Why Cities Are Hotter Than They Used to Be

Rafiq Omair

Walk through downtown Toronto on an August evening, and the air around you feels heavier than it should. Cross over to a leafy residential street twenty minutes later, and you can usually feel the difference on your skin. The buildings are still warm, but the air moves more easily, and the heat is not quite pressing down on you the same way.

Cities really are hotter than the countryside around them. Sometimes by a small amount, sometimes by a startling one. And in many cities, the gap has been getting wider over the last few decades.

The urban heat island effect

The phenomenon has a name. It is called the urban heat island effect, and it describes the consistent observation that urban areas are warmer than nearby rural areas, especially at night.

The temperature difference varies a lot depending on the city, the weather, and the time of day. On clear, calm summer nights, large cities can run 5 to 10 degrees Celsius warmer than the surrounding countryside. During the day, the gap tends to be smaller, but it is still real.

This is not the same thing as global warming, although the two interact. Urban heat islands are a local effect caused by how we build and use cities. They have been documented since the 1800s in places like London.

Where does the extra heat come from?

A few things conspire to make a city warmer than a forest or a field.

Dark surfaces like asphalt roads, dark roofs, and concrete absorb a lot of solar radiation during the day and reflect very little of it. By late afternoon, these surfaces are hot to the touch and slowly release that heat into the air through the evening and night.
Less vegetation means less cooling from a process called evapotranspiration, where water from leaves evaporates and pulls heat out of the air, similar to how sweating cools your skin. A city with fewer trees and less grass loses this natural cooling.
Less water means less natural cooling, since soil and vegetation hold water that evaporates and cools the local air, while concrete and asphalt do not. A city sheds rainwater quickly through storm drains rather than letting it soak in and re-evaporate over days.
Waste heat from air conditioners, vehicles, factories, and electronics all dumps into the surrounding air. An air conditioner cools your apartment by moving heat out to the street. Multiply by ten thousand units in a downtown block, and you are space-heating the city.
Building geometry traps heat and blocks breezes. Streets become canyons that absorb sunlight from many angles during the day and release it slowly at night, while restricting airflow that would otherwise carry heat away.
Less long-wave cooling at night is another factor. Open ground radiates heat to the sky at night and cools off, but in a city, surfaces also see other buildings and other warm surfaces, so they cannot radiate as efficiently. This is a big reason the urban-rural temperature gap is largest after sundown.

None of these is particularly mysterious. They are all just consequences of replacing natural surfaces with engineered ones.

Climate change layered on top

Now stack the urban heat island effect on top of broader climate change. Globally, average temperatures have risen by more than 1 degree Celsius since pre-industrial times. Heat waves are becoming more frequent and more intense in most parts of the world.

If a city is already 5 degrees warmer than the surrounding region, and the regional climate is also warming, the city experiences both increases. This is why cities are seeing record-breaking heat events more often than they used to. It is not just that the planet is warmer. The urban version of the planet was always warmer to begin with, and now its baseline has shifted up.

Why this is a public health problem, not just a comfort issue

Heat is the deadliest weather-related risk in most developed countries, well ahead of floods, storms, or cold. Extended heat waves stress hearts and kidneys, worsen respiratory conditions, and disproportionately affect the elderly, the very young, and people with chronic illness.

Cities make this worse in three specific ways.

They never cool down at night. The body needs the night to recover from heat stress. When indoor temperatures stay above 25 degrees Celsius around the clock, sleep suffers, and the body never gets a break.
Many residents do not have air conditioning or cannot afford to run it.
Heat is unevenly distributed. Lower-income neighbourhoods often have less tree canopy, fewer parks, and more pavement, meaning the people with the fewest resources to cope are also the most exposed.

Heat also drives up demand on the electrical grid as everyone runs their AC at once, which can cause brownouts, which makes the situation worse for people who lose power.

What helps

The good news is that the urban heat island is a designed problem, which means it can be redesigned.

Cool roofs coated with light, reflective materials bounce sunlight back instead of absorbing it. A white or reflective roof can run 30 degrees Celsius cooler than a dark roof on a sunny day, and the building below uses less air conditioning.
Green roofs planted with vegetation provide insulation, capture rainwater, and add evapotranspiration cooling.
Cool pavements made from lighter coloured asphalt, permeable pavement, and reflective coatings reduce heat absorption at street level.
Tree canopy provides shade and evapotranspiration. Doubling the tree canopy in a residential block can drop peak air temperatures by several degrees on hot days.
Water features like fountains, ponds, and reflecting pools cool the air directly through evaporation.
Building orientation and ventilation matter too. Designing new districts to allow airflow through them, with breezeways and shaded corridors, helps move heat out of dense areas.

None of these are silver bullets on their own, but together they bend the urban heat curve significantly.

Why this matters for students

Heat is going to shape city planning, building design, public health, and energy policy for the rest of your career. Engineers in mechanical, civil, environmental, electrical, and architectural fields all touch on this problem.

The next time you cross from a quiet tree-lined street into a hot strip of pavement, you are walking through a real piece of physics. The materials underneath you, the shape of the buildings around you, and the choices about what to plant, where, all combine to set the temperature of the air you breathe.

Cities do not have to be hotter. They are hotter because of how we built them. That can be changed.