Detailed Chapter Notes (Part - 2) - Climate - Practice - Class 9 PDF Download

➢ Mechanism of the Monsoon

• The word monsoon denotes a season in which the wind regime is completely reversed. The southeast winds, after crossing the equator in the Indian Ocean, take a southwesterly direction. 

• The dry and hot land bearing trades are thus completely replaced by sea bearing winds full of moisture. This phenomenon of complete reversal of winds is confined to tropical lands lying between 20^º N and 20^º S. This phenomenon account for 75 to 90 per cent of the annual rainfall just from June to September.

➢ Characteristics of the Monsoon

•  Almost all over the country, the rains occur from June to September.
• 75% to 90% of the total annual rainfall is concentrated over this period.
• There is great variation in the advance and withdrawal dates of the monsoons.
• The monsoons occur in wet spell, interspersed by dry spells.
• The amount of rainfall also varies, causing floods and drought conditions.
• “Distribution of rainfall received from the southwest monsoons is governed mainly by the relief of the country.”
• The windward side of the Western Ghats receives rainfall of over 250 cm. On the other hand, the leeward side of the Western Ghats receives less than 50cm.
• The heavy rainfall in the northeastern states can be attributed to the hill and mountain ranges.
• Rainfall in the Northern Plains decreases westward.
• “Monsoon has a tendency to have breaks in rainfall. Thus it has wet and dry spells”.
• Breaks in monsoons are related to the frequency and intensity of tropical depressions. They are formed at the head of the Bay of Bengal and cross over the mainland. 
• The depressions follow the axis of the monsoon trough of the low pressure. For various reasons, the trough and its axis keep on moving northward or southward, which determines the spatial distribution of rainfall. 
• When the axis of the monsoon trough lies over the plains, rainfall is good in these parts. On the other hand, whenever the axis shifts closer to the Himalayas, there are longer dry spells in the plains and widespread rains in the mountainous catchment area of the Himalayan Rivers.
• “Rainfall decreases from east to west in the Northern Plains while it increases in Peninsular India”.

➢ Pattern of Rainfall in the Northern Plains

• The Northern Plains get much of their rainfall by Southwest Monsoons which strike the eastern part of the country first and give heavy rainfall there. So eastern parts of the country like Assam, Meghalaya, Bengal etc. get much rainfall.
• Then the monsoons arising from the Bay of Bengal move westwards along the Himalayas. Their capacity to cause rain become lesser and lesser as they move westward because they continue to become drier and drier.

➢ Pattern of Rainfall in Peninsular India

• The Peninsular India also gets much rainfall because of another branch of the South-West Monsoons which rises from the Arabian Sea. These monsoon winds first of all strike the Western Ghats and cause much rain there.
• These winds while reaching the other side of the Western Ghats, become dry and cause less rain. As they go on moving to the eastern side, they become drier and drier and so the rain goes decreasing from west to east.

➢ Mumbai receives rainfall in summer while Chennai has in winter 

• Mumbai receives more rainfall in summer as it is situated on the Arabian Sea coast, and receives all its rains from the Arabian Sea branch of the southwest monsoons from June to September only. 
• Chennai receives two-third of the rainfall from the retreating southwest monsoons aided by cyclones from October to December.

➢ Western Rajasthan has the desert type of climate

• Western Rajasthan lies in the rain shadow areas of the Aravalli Mountains. (Leeward side).
• Arabian sea branch of S.W. monsoons blows parallel to the Aravalli range; hence the Aravallis fail to check it.
• By the time the Bay Bengal branch reaches here it is almost dry; moreover it lies on the leeward side of the Aravallis.
• Monsoon winds become warmer and increase their capacity to hold moisture instead of causing rain.

4. Retreating Monsoons

During October-November, the monsoon trough of low pressure becomes weaker and is gradually replaced by high pressure. The outreach of the monsoon becomes unsustainable and it starts withdrawing gradually. This is known as the retreat of monsoon.

➢ October Heat

• The retreat of the monsoons is marked by clear skies and rise in temperature.
• The land is still moist.
• Owing to the conditions of high temperature and humidity, the weather become oppressive, which is known as October heat in northern India.

➢ Cyclonic Depressions which originate over the Andaman Sea

• In October and November, the shift of the low pressure from land to sea is far from smooth. The period is associated with the occurrence of cyclonic depressions which originate over the Andaman Sea.
• The cyclones affect the eastern coasts of southern peninsular.
• The cyclones affect the Sundarban Delta, Mahanadi, Godavari, Krishna and Kaveri deltas too.
• These tropical cyclones cause heavy and widespread rain.
• One adverse effect of the cyclones is, it is very often destructive. No year is found disaster-free, it affects one or the other deltas of the Eastern Coast.

➢ Vagaries of the Monsoons in India

• At times the monsoons come in full swing, or it may fail altogether. Thus, causing the twin problems of floods and famines. 
• The alternation of dry and wet spells keeps on varying in intensity, frequency and the duration.
• Implications of vagaries of the monsoons:
  (i) Due to the late arrival, the crops dry up.
  (ii) Due to excessive rains, floods are caused, leading to destruction.
  (iii) The amount of rain may vary, causing drought conditions.
  (iv) Sometimes the monsoons come early, normal or late.
  (v) The monsoons may retreat early, normal or late.

➢ Distribution Of Precipitation In The Country

• Areas of Heavy Rainfall: Areas which get rainfall of 200 cms and above are Assam, the Ganga Delta, the Western Ghats and the Western Coastal regions and the mountainous regions of Himachal Pradesh.

• Areas of Moderate Rainfall: Areas which get annual rainfall between 100 cms to 200 cms are Madhya Pradesh, Orissa, Chhota Nagpur, Western Bengal, Bihar, Eastern U.P, North-Eastern Punjab; Eastern parts of Tamil Nadu and Eastern slopes of Western Ghats receive moderate rainfall.
• Areas with Low Rainfall: Areas receiving annual rainfall between 50 cms to 100 cms are the Deccan Plateau, Western U.P, South-Eastern Punjab, Eastern Rajasthan and parts of Kashmir get low rainfall.
• Areas with Scanty Rainfall: Areas which get less than 50 cms of rainfall annually are western Rajasthan, Kutch, Southeastern parts of Haryana, Northeastern Kashmir get scanty rainfall.