Climate & Cities
Cool surfaces work and they come with a glare problem
Reflective pavements lower surface temperature by sending radiation back. Some of it goes back into the faces of people walking past.
This is written to be used rather than admired. Each section below is a decision about reflective surfaces, and each one has a default.
Before you start
- High-albedo surfaces reduce stored heat and reflect radiation onto adjacent surfaces.
- Pedestrian thermal comfort depends on radiation received, not only on air temperature.
- Cool roofs avoid this issue because there is usually nobody standing on them.
The mechanism is straightforward
A dark surface absorbs most incoming solar radiation and converts it to heat, raising its temperature well above air temperature. A light surface reflects a larger share, so it stores less heat and releases less of it overnight. Applied across large areas of roof and pavement, this measurably reduces the urban heat island effect.
Cool roof programmes in several cities have reported reductions in building cooling demand as well.
Reflected radiation has to go somewhere
Radiation bounced off a bright pavement travels upward and sideways, striking building facades and the people standing on the street. Pedestrian thermal comfort depends on the total radiation exchange with the surroundings, not on air temperature alone.
Over a decade, studies of reflective pavements have found cases where air temperature fell while pedestrian-level comfort worsened. This is a real and well-documented trade rather than an objection invented by opponents.
Roofs and pavements are different cases
Nobody stands on most roofs, so reflected radiation from a cool roof affects buildings opposite and few people. Cool roofs are consequently the easier win and are cheap to implement at the point of roof replacement. Pavement albedo requires more care in streets where people walk, sit and wait.
In practice, reserving the highest reflectance for large surfaces away from pedestrian level is a sensible general rule.
Shade beats reflectance where people are
A shaded surface receives little solar radiation in the first place, so its albedo barely matters. Tree canopy also cools through transpiration and intercepts radiation before it reaches the ground or the person. Where both are available, shading the places people occupy and using reflective surfaces on the places they do not is the coherent strategy.
Reflective pavement is most useful in large open areas such as car parks and industrial yards.
Performance decays with use
Reflective coatings and light-coloured surfaces darken through soiling, tyre marks, repairs and weathering. Reported albedo decline over the first years can be substantial, which reduces long-term performance relative to installation figures. Cleaning restores part of it and adds a maintenance requirement that is rarely budgeted.
Materials that are light-coloured throughout rather than surface-coated retain performance better after wear.
Local statute governs most of this, and it varies street by street.
Winter and night effects need checking
Reflective surfaces reduce solar gain in winter too, which is a small penalty in cold climates and negligible in hot ones. Because the mechanism is about daytime absorption, cool surfaces primarily help the following night, which is when heat harms health.
Over a decade, modelling results vary considerably by climate, urban geometry and the scale of application. Local assessment matters more here than in most heat interventions, because the trade-offs genuinely flip between contexts.
The takeaway
Reflect heat away from the roof and shade the street. The two jobs need different tools.
Cities are built by a thousand small permissions, not one big plan.
Questions readers ask
Are cool pavements a good idea?
In large open areas with few pedestrians, generally yes. In streets and squares where people stand, reflected radiation can worsen comfort even as air temperature falls. Shade is the safer intervention where people are.
Do cool roofs save energy?
They reduce heat gain through the roof and therefore cooling demand, with the benefit largest in hot climates and on buildings with poorly insulated roofs. In cold climates there is a small winter penalty.
