Temperature and the factors controlling it

Detail

Heat and temperature: two different things

• Heat is the energy of moving molecules. The faster the molecules in a body move, the more heat it holds.
• Temperature is the measure, in degrees (°C or °F), of how hot or cold something is.

• Heat is the energy. Temperature is the reading on the thermometer.

• How the air gets its heat:

• Sunlight (insolation) passes through the air and heats the ground first.
• The ground then warms the air above it by conduction, convection and long-wave (terrestrial) radiation.
• So the air is heated from below, not from the top. This is why it is usually warmer at the surface than higher up.

Factors controlling temperature distribution

Factors controlling temperature distribution are the conditions that decide how hot or cold a place is. There are six main ones.

Factor	How it works	Example
Latitude	Near the equator the Sun is high and insolation is strong. Towards the poles the rays are slanting and weak.	Thiruvananthapuram (~8.5°N) is warmer in winter than Amritsar (~31.6°N)
Altitude	Temperature falls with height (lapse rate)	Darjiling is colder than Agra
Distance from the sea	The sea keeps coasts mild. Interiors become extreme.	Mumbai (mild) vs Delhi (extreme)
Air masses	Warm or cold air masses bring their own temperature with them	Cold winds from the north-west chill north India in winter
Ocean currents	Warm currents heat coasts. Cold currents cool them.	Norway (warm), Peru (cold)
Local aspects	Slope, relief and land cover change the heat a place receives and keeps	Sun-facing slopes, valley floors, cities

Altitude and the normal lapse rate

• Normal lapse rate (also called the environmental lapse rate): the rate at which temperature falls as you go up in still air. It is 6.5°C per 1,000 m, or about 1°C for every 165 m.
• Why temperature falls with height:
• The air is heated from below by the earth's surface.
• Higher up, you are farther from this heat source.

• The air is also thinner and holds less water vapour and dust, so it traps less heat.

• Indian example (same latitude, different height):

• Darjiling (West Bengal, about 2,000 m, ~27°N) averages about 4°C in January.
• Agra (Uttar Pradesh, ~27°N, on the plains) averages about 16°C in January.

• The gap of about 12°C comes mostly from height, not latitude.

• Hill stations have cool summers because they are high up: Shimla (Himachal Pradesh), Ooty (Nilgiris, Tamil Nadu) and Shillong (Meghalaya).

• Leh (Ladakh, about 3,500 m) has very cold winters, with monthly means well below 0°C. It is also dry and far from the sea (Class 9, Table 3.3).
• Exception: in a temperature inversion, temperature rises with height for a while instead of falling. This happens on long, clear winter nights and in valleys.

Adiabatic lapse rates and stability (beyond NCERT)

• Adiabatic lapse rate: the rate at which a rising parcel of air cools only because it expands, without gaining or losing heat to the air around it. "Adiabatic" means no heat comes in or goes out.
• Process: cause → process → result
• Air rises → air pressure falls with height → the parcel expands.
• Expansion uses up the parcel's own energy → the parcel cools.
• When air sinks, the opposite happens: it is squeezed and warms up.

Rate	Value	Why
Normal (environmental) lapse rate	6.5°C per km	Real temperature of the still air around the parcel
Dry adiabatic lapse rate	~9.8°C per km	Unsaturated air cools only by expanding
Saturated (wet) adiabatic lapse rate	~4–7°C per km	Condensation releases latent heat (hidden heat stored in water vapour), which slows the cooling

• Stability (compare the surrounding air with the rising parcel):
• Stable air: the surrounding air cools more slowly than the rising parcel. The parcel soon becomes colder and heavier than its surroundings, so it sinks back. The sky stays clear, or there are flat layer clouds.

• Unstable air: the surrounding air cools faster than the parcel. The parcel stays warmer and lighter, so it keeps rising. This builds tall cumulonimbus clouds, thunderstorms and pre-monsoon storms such as the Kal Baisakhi (Nor'westers) of West Bengal.

• Adiabatic processes are covered in detail in atmospheric-moisture-precipitation.

Distance from the sea: continentality vs equable climate

• Why land and sea behave differently:
• Water has a higher specific heat (the heat needed to raise 1 g of a substance by 1°C) than land. So the sea warms and cools slowly.
• Sunlight passes deep into water, and waves and currents mix the heat down. On land, heat stays in a thin top layer.
• Water also loses heat through evaporation.

• Result: land heats and cools fast, and the sea heats and cools slowly.

• Sea and land breezes carry this effect to the coast:

• By day, the land is hotter → low pressure over land → a cool sea breeze blows in.
• By night, the land is cooler → a land breeze blows out to sea.

• So coasts never get very hot or very cold.

• Continentality: the effect of being far from the sea. Inland places have large daily and yearly ranges of temperature (hot summers, cold winters, hot days, cold nights).

• India: Delhi, Kanpur, Amritsar; Jodhpur (Thar desert, Rajasthan) and Leh (Class 9, Table 3.3).

• World: Siberia (Russia) has one of the largest yearly ranges on Earth.

• Equable climate (also called maritime climate): a climate with only a small gap between summer and winter.

• India: Mumbai, the Konkan coast, Kerala, the Andaman and Nicobar Islands.
• Thiruvananthapuram (Kerala, ~8.5°N): monthly mean temperatures stay near 26–29°C all year (Class 9, Table 3.3). The latitude and the sea work together here.

Feature	Continental (interior)	Maritime / equable (coast, island)
Daily range	Large	Small
Yearly range	Large	Small
Indian examples	Delhi, Kanpur, Amritsar, Jodhpur, Leh	Mumbai, Konkan, Kerala, Andamans
World example	Siberia	Coast of north-west Europe

Air masses and ocean currents

• Air mass: a very large body of air that has nearly the same temperature and moisture throughout. It takes these from the region where it formed (its source region).
• Warm air masses raise the temperature of the places they move over. Cold air masses lower it.

• Example: cold waves in north India in winter come with cold, dry air from the north-west and from behind western disturbances.

• Warm ocean currents raise coastal temperatures:

• The Gulf Stream starts near Florida (USA) and flows up the east coast of North America. Its extension, the North Atlantic Drift, flows across the Atlantic to north-west Europe.

• These currents keep Norway's coast ice-free in winter, even though much of it lies north of 60°N.

• Cold ocean currents cool coasts and dry the air above them:

Cold current	Ocean	Coast it cools	Linked coastal desert
Humboldt (Peru) Current	South Pacific	Peru, Chile	Atacama
Benguela Current	South Atlantic	Namibia, South Africa	Namib
California Current	North Pacific	West coast of the USA	Sonoran / Baja California deserts

• How a cold current makes a desert:
• Cold water cools the air near the surface.
• Cool air under warm air makes the air stable, so it does not rise.
• The result is fog but almost no rain, so dry deserts form along these coasts.

Local controls: slope, relief and land cover

• Slope and aspect (aspect means which direction a slope faces):
• In the northern hemisphere, south-facing slopes get more sunlight and are warmer.

• In the Himalaya, villages, farms and orchards are often on the sunny south-facing slopes (called adret slopes).

• Relief: on calm winter nights, cold heavy air drains down into valleys. Valley floors become colder than the slopes and get frost and fog (temperature inversion).

• Land cover: forests, water bodies and wet soil stay cooler. Bare rock, sand and concrete get hotter.

Microclimate (beyond NCERT)

• Microclimate: the climate of a small area, such as a valley floor, a forest, a lake shore or a city block, that differs from its region in temperature, humidity or wind.
• Why it matters:
• Crops: frost pockets on valley floors damage tea, fruit and vegetables. Orchards are planted on slopes instead.
• Frost: cold air settles at the lowest point.
• Comfort and health: shaded, green and windy places stay cooler in a heat wave.

Urban heat island (beyond NCERT)

• Urban heat island (UHI): a city is markedly warmer than the countryside around it, often by several degrees. The difference is greatest at night.
• Causes:
• Concrete and asphalt absorb sunlight by day and give the heat back slowly at night. Unshaded roads and buildings gain heat during the day and radiate it into the air around them [4].
• Little vegetation and water, so there is little cooling by evaporation.
• Tall buildings trap radiation in "street canyons" and block the wind.

• Waste heat comes from vehicles, industry and air-conditioners. An air-conditioner cools the inside of a room but sends more heat outside.

• Result: hotter nights, a higher risk of heat stroke, more demand for power, and worse air pollution.

• Heat waves in India. The IMD (India Meteorological Department) uses these criteria [3]:
• A heat wave is considered when the maximum temperature reaches at least 40°C in the plains, 37°C in coastal areas or 30°C in hilly regions [3].

• Based on departure from normal, it is a heat wave when the departure is 4.5–6.4°C and a severe heat wave when it is more than 6.4°C [3].

• Responses:

• Ahmedabad Heat Action Plan (2013): the first in South Asia, and the first municipal heat action plan in India. It became a model for other Indian cities [3].
  • It covers early warning, public awareness, training of health workers and cooling measures.
• NDMA guidelines for preparing heat-wave action plans (NDMA: National Disaster Management Authority, India's top body for disaster management, headed by the Prime Minister). They name the urban heat island and cool roofs as key issues for cities [3].
• Cool roofs (white or reflective roof surfaces that reflect sunlight and keep buildings cooler), as in the Telangana Cool Roof Policy (2023) [3].
• Urban greening (trees, parks, green roofs) and water bodies, which cool by shade and evaporation.
• This links to human-settlements and urban planning.

Prelims Hooks

• Normal (environmental) lapse rate = 6.5°C per 1,000 m (about 1°C per 165 m). It is not 9.8°C, which is the dry adiabatic rate.
• Ordering by rate of cooling, highest to lowest: dry adiabatic (~9.8°C/km) > normal lapse rate (6.5°C/km) > saturated adiabatic (~4–7°C/km).
• The saturated rate is lower than the dry rate because condensation releases latent heat.
• Darjiling and Agra are at about the same latitude (~27°N), but January means are about 4°C and about 16°C. The difference is due to altitude.
• Thiruvananthapuram has a small yearly range (monthly means about 26–29°C). Leh and Jodhpur have large ranges because of continentality.
• Warm current–effect pair: Gulf Stream and North Atlantic Drift → ice-free Norwegian coast.
• Cold current–desert pairs: Humboldt–Atacama, Benguela–Namib, California–Sonoran/Baja.
• The urban heat island is strongest at night, not at midday.
• First heat action plan in South Asia: Ahmedabad (2013). Cool Roof Policy: Telangana (2023).
• IMD heat-wave threshold: 40°C in the plains, 37°C on the coast, 30°C in the hills. A departure of more than 6.4°C = severe heat wave [3].

Mains Points

• Controls of temperature across India: latitude sets the north–south gradient. Altitude makes the Himalaya and the Western Ghats cooler. Continentality gives the north-west plains (Delhi, Amritsar, Jodhpur) extreme ranges. The sea gives the west coast, Kerala and the islands an equable climate. Together they explain why heat waves mainly hit the north-west and central plains (GS-I).
• Atmospheric stability as a weather control: an environmental lapse rate steeper than the parcel's rate makes air unstable, with thunderstorms and Nor'westers. Cold currents and night-time inversions make air stable, with fog, smog and coastal deserts. Useful for questions linking temperature with rainfall and pollution (GS-I, GS-III).
• Urban heat island as a disaster-risk problem: concrete, loss of greenery and waste heat make city nights hotter. This raises heat-stroke deaths, power demand and harm to outdoor workers and slum dwellers (GS-III).
• Policy responses: heat action plans (Ahmedabad 2013, NDMA guidelines), IMD early warnings, cool roofs (Telangana 2023), urban forests and the revival of water bodies. Gaps remain in funding, in night-time heat thresholds and in reaching the most vulnerable people (GS-III).