Chapter 3: Atmosphere and Climate (Geography)

The atmosphere is the blanket of air that surrounds the Earth and is held to it by gravity. It is a mixture of gases, water vapour and dust particles. It protects us from the Sun's harmful (ultraviolet) radiation, keeps the Earth warm by trapping heat, and is where all weather happens.

Composition (by volume):

• Nitrogen: 78% (the most abundant gas; important for plants)
• Oxygen: 21% (needed for breathing and burning)
• Argon: 0.93%
• Carbon dioxide: 0.04% (traps heat; used by plants for photosynthesis)
• Others: 0.03% (helium, neon, krypton, xenon, ozone, hydrogen)
• Water vapour: varies from 0.1% to 0.4%; it forms clouds and rain. There are also tiny dust particles.

The atmosphere is the layer of air around the Earth held by gravity. It is 78% nitrogen, 21% oxygen and about 1% other gases (argon 0.93%, CO₂ 0.04%, others 0.03%), plus variable water vapour and dust.

The atmosphere is divided into five layers on the basis of changes in temperature and density with height.

From the ground up: troposphere (to ~12 km), stratosphere (to 50 km, ozone layer), mesosphere (to 80 km), thermosphere (80–700 km, ionosphere) and exosphere; separated by the tropopause, stratopause and mesopause.

The India Meteorological Department (IMD) recognises four seasons:

• Winter (cold weather) season: December to early April; December and January are the coldest months.
• Summer or pre-monsoon (hot weather) season: April to June (up to July in north-west India).
• Monsoon or rainy season (advancing monsoon): June to September, dominated by the south-west monsoon.
• Post-monsoon season (retreating monsoon): October to December.

Winter, summer (pre-monsoon), monsoon (rainy, advancing monsoon) and post-monsoon (retreating monsoon).

The air above us does press on our bodies with a large force. We do not feel it because:

• air presses on us equally from all sides, so the pushes balance one another; and
• the pressure inside our bodies (in the blood and body fluids) is equal to the atmospheric pressure outside and cancels it.

Because air presses on us from all sides and the pressure inside our bodies is equal to the outside air pressure, so the forces balance.

Aeroplanes (jet planes) fly in the lower stratosphere, just above the tropopause (about 10–12 km and above), because:

• the stratosphere is free of clouds, rain, storms and other weather disturbances, which occur in the troposphere;
• the air is calm and stable, without vertical air currents, so the flight is smooth;
• the thinner air causes less resistance (drag), which saves fuel.

In the (lower) stratosphere, because it is free of clouds and weather disturbances, so flying is smooth and safe.

a.

b.

(a) Troposphere: lowest layer, temperature falls with height, all weather; stratosphere: up to 50 km, ozone layer, no weather, good for flying. (b) SW monsoon: June–Sept, moist sea-to-land winds, most rainfall; NE monsoon: Oct–Feb, dry land-to-sea winds, rain only on the south-east coast.

For each station, draw the months on the x-axis, rainfall as bars (scale on the right) and temperature as a line (scale on the left), as in Fig. 3.14. Example for Chennai:

Broad generalisations from the 10 graphs:

• Most stations (Mumbai, Kolkata, Delhi, Jodhpur, Nagpur, Shillong, Bengaluru) get most of their rain from June to September (south-west monsoon), and winters are dry.
• Chennai is different: it gets most rain in October–December (retreating/north-east monsoon).
• Coastal and southern stations (Thiruvananthapuram, Mumbai, Chennai) have a small temperature range; inland northern stations (Delhi, Jodhpur) have hot summers and cool winters, i.e. a large range.
• Altitude lowers temperature: Shillong and Bengaluru are cooler than places on the plains, and Leh (3506 m) is very cold, with sub-zero winters.
• Rainfall decreases from the east and the west coast towards the north-west: Shillong (225 cm) and Mumbai (183 cm) get far more than Delhi (67 cm), Jodhpur (37 cm) and Leh (8.5 cm).

Draw a bar-and-line climograph for each station. Generally, rain comes mostly in June–September (except Chennai, October–December), coastal places have equable temperatures, inland places have extremes, and rainfall decreases towards the north-west.

Arranged by latitude, from nearest to farthest:

Thiruvananthapuram, Bengaluru, Chennai, Mumbai, Nagpur, Kolkata, Shillong, Jodhpur, Delhi, Leh.

a. The annual range of temperature (hottest month − coldest month) is largest at Leh (17.2 − (−8.5) = 25.7°C) and then Jodhpur (33.9 − 14.9 = 19.0°C), closely followed by Delhi (33.3 − 14.4 = 18.9°C). Leh and Jodhpur have the most extreme climate.

b. Chennai (heaviest rain in October 30.6 cm and November 35.0 cm) and Thiruvananthapuram (a second rainfall peak in October–November, 27.3 cm and 20.6 cm).

c. (i) February: Thiruvananthapuram (27.3°C) and Chennai (25.7°C). (ii) June: Jodhpur (33.9°C) and Delhi (33.3°C).

(a) Leh and Jodhpur (Delhi is almost as extreme) (b) Chennai and Thiruvananthapuram (c) February: Thiruvananthapuram and Chennai; June: Jodhpur and Delhi.

a. Shillong is on the Meghalaya plateau, at 1461 m, facing the moist winds of the Bay of Bengal branch of the monsoon. The hills force the winds to rise, cool and condense, giving heavy orographic (relief) rainfall. Kolkata lies on the flat delta plains, where the winds are not forced to rise as much.

b. Delhi receives the Bay of Bengal branch of the monsoon, which moves up the Ganga plains and still carries some moisture when it reaches Delhi. Jodhpur lies in the Thar desert, far from the Bay; the Arabian Sea branch blows parallel to the Aravalli hills, so it is not forced to rise and gives little rain. By the time the winds reach Jodhpur, most of their moisture has been used up.

(a) Shillong's hills force moist Bay of Bengal winds to rise and shed rain (orographic rain); Kolkata is on flat plains. (b) Delhi gets the Bay of Bengal branch along the Ganga plains; Jodhpur is in the Thar desert, where the winds pass parallel to the Aravallis with little moisture left.

a. Thiruvananthapuram is close to the equator (8°29′N) and on the sea coast. The sun is high all year, and the sea keeps the air from getting very hot or cold. So the temperature stays between about 26°C and 29°C all year (a range of only about 2.5°C): an equable climate.

b. Chennai lies on the Coromandel coast. During June–September it is in the rain-shadow of the Western Ghats for the Arabian Sea branch and the Bay of Bengal branch blows roughly parallel to the coast, so it gets little rain. In October–December, the retreating (north-east) monsoon winds pick up moisture over the Bay of Bengal and bring heavy rain (and cyclones) to the Tamil Nadu coast.

c. Leh is a cold desert in the rain-shadow of the Himalaya, so its total precipitation is very low (8.5 cm a year). But the little it gets is spread fairly evenly through the year (0.5–1.3 cm each month): in winter it comes as snow from western disturbances, and in summer from weak remnants of the monsoon.

(a) Nearness to the equator and the sea keeps temperatures nearly the same all year. (b) Chennai is sheltered from the SW monsoon and gets its rain from the NE (retreating) monsoon in Oct–Dec. (c) Leh is a rain-shadow cold desert: very little precipitation, but spread evenly (winter snow from western disturbances, weak summer rain).

Yes. The monsoon gives the whole country a common rhythm:

• Same seasonal pattern: almost every station gets most of its rain in a few months (June–September; for Chennai, October–December), with a dry period in winter and spring. The table shows this for 9 of the 10 stations.
• Seasonal reversal of winds: all of India experiences the change from the south-west monsoon in summer to the north-east monsoon in winter.
• Common seasons: the four IMD seasons (winter, summer, monsoon, post-monsoon) are found across the country.
• The Himalaya block cold winds from Central Asia and trap the monsoon winds, so the whole subcontinent has a monsoon type of climate.
• Life follows the monsoon everywhere: sowing and harvesting (kharif and rabi crops), water supply in rivers and reservoirs, and festivals are all linked to its arrival and retreat.

Yes: nearly all stations share one rhythm of a wet season and a dry season, with seasonal reversal of winds and the same four seasons; agriculture and life everywhere depend on the monsoon. This gives India climatic unity despite regional differences.

They clearly do. Some examples to collect and compare:

Conclusion: houses and clothing are adapted to temperature, rainfall, snowfall and the materials available in the relief of each region.

Yes: thick-walled, small-windowed houses and loose cotton clothes in hot deserts, sloping roofs and raised floors in rainy areas, stone-and-wood houses and woollens in the cold mountains show how people adapt to climate and relief.