Urban heat island

Also called: UHI · Topic: Solar Radiation, Heat Balance and Temperature · NCERT: Class 7, Ch 3 "Climates of India"

Meaning

An urban heat island (UHI) is a city that is clearly warmer than the countryside around it, often by several degrees. The gap is largest at night. It happens because concrete and asphalt store heat, there is little greenery and water, tall buildings trap heat and block wind, and vehicles, industry and air-conditioners give off waste heat.

It matters because hotter city nights raise the risk of heat stroke, push up power demand and make air pollution worse. Indian cities now plan for it in their heat-wave action plans.

Explanation

How it forms (cause → process → result)

• Concrete and asphalt store heat
• By day, roads, roofs and walls absorb sunlight.
• At night, they give this heat back slowly to the air.

• Unshaded roads and buildings gain heat during the day and radiate it into the air around them [3].

• Little vegetation and water

• Trees, wet soil and water bodies cool the air by evaporation (water turning into vapour takes in heat).

• Cities have few of these, so there is little cooling by evaporation.

• Tall buildings form "street canyons"

• Heat given off by one wall is caught by the next wall instead of escaping to the sky.

• The buildings also block the wind, so the hot air is not carried away.

• Waste heat

• Vehicles, factories and air-conditioners all release heat.

• An air-conditioner cools the inside of a room but sends more heat outside.

• Result: the city stays warm after sunset, while the countryside cools quickly.

Why it is strongest at night

• By day, both the city and the countryside are heated by the Sun.
• After sunset, the countryside loses its heat quickly.
• The city keeps releasing the heat stored in concrete and asphalt, and waste heat keeps coming from machines.
• So the gap between city and countryside is greatest at night, not at midday.

Where it fits among the controls of temperature

• Temperature depends on six main factors: latitude, altitude, distance from the sea, air masses, ocean currents and local aspects.
• The UHI is a local aspect. It comes from land cover:
• Forests, water bodies and wet soil stay cooler.

• Bare rock, sand and concrete get hotter.

• A UHI is a type of microclimate (the climate of a small area that differs from its region in temperature, humidity or wind). Here the "small area" is a city.

• Recall that air is heated from below. The ground warms the air by conduction, convection and long-wave (terrestrial) radiation. A city's hot surfaces therefore heat the air above them.

Effects and links to heat waves

• Effects: hotter nights, a higher risk of heat stroke, more demand for power and worse air pollution.
• People hurt most are outdoor workers and slum dwellers.
• IMD (India Meteorological Department) heat-wave criteria [2]:
• 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 [2].

• Heat wave: departure from normal of 4.5–6.4°C. Severe heat wave: departure of more than 6.4°C [2].

• A UHI makes a heat wave worse inside a city, because nights do not cool down enough for the body to recover.

Where it occurs

• Cities everywhere in the world. Wherever concrete, asphalt, dense buildings and traffic replace fields and forests, the city becomes warmer than its surroundings.
• India: the north-west and central plains. Heat waves mainly hit these regions because of continentality (the effect of being far from the sea). Cities here get both the regional heat wave and their own extra urban heat.
• Ahmedabad (Gujarat): the Ahmedabad Heat Action Plan (2013) was the first in South Asia and the first municipal (city-level) heat action plan in India. It became a model for other Indian cities [2].

• It covers early warning, public awareness, training of health workers and cooling measures.

• Telangana: the Telangana Cool Roof Policy (2023) promotes cool roofs (white or reflective roofs that reflect sunlight and keep buildings cooler) [2].

• All of India: NDMA guidelines. The NDMA (National Disaster Management Authority, India's top body for disaster management, headed by the Prime Minister) has guidelines for preparing heat-wave action plans. They name the urban heat island and cool roofs as key issues for cities [2].

Don't confuse with

• Heat wave: a heat wave covers a whole region for some days and is defined by IMD temperature thresholds [2]. A UHI is a city-only effect that comes from land cover and is present all the time, strongest at night.
• Temperature inversion: in an inversion, temperature rises with height for a while. It forms on long, clear winter nights and in valleys, where cold air settles at the bottom. A UHI is about a warm city compared with cool countryside, not about change with height.
• Continentality: this is a regional effect of being far from the sea, giving large daily and yearly ranges (Delhi, Jodhpur, Leh). A UHI is a local effect of concrete and waste heat, and it reduces night-time cooling.
• Microclimate: this is the wider idea, covering valley floors, forests, lake shores and city blocks. A UHI is one type of microclimate.

Prelims Hooks

• The urban heat island is strongest at night, not at midday. This is a common trap.
• Causes to tick: concrete and asphalt, little vegetation and water, street canyons, waste heat from vehicles, industry and air-conditioners. An air-conditioner sends more heat outside.
• First heat action plan in South Asia: Ahmedabad (2013). It was also the first municipal heat action plan in India [2].
• Cool Roof Policy: Telangana (2023) [2].
• IMD heat-wave threshold: 40°C in the plains, 37°C on the coast, 30°C in the hills. A departure from normal of more than 6.4°C = severe heat wave [2].
• Among the factors that control temperature, the UHI comes under local aspects (land cover). It does not come under latitude or altitude.

Mains Points

• UHI as a disaster-risk problem (GS-III): concrete, loss of greenery and waste heat keep city nights hot.
• This raises heat-stroke deaths and power demand.
• It hurts outdoor workers and slum dwellers most.

• It adds to the heat waves of the north-west and central plains.

• Policy responses (GS-III):

• Heat action plans: Ahmedabad (2013), and NDMA guidelines that name the urban heat island and cool roofs [2].
• IMD early warnings.
• Cool roofs, as in the Telangana Cool Roof Policy (2023) [2].

• Urban greening (trees, parks, green roofs) and the revival of water bodies, which cool the city through shade and evaporation.

• Gaps and the way forward (GS-I/GS-III):

• Plans lack enough funding.
• There are no proper thresholds for night-time heat, even though the UHI is worst at night.
• Help often does not reach the most vulnerable people.
• Urban planning should keep green cover and water bodies and limit waste heat.

Related concepts

• Factors controlling temperature distribution
• Normal lapse rate
• Adiabatic lapse rate
• Continentality
• Equable climate
• Microclimate

Read more

• Temperature and the factors controlling it