Climate Notes Class 9 NCERT Geography: The Notes of the chapter ‘Climate’ covers the topics – Weather and Climate, the Seasons, climatic Control and Factors affecting India’s Climate, the Indian Monsoon, the Onset of the Monsoon and withdrawal, seasons, distribution of rainfall and Monsoon as a Unifying Bond.
- Weather refers to the state of the atmosphere over an area at any given point of time i.e. variations and fluctuations may be observed over short period of time from minute to minutes and hours to hours i.e. the day-to-day changes that we experience are referred to as Weather.
- The elements of weather are temperature, atmospheric pressure, winds, humidity and precipitations.
- Common weather patterns observed are – sunshine, clouds, winds, snow, fog, rain.
- The study of weathers is called meteorology.
- It is the average of weather patterns of an area lasting for a longer time of over 30 years. variations are there but some common pattern is almost same throughout the mentioned longer duration of 30 or more years.
- Elements of climate are same as the elements of weather – wind, temperature, air pressure, precipitation and moisture.
- The two important elements of climate are temperature and precipitation varying from place to place and season to season.
- Climate is also affected by some other factors like location and relief features.
- Based on climatic differences, the world can be divided into a number of climatic regions. Each climatic region has its own characteristic vegetation and wildlife. The climatic conditions also influence the lifestyles of the people living in these regions.
- India has monsoon type climate.
- The study of climate is called climatology.
- A year is divided into seasons based on general weather conditions lasting through an average period of three months.
- Seasons are affected by the revolution of the earth and its changing distance from the sun which is responsible for the availability of sunshine on any location on the earth.
- There are four seasons – summer, winter, spring and autumn but all are not observed everywhere on the earth.
- The northern hemisphere and the southern hemispheres have opposite seasons.
- The word monsoon is derived from the Arabic word ‘mausim’ which literally means seasons.
- Monsoon refers to the seasonal reversal in the wind direction during a year.
India has monsoon type climate.
- In Asia this type of climate is mainly found in the south and the south-east Asia.
Regional Variations in Climatic Conditions in India
- Seasonal contrast is more and extreme (Continentality) in the interior of the country (away from the sea) than in the coastal areas where equable climate is found with less contrast in summer and winter seasons.
- In summer, the mercury occasionally touches 50°C in some parts of the Rajasthan desert, whereas it may be around 20°C in Pahalgam in Jammu and Kashmir.
- On a winter night, temperature at Drass in Ladakh may be as low as minus 45°C. On the other hand, in Thiruvananthapuram, it may have a temperature of 22°C.
- There are variations not only in the form and types of precipitation but also in its amount and the seasonal distribution.
- The annual precipitation varies from 400 cm in Meghalaya to less than 10 cm in Ladakh and western Rajasthan.
- Precipitation is mostly in the form of snowfall in the upper parts of the Himalayas but it rains over the rest of the country.
- Most parts of the country receive rainfall from June to September but some parts like the Tamil Nadu coast gets most of its rains during October and November.
- There is decrease in rainfall generally from east to west in the northern plains.
Effect of seasonal variations on lives of people
- These variations have given rise to variety in lives of people – in terms of the food they eat, the clothes they wear and also the kind of houses they live in.
- For example, people in Rajasthan build thick mud houses with thatched roofs to keep the interior cool, whereas people in hilly areas build wooden houses with sloping roofs to keep them warm.
Latitude: Due to the curvature of the earth insolation or sun rays are not equally focused on every latitude and air temperature decreases from the equator towards the poles.
Altitude: At higher altitudes, the atmosphere becomes less dense and temperature decreases with increasing altitudes. Hills are cooler during summers, e.g., climate of Shimla is cooler than that of Delhi.
The pressure and wind system: It depends on the latitude and altitude of the place. Thus, it influences the temperature and rainfall pattern.
Distance from the sea: As the distance from the sea increases, its moderating influence decreases and the people experience extreme weather conditions known as continentality.
Ocean currents: Any coastal area with warm or cold currents flowing past it will be warmed or cooled if the winds are onshore.
Relief: High mountains act as barriers for cold or hot winds. They may also cause precipitation if they are high enough and lie in the path of rain-bearing winds. The leeward side of mountains remains dry.
Factors Affecting Climate of India
- The Tropic of Cancer passes almost from the middle of the country.
- Almost half of the country, lying south of the Tropic of Cancer, belongs to the tropical area.
- All the remaining area in the north of the Tropic of Cancer lies in the sub-tropical area. Therefore, India’s climate has characteristics of tropical as well as sub-tropical type of climate.
- India has mountains to the north which have an average height of about 6,000 metres.
- The Himalayas prevent the cold winds from Central Asia, from entering the subcontinent.
- It is due to these mountains that the Indian subcontinent experiences comparatively milder winters as compared to Central Asia.
Pressure and winds
Pressure and surface winds, upper air circulation, and western cyclonic disturbances, and tropical cyclones are the forces that together regulate India’s climate and weather conditions.
Pressure and surface winds
- India lies in the region that is affected by north-westerly winds.
- These winds originate in the sub-tropical high-pressure belts in the northern hemisphere.
- When westerly winds blow southwards, they are deflected to the right due to Coriolis force.
- They move towards the equatorial low-pressure area.
- Westerly winds carry very little moisture as they blow over the land and do not encounter any large water bodies in their path.
- They cause very little rain over the regions where they blow.
- Coriolis Force: This force is caused by the rotation of the earth. This force deflects winds towards the right in the northern hemisphere and towards the west in the southern hemisphere. It is also known as Ferrel’s Law.
- During the winter months, high pressure prevails to the north of the Himalayas.
- Cold, dry winds blow from this high-pressure region towards the oceans in the south, where low pressure persists.
- During summer, there is a complete reversal in the direction of winds.
- Air from the high-pressure region over the oceans blows in the south-easterly direction.
- From the ocean, the winds blow to the low-pressure areas of the Indian subcontinent.
- These winds that blow from the ocean to the land are known as southwest monsoon winds.
- Since these winds blow over warm oceans, they gather enough moisture and cause the most rainfall over the mainland of the country.
Upper air circulation
- Upper air circulation in the region of southwest monsoon winds is dominated by a westerly flow.
- The jet stream is the unique feature of this circulation.
- Jet streams are found between 27°–30° north latitudes and are known as subtropical westerly jet streams.
- Throughout the year, these jet streams blow south of the Himalayas except during the summer months.
- This westerly flow brings western cyclonic disturbances to the north and north-western parts of the country.
- The subtropical westerly jet moves north of the Himalayas with the apparent movement of the sun in summer.
- During the summer months, subtropical easterly jet streams blow over 14°N in the peninsular parts of India.
Western cyclonic disturbances and Tropical Cyclones
- Western cyclonic disturbances weather phenomenon occurs in winter months.
- These disturbances are brought about by the westerly flow from the Mediterranean region.
- Western cyclonic disturbances affect the weather of the north and north-western regions of India.
- Tropical cyclones that are part of easterly flow occur during the monsoon and in October and November.
- Tropical cyclones have the greatest effects on the coastal areas of the country, such as Odisha, Andhra Pradesh, Tamil Nadu.