comes from the Arabic word ‘mausam’ which means season. Monsoon is thus rain bearing winds which reverse their direction with the change of ‘mausam’. Facts about mechanism of Monsoons : (A) The differential heating and cooling of land and water creates low pressure on the landmass of India while the sea around experiences comparatively high pressure. (B) The shift of the position of Inter Tropical Convergence Zone (ITCZ). In summer over the Ganga plain (This is the equatorial trough normally positioned about 5º N of the equator - also known as the monsoon trough during the monsoon season). (C) The presence of high pressure area, east of Madagascar approximately at 20ºS over the Indian ocean. The intensity and position of this high pressure area affects the Indian monsoon. Downloaded from www.studiestoday.com (D) The Tibetan plateau gets intensely heated during summer which results in strong vertical air currents and the formation of high pressure over the plateau at about 9 km above sea level. (E) The movements of the westerly jet stream to the north of the Himalayas and the presence of the tropical easterly jet stream over the Indian peninsula during summer. Southern Oscillation or SO : When the tropical eastern south Pacific Ocean experiences high pressure the tropical eastern Indian Ocean experiences low pressure. But in certain years, there is a reversal in the pressure conditions and the eastern Pacific has lower pressure in comparison to the eastern Indian Ocean. This periodic change in pressure condition is known as Southern Oscillation or SO. El Nino Southern Oscillations : A warm ocean current EIL-Nino, flows past the Peruvian Coast, in place of the cold Peruvian current, every 2 to 5 years. The changes in pressure conditions are connected to the El Nino. Hence the phenomenon is referred to as ENSO. The presence of the EIL-Nino leads to an increase in sea surface temperatures and weakening of the trade winds in the region. THE ONSET OF THE MONSOON The monsoon is pulsating in nature. affected by different atmospheric conditions encountered by it on way over the warm tropical seas. Around the time of its arrival the normal rainfall increases suddenly and continues constantly for several days. This is known as the 'burst of the monsoon'. The onset of the monsoon : The monsoon arrives at the southern tip of the Indian Peninsula generally by the first week of the June. Subsequently, it divides into two - 1. The Arabian sea Branch. 2. The Bay of Bengal Branch. 1. The Arabian sea Branch reaches Mumbai about 10 days later on approximately the 10th of June. This is fairly rapid advance. 2. The Bay of Bengal branch also advances rapidly and arrives in Assam in the first week of June. 3. The lofty mountains causes the monsoon winds to deflect towards the west over Ganga plains. 4. By mid June the Arabian sea branch of the monsoon arrives over Saurashtra-Kuchchh and the central part of the country. 5. Both the branches of the monsoon merge over the northwestern part of the Ganga plain. 6. Delhi receives the monsoon shows from the Bay of Bengal branch by the end of June. 7. By the first week of July, Western Uttar Pradesh, Punjab, Haryana and Eastern Rajasthan experience the monsoon. 8. By mid July the monsoon reaches Himachal Pradesh and rest of the country. WITHDRAWAL OF THE MONSOON 1. It is a gradual process. It begins in northwestern states of India by early September. 2. By mid October, it withdraws completely from the northern half of the peninsula. 3. The withdrawal from the southern half of the peninsula is fairly rapid. 4. By early December, the monsoon has withdrawn from the rest of the country. Downloaded from www.studiestoday.com THE SEASONS The year can be divided into the following four seasons on the basis of the monsoon variations. 1. The cold weather season : The cold weather persists from December to February throughout India. January is the coldest month. Temperature is between 10º C to 15ºC in northern plains and 25ºC in southern parts of the country. There is high pressure in northern plains due to the cold climatic conditions. The cold dry winds blow from the high pressure towards the low pressure equatorial regions. These winds are north westerly in the Ganga plains and north easterly over the Bay of Bengal. Only two parts of the country receive rain in this season. First is the north west part of India which receives rainfall from the Mediterranean cyclone. Second is the coast of Tamil Nadu which gets rainfall in winter. 2. The Hot Weather Season : During this period there is a rapid shifted belt of heat from south to north. In May, the rising temperature leads to a low pressure in a wider area from the Thar desert to Chhota Nagpur. Because of this low pressure, the moisture laden winds from the Arabian Sea are attracted toward this area and consequently there is rain fall. Kerala and Western Ghats also experience the pre monsoons showers. Assam and Bengal also receive some rainfall by means of evening thunderstorms. But the north west remains dry and hot winds called the loo, engulf the whole area.