Cities and Urbanization

Quick Recap from Unit 8

Unit 8 ended with a section on how cities grow. Here are its main points in brief. Review "How Cities Grow" in Unit 8.

• Three sources of growth: births, people moving in and reclassification, when a town is counted as a city for the first time.
• Informal settlements: homes people build for themselves when a city grows faster than its housing.
• Sprawl: low-density building that spreads onto farmland at a city's edge.

Site and Situation

Geographers describe a city's location in two ways. Its site is the land the city stands on: the ground, the water, the hills and the climate. Its situation is where the city is compared with other places: the rivers, seas, roads and cities around it. A good site helps a town get started. A good situation helps it keep growing.

Notice the pattern. Chicago and Singapore had ordinary or cramped sites, but their situations made them great. Situations can also change. A new canal, railroad or airport can turn a quiet town into a hub, while a closed mine or a shifted trade route can leave a city shrinking.

Why Cities Grow Where They Do

Look at a map of the world's big cities, and some common reasons for their locations stand out.

• Fresh water and farmland. Early cities grew in river valleys such as the Nile, the Tigris and Euphrates, and the Indus, where farms could feed many people.
• Crossings. A shallow spot or a chain of islands made a river easy to bridge, as in Paris.
• Harbors. A sheltered bay lets ships load and unload safely. Singapore, Sydney and New York grew around natural harbors.
• Break-of-bulk points. A break-of-bulk point is a place where goods must switch from one kind of transport to another, such as from ship to train. Workers are needed to unload, store and reload the goods, so jobs and warehouses gather there. Chicago grew as one.
• Defense. Hills and islands were easier to defend, as with Tenochtitlan in its lake.
• Government. Capitals attract offices, jobs and people. Some, such as Brasília and Canberra, were planned and built to be capitals.

Once a city is big, it tends to keep growing. Jobs draw workers, workers draw shops and services, and those draw more jobs. The UN found that more than three-quarters of the 1.3 billion new city dwellers between 2000 and 2025 were added to cities that already existed, not to new ones (UN, 2025).

Primate Cities and the Rank-Size Rule

In 1939 the geographer Mark Jefferson wrote about the primate city. That is a country's leading city when it is at least twice as large as the next largest city. A primate city usually leads in money, government and culture too. Jefferson noted that Mexico City was about five times the size of Guadalajara, Mexico's second city at the time.

The rank-size rule describes a different pattern. It says the second-largest city has about half the people of the largest. The third has about a third, the fourth about a quarter, and so on. The idea is linked to the linguist George Zipf, who wrote about it in the 1940s. Real countries never fit it exactly.

Worked example: the United States

The Census Bureau counted these urban areas in 2020: New York–Jersey City–Newark had 19,426,449 people, Los Angeles–Long Beach–Anaheim had 12,237,376 and Chicago had 8,671,746.

• The rule predicts the second city has 19.4 ÷ 2 = about 9.7 million. Los Angeles has 12.2 million, more than predicted.
• The rule predicts the third city has 19.4 ÷ 3 = about 6.5 million. Chicago has 8.7 million, also more.
• New York is about 1.6 times the size of Los Angeles. That is less than double, so New York is not a primate city.

A country with a primate city, such as Mexico in 1939, has one city that stands far above the rest. A country with several strong cities, such as the United States, spreads its jobs and people more evenly.

Megacities and Metacities

A megacity has more than 10 million people. There were 8 megacities in 1975 and 33 in 2025, and the UN expects 37 by 2050. Nineteen of today's megacities are in Asia. Megacities held under 3% of the world's people in 1975 but nearly 8% in 2025 (UN, 2025). Still, they are rare: 96% of the world's 12,000-plus cities have fewer than 1 million people.

UN-Habitat has used the word metacity for a city of more than 20 million people. By the UN's 2025 estimates, each of the ten largest cities passes that mark.

1. Jakarta, Indonesia: 41.9 million
2. Dhaka, Bangladesh: 36.6 million
3. Tokyo, Japan: 33.4 million
4. New Delhi, India: 30.2 million
5. Shanghai, China: 29.6 million
6. Guangzhou, China: 27.6 million
7. Cairo, Egypt: 25.6 million
8. Manila, Philippines: 24.7 million
9. Kolkata, India: 22.5 million
10. Seoul, South Korea: 22.5 million

Why is Jakarta first and not Tokyo, which tops many older lists? In 2025 the UN started measuring cities in a new way. It uses satellite maps to find the whole area of dense, connected settlement, no matter where city lines are drawn. That gives Jakarta a much larger count. Tokyo was the largest city in 2000, with 30.3 million people. Different methods give different rankings, so always check how a list was made. By 2050 the UN expects Dhaka to be the largest, with about 52 million people, while Tokyo shrinks slightly.

Three Models of a City

In the early 1900s, sociologists and economists tried to explain how land is used inside a city. Their three famous models are simplified. All three were based on cities in the United States, and no real city matches any of them exactly. They are useful because they give you questions to ask about a city you know.

Concentric zone model (Burgess, 1925)

Ernest Burgess studied Chicago. He pictured the city as rings around a central business district (CBD), the downtown of offices and stores. Next came a zone of transition with old housing and small factories, then working-class homes, then middle-class homes, and finally a commuter zone. As a city grew, each group pushed outward into the next ring.

Sector model (Hoyt, 1939)

Homer Hoyt studied rents in many U.S. cities. He found that neighborhoods grow in wedges, or sectors, along railroads, rivers and main roads. A factory district follows the rail line out from downtown, while wealthy families keep building farther out along the most attractive corridor.

Multiple nuclei model (Harris and Ullman, 1945)

Chauncy Harris and Edward Ullman argued that a big city grows around several centers, or nuclei, not just one. A port, a university, an airport or an old town center can each attract its own mix of businesses and homes. This model fits cities that grew in the age of the car.

What do the models leave out? They say little about hills, rivers, zoning laws or public housing. They also describe North American cities. In many cities in Latin America, Africa and Asia, wealthier families live near the center, and newer, poorer neighborhoods grow at the edges.

Informal Settlements

Unit 8 introduced informal settlements. UN-Habitat uses a more exact test for a slum household. A household counts if it lacks any one of these five things:

1. Clean drinking water
2. A decent toilet
3. Enough living space, so that no more than three people share a room
4. A sturdy, lasting home
5. Secure tenure, meaning protection from being forced out

Every item on the list is a missing service or right, not a fault of the people who live there. Many residents work in the city, run small businesses and build strong communities. For this reason, many cities now upgrade settlements instead of tearing them down. They add water pipes, drains, paths, lights and legal ownership. UN Sustainable Development Goal 11 asks all countries to upgrade slums and make sure everyone has adequate, safe and affordable housing by 2030.

Suburbs, Sprawl and Edge Cities

A suburb is a mostly residential area outside a city's center. In the United States, suburbs grew fast after World War II. Cars, new highways and cheap home loans helped. Levittown, on Long Island in New York, became the symbol. Planned in 1946 and built from 1947 to 1951, it got more than 17,000 low-cost homes from a company called Levitt and Sons, using the same few house plans over and over.

Suburbs can bring yards and quieter streets. But low-density growth, or sprawl, has costs. People must drive farther, roads and pipes must stretch longer, and farmland and forest are paved over. Unit 8 showed how fast the world's built-up land has grown.

How the U.S. counts "urban"

In 2020, 80.0% of people in the United States lived in urban areas. For that census, the Census Bureau raised the test for an urban area to at least 5,000 people or 2,000 housing units. Before, it was 2,500 people. Every country sets its own test, which is one reason urban figures are hard to compare.

Edge cities

The writer Joel Garreau coined the term edge city in 1991. An edge city is a big cluster of offices and shops that grew up at the edge of an older city, usually near a highway interchange. Garreau's test was:

• at least 5 million square feet of office space;
• at least 600,000 square feet of stores;
• more jobs than bedrooms;
• people see it as one place;
• 30 years earlier, it was not a city at all, just farms or homes.

Tysons, in Virginia outside Washington, D.C., is a classic example. Garreau described how a country crossroads with a cow pasture and a gravel pit filled with office towers, malls and hotels after the Capital Beltway highway opened nearby in the early 1960s. By the late 1980s it had more office space than downtown Miami. Edge cities like Tysons are the multiple nuclei model in action.

Transit

Public transit means buses, trains, trams and ferries that anyone can ride. Transit shapes cities as much as cities shape transit. Hoyt's sectors grew along rail lines, and streetcar suburbs grew along tram lines.

Subways are expensive, so many cities have looked for cheaper choices. In 1974 Curitiba, Brazil, opened an early bus rapid transit (BRT) system. Buses ran in their own lanes, so traffic did not slow them. Curitiba also planned dense housing and shops along the bus corridors. Later, Curitiba added raised tube stations where riders pay before boarding. Bogotá, Jakarta and many other cities have built BRT systems since. Medellín, Colombia, which you met in Unit 8, uses cable cars to reach hillside neighborhoods.

Goal 11 includes a target for transit too: by 2030, give everyone access to safe, affordable and sustainable transport, with special attention to children, older people, women and people with disabilities.

Sustainable and Smart Cities

UN Sustainable Development Goal 11 is to "make cities and human settlements inclusive, safe, resilient and sustainable." In plain words: cities that work for everyone, that are safe, that can recover from disasters and that do not use up resources faster than they can be replaced.

Some ideas that help:

• Density near transit. More people can walk to a bus or train, and less farmland is paved.
• Affordable housing. In Singapore, about four out of five people live in flats built by a public agency, the Housing and Development Board.
• Green space. Parks and trees cool the air and soak up rain.
• Safe streets for walking and cycling.

A smart city uses sensors, data and connected devices to run services. Examples include traffic lights that adjust to traffic, apps that show when the next bus will arrive and meters that find water leaks. UN-Habitat says smart-city plans must put people first. That includes closing the digital divide, the gap between people who can use the internet and new technology and people who cannot. A city can be "smart" without being sustainable, and sustainable without many gadgets.

Urban Heat Islands

An urban heat island is a city area that is warmer than the land around it. Asphalt, concrete and dark roofs soak up sunlight during the day and give off heat at night. Cities also have fewer trees and less moist soil to cool the air, and cars and air conditioners give off heat of their own.

• NASA reports that cities are often 1 to 3 °C (about 2 to 5 °F) warmer than their surroundings.
• In the United States, the EPA found daytime temperatures in cities about 1 to 7 °F higher than in outlying areas, and nighttime temperatures about 2 to 5 °F higher.
• A NASA study of the whole continental United States found that areas with lots of pavement and roofs were on average 1.9 °C warmer than nearby rural areas in summer, and 1.5 °C warmer in winter.
• Not every city is warmer. NASA found that desert cities such as Phoenix can be cooler than the bare desert around them, because watered lawns and trees cool the air.

Heat islands matter because heat can make people sick, especially older people, young children and people without air conditioning. Cities fight heat with trees, parks, green roofs covered in plants and light-colored "cool" roofs and pavements that reflect sunlight.

Links to Earlier Units

• Population and Migration: how cities grow, informal settlements, sprawl and the push and pull factors that bring people to cities.
• People and Cultures: density, urban and rural life and the basics of megacities.
• Resources and Trade: ports, shipping routes and trade, which explain the situations of cities like Singapore.
• Climate and Biomes: why heat islands matter more in hot climates, and why desert cities behave differently.

Game: Put It in Order

Each round shows three things. Tap them in the right order, from first to last. Every answer comes from this page.

Quiz: Cities and Urbanization

Ten questions. Choose one answer for each, then check your score. You can try as many times as you like.

Credits

Written by Marco for Jugami's World Geography course. The map of the largest cities, the rank-size chart, the three urban model diagrams and the heat island sketch are original SVG illustrations made for this page. The map uses public-domain land shapes from Natural Earth and city figures from UN World Urbanization Prospects 2025. The chart uses U.S. Census Bureau 2020 figures. The model diagrams and the heat island sketch are simplified drawings, not real data.