Summer temperatures can differ by several degrees between neighbourhoods a few kilometres apart. The pattern is stable year to year because it is built into the physical fabric.
Surfaces determine how much heat is stored
Dark paving and roofing absorb solar radiation and release it slowly, keeping temperatures elevated well into the night when the body would otherwise recover.
Vegetation cools by shading surfaces and by evaporating water through leaves. Both effects scale with the amount and maturity of the canopy.
Areas dominated by roads, parking and rooftops with little planting therefore run consistently hotter than areas with established trees, under identical weather.
Canopy distribution reflects past investment
Street trees require planting, watering during establishment and decades of maintenance. Their presence records sustained municipal spending over a long period.
Districts that received less investment historically have thinner canopy today, since a tree not planted forty years ago cannot be present now.
Because canopy takes decades to mature, planting programmes started now address the problem for the next generation rather than the current one. Maintenance funding matters as much as planting, since young trees die without watering.
Building stock changes exposure indoors
Insulation, window orientation, ventilation and ceiling height determine how far outdoor heat translates into indoor conditions.
Older housing with poor insulation and limited ventilation can be hotter inside than outside during a heat episode, and upper floors are worst affected.
Cooling equipment shifts the burden to energy cost, so households unable to afford running it remain exposed even where equipment is installed. Rules restricting disconnection during heat episodes exist in some places for that reason.
Tenure complicates matters further, since insulation and shading are improvements a tenant cannot make and a landlord has limited reason to fund.
Heat affects health through existing conditions
Heat places load on the cardiovascular system and interacts with respiratory conditions, kidney function and several common medications.
Risk concentrates among older people, infants, outdoor workers and those living alone, and mortality during heat episodes typically rises without heat appearing as the recorded cause.
Because the effect works through existing conditions, areas with higher baseline illness experience worse outcomes from the same temperature.
Measurement has shaped the response
Fixed weather stations are often sited at airports and parks, which understates conditions in dense built-up areas.
Community mapping campaigns using vehicle-mounted sensors have produced fine-grained temperature maps, and those maps have been used to prioritise planting and cooling centres.
Heat planning duties, building standards and utility disconnection rules differ by jurisdiction and are being revised as heat episodes receive more attention.