About the Urban Heat Island Simulation
This simulation paints a 60×60 grid of city surfaces and solves a simple heat balance on every cell. Each cell follows C·dT/dt = α·Qsun − k·(T − Tamb) − L·max(0, T − Tamb) + D·∇²T, where α is solar absorptivity, k convective loss, L latent evaporative cooling and D·∇²T diffusion from neighbours. The live heatmap spans 20–36 °C around a 20 °C ambient.
You choose a draw material (asphalt, concrete, grass, water, trees or roof, each with its own α), then paint cells while toggling between the temperature and material views. The hour-of-day slider drives a sine-shaped solar input and the solar-intensity slider scales it from 0 to 200%. Three preset cities and a live UHI index show why dense, dark, dry surfaces run far hotter than green or watered ones — the core challenge of urban climate design.
Frequently Asked Questions
What is the urban heat island effect?
It is the tendency of built-up areas to be warmer than surrounding countryside because asphalt, concrete and rooftops absorb and store more solar energy than vegetation or water. In real cities this can raise temperatures 5–12 °C above nearby rural areas, especially at night.
How does the simulation calculate each cell's temperature?
Every cell integrates a heat-balance equation each step: solar gain (α·Qsun) heats it, convective loss and latent evaporative cooling remove heat, and diffusion exchanges heat with its four neighbours. The result is divided by the material's heat capacity, so water and vegetation warm slowly while dark asphalt heats quickly.
What does solar absorptivity (α) mean here?
Alpha is the fraction of incoming solar energy a surface absorbs rather than reflects. Asphalt is set to 0.95 (very dark and absorbent), concrete 0.88, roof 0.85, water 0.75, grass 0.70 and trees 0.62. A higher α means the cell soaks up more of the sun's energy and runs hotter.
What do the controls do?
The draw-tool buttons pick which material your brush paints. The view buttons switch between a temperature heatmap and a flat material map. The hour-of-day slider sets the solar angle (peaking near midday), while the solar-intensity slider scales the sunlight from 0 to 200%. Three preset buttons rebuild a dense, green or industrial city layout.
Why do parks and water stay cooler?
Grass, trees and water carry a latent-cooling term that removes heat as moisture evaporates, mimicking evapotranspiration. Water also has the highest heat capacity, so it resists temperature swings. Together these keep vegetated and watered cells several degrees below surrounding asphalt, just as real forests run 5–10 °C cooler than city centres on a hot day.
What does the UHI index statistic represent?
The UHI index is the average temperature of the built-up cells (asphalt, concrete, roof) minus the average of the green and water cells (grass, trees, water), floored at zero. It gives a single number for how much hotter the developed parts of your city run compared with its green spaces.
How does the hour-of-day slider affect the result?
Solar input follows a sine curve that is zero before 6:00 and after 18:00 and peaks at midday. Move the slider into night-time hours and the solar term vanishes, so cells cool toward ambient through convection; move it to noon and dark surfaces heat fastest, widening the temperature spread across the grid.
Is this simulation physically accurate?
It is a qualitatively correct teaching model, not a calibrated forecast. It captures the right mechanisms — absorptivity, heat capacity, convective loss, evaporative cooling and diffusion — but uses simplified relative coefficients, a fixed 20 °C ambient and a clamped 20–36 °C colour scale rather than real meteorological data.
Why does the temperature view diffuse over time?
The D·∇²T term spreads heat from each cell to its four neighbours, smoothing sharp edges between hot and cool patches. Materials have different diffusivities — water spreads heat most readily, dense vegetation least — so a small park surrounded by asphalt slowly warms at its edges while its centre stays cool.
What real-world strategies reduce urban heat?
The same levers appear in the model: adding trees and grass introduces evaporative cooling, water bodies buffer temperature swings, and lighter, more reflective surfaces lower absorptivity. Real cities use green roofs, street trees, ponds and high-albedo "cool pavements" to cut peak temperatures and reduce air-conditioning demand.
Who first described the urban heat island?
The chemist Luke Howard documented London running warmer than its surroundings in 1818, making it one of the earliest recorded urban climate effects. Today cities such as Tokyo and Phoenix can be more than 10 °C hotter than nearby rural land at night, which is why UHI research is central to climate-resilient city planning.