Weather, climate and the monsoon: unity and diversity

Detail

1. Weather and climate: the basic difference

• Weather is the state of the air at one place at one moment. It is measured by five elements:
• temperature (how hot or cold)
• pressure (the weight of the air above)
• wind (its speed and direction)
• humidity (the water vapour in the air)

• precipitation (rain, snow, hail)

• Weather can change within hours. A sunny morning can turn into a thunderstorm by the afternoon.

• Climate is the average weather of a place over a long period.
• Class 11 NCERT: a change in climate shows only after 50 years or more.

• Class 7 and Class 9 NCERT: climate is the weather pattern over 30 years or more (three decades).

• World standard: the WMO (World Meteorological Organization) works out climatological standard normals (official long-term averages) over 30-year periods. The current period is 1 January 1991 – 31 December 2020. Each new normal uses the latest 30-year block that ends in a year ending in 0 [2].

• The older block 1961-1990 is still kept as the fixed baseline for studying long-term climate change [2].

• Normals work like a ruler. Forecasters measure today's weather against them and call it "normal", "above normal" or "below normal" [2].

• Seasons link weather and climate. They are the parts of the year with a typical type of weather (summer, rainy season, winter), and they come back every year in the same order.

Feature	Weather	Climate
Time scale	Hours to days	30-50+ years
Nature	Changes fast	Stable, changes slowly
Example	"Delhi is 44°C today"	"Delhi has hot summers and cool, dry winters"
Scale used	Daily forecast	Long-term averages (WMO normals 1991-2020 [2])

2. What "monsoon" means

• Monsoon: a seasonal reversal of winds. The main wind direction flips between summer and winter.
• Word origin: from the Arabic mausim, "season".

• Arab sailors and traders gave it the name. Their sailing ships used the winds to cross the Arabian Sea: the south-west wind in summer took them to India, and the north-east wind in winter brought them home.

• How the reversal works (simple chain):

• Summer: land heats faster than sea → low pressure forms over north-west India → moist south-west winds are pulled in from the Indian Ocean → heavy rain (June-September).

• Winter: land cools faster than sea → high pressure forms over the land → dry north-east winds blow from land to sea → little rain, except on the Tamil Nadu coast.

• Monsoon climate: a climate with wet summers, dry winters and a seasonal reversal of winds.

• India has a hot (tropical) monsoon climate. So does the rest of South and Southeast Asia.
• Class 7 NCERT notes that monsoon systems are also found over northern Australia and Africa.

3. Unity: one monsoon rhythm

• Unity and diversity of the monsoon climate: the whole of India follows one yearly monsoon rhythm, but the regions differ a great deal in temperature, rainfall and season timing. So there are many sub-types inside one monsoon climate.
• Sources of unity:
• The same monsoon winds reverse over the whole country.
• The whole country shares the same order of seasons: cold weather, hot weather, the south-west monsoon, then the retreating monsoon.

• The Himalaya works like a wall. It keeps the very cold Central Asian winds out in winter and traps the monsoon winds over the subcontinent in summer.

• Class 11 NCERT therefore says India's climate is "monsoonal in rhythm and character". The monsoon gives the country "a very strong framework" of unity.

• How much rain the monsoon brings: IMD measures each season against an LPA (Long Period Average: the average June-September rainfall for India as a whole).
• In 2019, India got 110% of the LPA. That LPA was 88 cm [3].
• In the same 2019 season, 5 of the 36 meteorological subdivisions still had deficient (below-normal) rain. Haryana, Delhi and Chandigarh were 42% short [3]. One rhythm does not mean the same rain everywhere.

4. Diversity (i): temperature

Contrast	Hot end	Cold / mild end
Extreme range	55°C in western Rajasthan	−45°C around Leh (Ladakh)
A June day	Churu (Rajasthan) can touch 50°C	Tawang (Arunachal Pradesh) barely reaches 19°C
A December night	Drass (Ladakh) can fall to −45°C	Thiruvananthapuram / Chennai stay at 20-22°C

• Why the temperatures differ so much:
• Latitude: the Tropic of Cancer (23.5°N) runs through the middle of India. The south is tropical; the north is sub-tropical.
• Altitude: air cools as you go higher. That is why Leh, Drass and Tawang stay cold.

• Distance from the sea (continentality): coasts stay mild because the sea warms and cools slowly. Inland areas like Churu swing to extremes.

• Daily (diurnal) temperature range: the gap between the day's highest and lowest temperature.

• Kerala and the Andaman and Nicobar Islands: only 7-8°C. The sea all around keeps day and night nearly the same.
• Thar Desert (Rajasthan): about 50°C by day and 15-20°C at night. Dry air and bare sand heat up fast by day and lose heat fast at night.

5. Diversity (ii): precipitation

• Form: snow falls in the Himalaya. The rest of the country gets rain.
• Amount: the extremes
• Mawsynram and Cherrapunji (Sohra), in the Khasi Hills, Meghalaya: over 1,080 cm a year (NCERT).
• Mawsynram's average is about 1,187 cm (467 inches) a year, most of it in the monsoon season [4]. It lies just west of Cherrapunji, and its rain may be even higher than Cherrapunji's [4].
• Jaisalmer (western Rajasthan): under 9 cm a year.

• Tura (Garo Hills, Meghalaya) can get in one day as much rain as Jaisalmer gets in ten years.

• Why Meghalaya is so wet:

• The moist Bay of Bengal branch of the monsoon flows north over the plains of Bangladesh.
• It meets the funnel-shaped Khasi and Garo hills. The winds are forced to rise.

• Rising air cools, its water vapour condenses, and very heavy rain falls on the side facing the wind (the windward side).

• Wider regional contrast:

• Under 10 cm: the north-west Himalaya (Ladakh) and the western deserts.

• Over 400 cm: Meghalaya.

• Timing and frequency:

• In July-August, the Ganga delta and the Odisha coast are hit by strong rain storms every third to fifth day. At the same time the Coromandel coast (Tamil Nadu), only about a thousand kilometres to the south, stays dry.

• Tamil Nadu gets most of its rain at the start of winter (October-December).

• Tamil Nadu's winter rain in numbers:

• During the north-east monsoon (October-December), Tamil Nadu and Puducherry get about 44 cm. This is about 48% of their annual rain [5].
• South Peninsular India as a whole gets about 30% of its annual rain in this season [5].

6. Station data: reading the climograph

• Climograph: a single chart that shows a station's average monthly temperature (a line) and rainfall (bars) together.
• Class 9 NCERT gives monthly data for ten stations, including Delhi, Kolkata, Jodhpur, Chennai, Nagpur, Shillong, Thiruvananthapuram and Leh. Students draw climographs from it.

Exam question	Answer in steps
Why does Shillong get more rain than Kolkata?	Shillong sits on the Meghalaya plateau, in the path of the Bay of Bengal branch → the hills force the moist air to rise (orographic rain: rain caused by mountains lifting the air) → more rain. Kolkata is on the flat Ganga delta, where nothing pushes the air up.
Why does Delhi get more rain than Jodhpur?	The Bay of Bengal branch moves west along the Ganga plain and gets weaker as it goes → Delhi still gets some of it. Jodhpur is in the Thar. The Arabian Sea branch runs parallel to the Aravalli range, so no hills lift it → very little rain.
Why does Chennai's rain peak after the main monsoon (October-December)?	In June-September, the Coromandel coast lies in the rain shadow (the dry side behind a mountain) of the Western Ghats, and the Bay branch runs parallel to the coast → little rain. From October, the retreating / north-east monsoon winds cross the Bay of Bengal, pick up moisture and hit the coast directly. Cyclones add more rain → the peak comes in October-November.

Prelims Hooks

• "Monsoon" comes from the Arabic mausim ("season"). Arab sailors named it because they used the wind reversal to sail to India.
• Monsoon climate = wet summer + dry winter + seasonal reversal of winds. Found in South and Southeast Asia, northern Australia and Africa.
• Highest rain → lowest: Mawsynram (~1,187 cm [4]) > Cherrapunji > Tura (Meghalaya) … Jaisalmer (<9 cm).
• Place–state pairs: Churu – Rajasthan; Tawang – Arunachal Pradesh; Drass, Leh – Ladakh; Tura – Garo Hills, Meghalaya; Mawsynram – Khasi Hills, Meghalaya.
• Smallest daily temperature range: Kerala and the Andamans (7-8°C). Largest: Thar (about 50°C by day vs 15-20°C at night).
• Trap: Tamil Nadu gets most of its rain from the north-east monsoon (October-December), about 48% of its annual rain [5], not from the south-west monsoon.
• WMO climatological standard normals = 30-year averages. The current period is 1991-2020; 1961-1990 is kept for climate-change studies [2].
• Climate change shows after 50+ years (Class 11) and climate means 30+ years of weather (Class 7/9). Do not mix up the two figures.

Mains Points

• Unity in diversity (GS-I): one monsoon rhythm (wind reversal, the same order of seasons, the Himalayan barrier) shapes farming, festivals and the economy all over India. Latitude, altitude, distance from the sea and relief create sub-climates, from the cold desert of Ladakh to wet Meghalaya and the hot, dry Thar.
• Uneven rainfall in space and time: even in a surplus year (2019: 110% of LPA), 5 subdivisions had deficient rain [3]. Drought in one region and floods in another can happen together. India therefore needs water storage, inter-basin planning and drought-proofing that suit each region.
• Regional calendars: the Coromandel coast depends on the October-December rains (about 48% of Tamil Nadu's annual rain [5]), while most of India depends on June-September. Crop plans, reservoir operation and cyclone preparedness (GS-III disasters) must follow each region's own monsoon timing.
• Why the 30-year baseline matters: IMD's "normal" and the WMO's 1991-2020 normals [2] let us separate a single bad monsoon (weather) from a lasting shift in rainfall (climate change). This is key for adaptation policy.