The chapter notes will familiarise students with factors that affect climate and weather. The notes will also introduce students to atmospheric circulations and movement of ITCZ along with phenomena such as ElNino and Western Disturbances. We will also learn about India's climatic patterns, seasons and the arrival and retreat of monsoons in India.
Climates and Seasons of India
What is Climate?
Climate is the average weather conditions and variations over a large area for a long period of time - 30 years or more.
For example, regions near the equator are hotter as they receive direct sunlight throughout the year whereas the polar regions are cold due to oblique sun rays that fall on the surface.
Köppen's classification of Climate
Based on factors such as temperature and humidity, amount and times of precipitation and latitudinal extent of a region, a German climatologist Wladimir Köppen categorised climates into 5 major types:
Tropical
Dry
Temperate
Continental
Polar
These categories are further divided into subgroups based on the amount of precipitation such as rainforest, monsoon, savanna, steppe, desert and tundra.
A further classification level is based on the temperature and season such as hot, cold, dry winter, dry summer, wet winter, wet summer etc.
What is Weather?
Weather is the state of atmospheric conditions over an area at any point of a time. Weather conditions can fluctuate very often within a day. It could be sunny in the morning and rainy in the evening.
What are Seasons?
Weather has a common pattern over a period of a few weeks or months. This generalised monthly atmospheric condition is called a season. We have seasons such as Summer, Rainy, Winter, Autumn, Fall etc.
What are Climatic Controls?
There are six major controls of the climate of any place:
1. Latitude - The temperature generally decreases as we move from the equator towards the poles. This is due to curvature of the earth which leads to direct overhead sunlight at the equator and oblique less intense rays near the poles.
2. Altitude - At higher altitudes the atmosphere becomes thinner and the temperature decreases. This is why hills are cooler during summers.
3. Pressure and wind system - They influence the rainfall patterns and temperature of a place and vary with latitude and altitude.
4. Continentality (distance from the sea) - Sea exerts a moderating influence on weather. Places In the interiors of continents have extreme weather conditions.
5. Ocean currents : Warm or cold ocean currents along in onshore winds impact the temperature and precipitation in coastal areas.
6. Relief features : A relief feature interacts with the other controls and results in varied weather conditions. A mountain acts as a barrier for moisture laden winds causing orographic rainfall on the windward side. The other side of the mountain called the leeward side remains relatively dry and is also referred to as a rain shadow region.
Climate of India
India has a monsoon type of climate which is characterised by seasonal reversal of winds which bring rain to the Indian subcontinent. This type of weather is common in south and southeast Asia.
The apparent shifting of sun due to earth's tilt results in differential heating and precipitation across latitudes. The creation of high and low pressure zones due to differential heating drives moisture laden winds to flow towards and from the Indian subcontinent.
What type of regional climatic variations does India have?
Despite a general monsoonal pattern, there are regional variations in climatic conditions of India attributed to relief, latitude, continentality and altitude. These variations impact the lives of people in terms of the food they eat, the clothes they wear and also the kind of houses they live in.
Variations in Temperature: In summers, the temperature in India varies from 50°C in parts of Rajasthan to 20°C in Jammu and Kashmir. In winters, the temperatures in Drass could be as low as -45°C and Thiruvananthapuram could be at warm 22°C.
The coastal areas experience less contrasts in temperature conditions throughout the area, whereas variations are more in the interior of the country.
Variations in Precipitation: The annual precipitation varies from over 400 cm in Meghalaya to less than 10 cm in Ladakh and western Rajasthan. Most parts of the country receive rainfall from June to
September with advancing monsoons, but some parts like Tamil Nadu receive rain in October/November due to retreating monsoons.
There is a decrease in rainfall generally from east to west in the Northern Plains.
Factors Affecting India's Climate
1. Latitude
The Tropic of Cancer passes through the middle of the country and passes through Rann of Kuchh in the west to Mizoram in the east. It passes through Gujarat, Rajasthan, Madhya Pradesh, Chhattisgarh, Jharkhand, West Bengal, Tripura and Mizoram.
It divides the country into two climate regions; Tropical in the south and Subtropical in the north.
2. Altitude
The elevation in India varies from Himalayas having an average height of 6000 metres to its vast coastal plains having an elevation of only 30m.
Why are the winters in India milder as compared to Central Asia?
The Himalayas prevent cold winds from Central Asia from entering the Indian subcontinent thus having a moderating effect on the winters.
3. Pressure and Winds
The climate and associated weather conditions in India are governed by the following atmospheric conditions:
- Pressure and surface winds;
- Upper air circulation; and
- Western cyclonic disturbances and tropical cyclones.
Pressure and Surface Winds
India lies in the subtropical region above the equator. This region experiences north-easterly winds which originate and blow over land thus carrying little moisture. These winds blow southwards and are deflected slightly to the right due to rotation of earth. This deflection is said to have been caused by Coriolis Force.
During winters, the colder and dense air in the north of the Himalayas create a high pressure zone. These cold and dry winds blow towards the ocean and bring no rain. Their cooling effect is also moderated with Himalayas acting as barriers.
However, during summers a low pressure zone is created in interior of Asia. This causes a reversal of winds and now they flow from a high pressure zone in the southern Indian ocean. These winds flow in the south-easterly direction carrying moisture from the Indian ocean and bring rain to the Indian subcontinent. These winds are called Southwest Monsoons.
Upper Air Circulation
The differential heating of the earth's surface and its rotation results in creation of 3 cells or closed circulation loops of air in both the northern and the southern hemispheres. They act as a heat engine and help in redistribution of thermal energy over earth's surface.
Hadley Cell : It begins at the low pressure zone near the equator as hot air rises and begins to move toward the poles. As the air cools, it becomes denser and descends near 30°N and 30°S latitudes creating high pressure zones. The denser cool air now travels back to the equator closer to earth's surface completing the loop. Closer to the earth's surface the winds blow from east to west due to coriolis force. This phenomenon was used by George Hadley to explain the Trade Winds that blow between 30°N and 30°S latitudes.
Polar Cell : It exists as a conventional system of air circulation where air masses near the 60° latitudes rise and form a loop towards the poles. The loop closes as the cold denser air flows back to the 60° latitudes near the earth's surface. These are called as polar easterlies as they flow from north-east to southwest direction. Similar to the Hadley Cell, Polar Cell is also a direct consequence of surface temperatures.
Ferrel Cell : It is a secondary circulation feature which exists between the Hadley and Polar cells and derives its power from both of them. The air rises near the 60° latitudes and flows towards the tropical areas near 30° latitudes in the upper atmosphere and towards the poles near the surface. Unlike winds in Hadley and Polar cells, the winds in Ferrel cell blow from southwest to northeast thus making them westerlies. It is a weak cell as it is neither a significant heat source or sink. This is also called a 'Zone of Mixing'
What are Jet Streams?
Jet streams are a narrow belt of high altitude winds that blow from west to east due to eastward rotation of earth. They are found on margins of the 3 circulation cells at a height of around 12000 metres. They occur at margins of the circulation cells as its a boundary between hot and cold air masses. These margins keep on shifting in north-south direction due to the tilt of the earth and variable incident solar radiation that the earth's surface receives.
The jet stream at the margins of Polar and Ferrel Cell near 60° latitudes is called Polar jet stream while the ones between the Hadley and Ferrel Cells near 30° latitudes is called Subtropical westerly jet stream.
A number of separate jet streams have been identified. The most constant are the mid-latitude and the subtropical jet stream.
Why do the jet streams flow with greater speed during winters as compared to summers?
Since jet streams occur at boundaries of hot and cold air masses, the temperature difference between these masses is more pronounced in winters than in summers thereby affecting the speed and intensity of the jetstreams.
Their speed varies from about 110 km/h in summer to about 184 km/h in winter.
How do jet streams affect India's climate?
The upper air circulation over India is dominated by the subtropical westerly jet streams as they blow over 27° and 30° latitudes broadly near the northern extent of the Indian subcontinent.
During summers these subtropical westerly jet streams flow north of the Himalayas as the temperature in the northern hemisphere receives greater solar energy during the period. At the same time, another sub-tropical easterly jet stream blows over peninsular India, approximately over 14°N.
During the rest of the year and especially in winters, the subtropical westerly jet stream shifts south of Himalayas and brings with it the western cyclonic disturbances from the mediterranean region. These storms bring winter rain to the north and northwestern part of the Indian subcontinent.
Tropical cyclones on the other hand occur during the monsoon as well as in October-November, and are part of the easterly jet stream flow. These disturbances affect the eastern coastal regions of the country such as West Bengal, Odisha, Andhra Pradesh and Tamil Nadu.
Nature of Indian Monsoon
Monsoons unlike the trade winds are not steady. They are pulsating in nature since it is affected by variance in atmospheric conditions over the warm tropical seas.
India has a monsoon type climate influenced by the monsoon winds or the seasonal reversal of winds between the 20°N and 20°S latitudes. The following facts affect the intensity and nature of Indian monsoons.
- Differential Heating of Land and Water : Since land gets heated faster than water, the air above Indian landmass also gets warmer and less dense leading to development of a low pressure zone over the region. The seas surrounding India on the other hand develop a region of high pressure.
- Shifting of Inter-Tropical Convergence Zone (ITCZ) : ITCZ is a region of low pressure in equatorial latitudes . This is where the northeast and the southeast trade winds converge. This convergence zone lies more or less parallel to the equator but moves north or south with the apparent movement of the sun.
In summer, it lies over the Ganga plain and is normally positioned about 5°N of the equator. It is also known as the monsoon trough during the monsoon season.
- Presence of Mascarene High : It is a high pressure region located around 20°S latitude to the east of Madagascar. The intensity and position of this region affects the Indian monsoons.
- Formation of Low Pressure Region over Tibetan Plateau: During summer, the Tibetan plateau gets heated intensely resulting in strong vertical air currents. This creates a low pressure region at an altitude of 9 km above sea level.
- Presence of jet streams : Movement 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 summers.
- El-Nino Southern Oscillation (ENSO) - These are periodic changes in pressure conditions over the southern oceans. Normally 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.
If the pressure differences were negative, it would mean below average and late monsoons.
Why is there periodic change in pressures over southern oceans? or What is El-Nino Southern Oscillation (ENSO)?
When the cold current along the western coast of Peru is replaced by a warm current, it leads to increase in sea surface temperatures and creates a low pressure zone in the tropical eastern south Pacific ocean. This phenomenon occurs every 2-5 years and is referred to as an El-Nino Southern Oscillation.
Seasons of India
The monsoon type of climate is characterised by a distinct seasonal pattern. The weather conditions greatly change from one season to
the other.
Four main seasons can be identified in India :
1. Advancing Monsoon
2. Retreating Monsoon
3. The Cold Weather Season - Winter
4. The Hot Weather Season - Summer
Advancing Monsoons
The duration of the monsoon is between 100- 120 days from early June to mid-September. 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, and can be distinguished from the pre-monsoon showers which are milder.
Breaks in Monsoons : Monsoons have a tendency to have breaks in between. These rainless intervals are due to shifting of the ITCZ in the north-south direction.
Pre-Monsoon Showers : As summers come to a close the states of Kerala and Karnataka experience pre-monsoon showers. They help in the early ripening of mangoes, and are often referred to as ‘mango showers’.
Carrying Moisture from Southern Oceans : By early June, the Indian landmass gets extremely heated and the low-pressure condition over the northern plains intensifies. This low pressure zone attracts the trade winds of the southern hemisphere. These south-east trade winds originate over the warm subtropical areas of the southern oceans and blow towards the Indian subcontinent bringing with them abundant moisture.
Entering the Indian subcontinent : Monsoons arrive at the southern tip of the Indian peninsula from the south east direction by early June. Here it splits into two branches:
1. Arabian Sea Branch : It advances rapidly northward and reaches Mumbai by approximately 10ᵗʰ June. It reaches Saurashtra-Kachchh and the middle of the country by mid-June.
2. Bay of Bengal Branch : It moves rapidly towards Assam reaching it by first week of June and reaches Delhi by end of June.
The Arabian Sea and the Bay of Bengal branches of the monsoon are blocked by the Himalayas and merge over the northwestern part of the Ganga plains before being deflected eastwards.
By mid-July the Monsoon reaches Himachal Pradesh effectively covering the entire Indian subcontinent.
How does the weather of India change with Advancing Monsoons?
The windward side of Western Ghats receives heavy rainfall averaging more than 250 cm. The Deccan plateau and parts of Madhya Pradesh also receive rainfall in spite of lying in rain-shadow region.
Northeast India receives maximum rainfall during monsoons. Mawsynram in the southern ranges of the Khasi Hills receives the highest average rainfall in the world.
Average rainfall increases in Ganga plains from west to east with parts of Gujarat and Rajasthan receiving little rainfall. The Aravallis lying parallel to the approaching southwest monsoon do not provide a barrier to shed moisture thereby making the western edges of India arid in weather.
Why is Monsoon known for its uncertainties?
The movement of low pressure trough over plains and mountains bring good rainfall in the region. When the trough moves to mountains, they bring heavy rains causing devastating floods causing widespread damage to life and property in plains.
The amount and time of monsoons is also affected by the intensity and frequency of tropical depressions formed at the head of Bay of Bengal as they follow the monsoons into the Indian mainland. It is often irregular in its arrival and its retreat.
Distribution of Rainfall
India has high variability in levels of precipitation across the country.
High Rainfall Regions : The western coast and northeast India receive over 400 cm of rainfall annually and thus have equatorial evergreen forest cover.
Snowfall is restricted to Himalayan region.
Low Rainfall Regions : Rajasthan, parts of Gujarat, Haryana and Punjab, interior of Deccan plateau, , leeward side of western ghats (Sahyadris) the cold deserts of Leh and Ladakh .
Retreating Monsoons
Withdrawal of the monsoon is comparatively a gradual process . It begins to withdraw from northwestern states of India by early
September and by mid-October, it withdraws completely from the northern half of the peninsula. The withdrawal from the southern
half of the peninsula is however fairly rapid.
As the apparent movement of sun turns towards the south, the
monsoon trough over the northern plains also becomes weaker and is replaced by a high-pressure system.
By the beginning of October, the monsoon withdraws from the Northern Plains and by December it has withdrawn from the entire country.
What is October Heat?
The months of October-November form a period of transition from hot rainy season to dry winter conditions. As the monsoon withdraws, the skies become clear and the temperature rises along with humidity from the moist land. This harsh weather in the beginning of October is referred to as 'October Heat'.
By mid-October the temperature starts to drop rapidly as winter approaches.
How do tropical cyclones affect India?
The low pressure conditions that originate over Andaman Sea are associated with tropical cyclones. These cyclones make landfall on eastern coast of India and are often very destructive. Most of the rainfall on Coromandel Coast is derived from cyclones and depressions.
The Cold Weather Season - Winter
During this period northeast trade winds blow over the Indian subcontinent and flow from land to sea lacking moisture. They pick up little moisture as they descend southward over Bay of Bengal and bring rain to the coastal region of Tamil Nadu.
Weather in Northern India
Winter in northern India begins from mid-November and stays till February end with December and January being the coldest months.
Over the northern parts of the country the temperature varies from 10°C-15°C and the higher slopes of Himachal receive snowfall. Frost is also common.
Light winds also blow along the Ganga valley from northwest and weather is marked by clear skies, low temperature and low humidity.
Western Disturbances and Winter Rains: Winters in the north also feature western cyclonic disturbances which are low pressure systems that originate in the Mediterranean and move into India along the westerlies. They bring winter rain to northern plains which although small is of great importance for cultivation of 'Rabi crops'. This winter rain is locally called 'Mahawat'.
Weather in Peninsular India
Winters in peninsular India are not well defined. The average temperature on the coast of Chennai during winters is 24°C-25°C with little variation. This can be attributed to moderating influence of the sea.
The Hot Weather Season - Summer
During summers, the heat belt shifts northward giving summer in the north their characteristic high and rising temperatures. The temperature in peninsular India shows little variation due to moderating influence of the seas.
By the end of May a low pressure region develops from the Thar desert in the northwest to Patna in the east and the Chota Nagpur plateau in the southeast. This region also has the characteristic strong, hot, dry and gusty winds called 'loo'. These hot winds can prove to be fatal on direct contact for long periods of time.
Summers are also associated with localised thunderstorms which bring heavy rain and are often accompanied by hail. They are called as 'Kal Baisakhi' in West Bengal
