Environment
The Environment map has two layers: Pollution, the smoke from factories and power plants, and Heat, the warmth pavement and roofs add to the city’s nights.
Air pollution
Busy factories, coal plants, paper mills, petrochemical plants and warehouses give off particles while they work; an idle one gives off nothing. Each spreads a plume the same way in every direction: coarse dust that settles within a few blocks, and a finer haze that thins with distance but reaches across the city, so a big industrial district is felt everywhere. Every home gets an air pollution score from 0 to 100, where 100 is living next door to a working factory. Sources stack, up to the cap of 100, and a coal plant running flat out counts as three factories.
The map reads this game score as Clear below 30, Elevated from 30 to 39 and High from 40. The map colours, exposure counts, health and Room to Breathe use this score.
Air quality index
The Pollution tab opens with an estimated daily US AQI for particle pollution. It samples occupied homes and workplaces, with each completed, usable building counted once, and takes the median PM2.5 and PM10 concentrations. Empty buildings and unoccupied land do not dilute the reading. A city without occupied places has no reading yet.
The concentration model adds a regional background of 5 µg/m³ PM2.5 and 15 µg/m³ PM10 to local emissions. A source’s intensity, particulate weight and activity set its output: at the fence, one unit of particulate output contributes 500 µg/m³ PM10 in the standard breeze. This makes the reference factory’s contribution 150 µg/m³, of which 25% is fine particles. These are game calibration choices, not measured emission factors. Fine and coarse particles disperse over their respective distances; today’s wind and local canopy affect the local contributions, which add without a cap. Each day’s conditions represent an estimated daily mean.
The index uses the EPA daily AQI breakpoints and interpolation rules. PM2.5 is truncated to tenths and PM10 to whole µg/m³ before calculating their indices. The higher index determines the headline and main pollutant; the figure beside that pollutant is its median concentration. Values above 500 continue on the hazardous slope. The six categories run from Good through Hazardous.
This is a median estimate of the places people use, rather than the highest-monitor reporting convention used for an official city AQI. It covers PM2.5 and PM10; the game does not yet estimate ozone or gaseous pollutants.
The standard breeze and today’s wind
Wind spreads smoke thinner: twice the wind, half the local contribution, within limits. So that the map holds still while the weather changes, the Environment map scores every home in a standard breeze of 8 km/h, with the factories and plants running as they actually ran today. The Air Quality section and the Today exposure count use today’s actual wind.
Health uses the real wind. Still days make the air worse and windy ones better, and on the windy map air pollution runs well below what the map shows. A resident at a score of 100 is about half again as likely to fall ill. Trees near homes take a little off, at most 12% under full canopy, but the answer to a factory is where it stands and how hard it runs.
Night heat added by the city
Pavement, concrete and roofs soak up the day’s sun and give it back after dark, so a built-up neighbourhood stays warmer at night than the countryside around it. The Heat layer shows this night heat added by the city: a few tenths of a degree in a leafy suburb, up to about 3.5 °C in a district paved from wall to wall on a calm night. Busy factories add a little of their own.
Less pavement near homes is the strong lever. Trees cool afternoons far more than nights, and only in groves: scattered trees barely help, and canopy starts to count once it covers about 40% of the ground within 80 m of a home. A river or lake cools hot afternoons along its banks, by 1 °C at the water’s edge fading to nothing 100 m away, but it does nothing for nights: after dark the water gives back the heat it held. Wind carries the heat away. Like Pollution, the layer draws homes in the standard breeze; tonight’s figures use today’s wind. Open water is left blank: a river holds no pavement or roofs for the city to warm, while smoke drifts over it like any air.
It matters on summer nights. A night whose low stays at 20 °C or above counts as a warm night, and warm nights make people ill, because the body recovers from a hot day overnight. The Heat layer counts the residents the city’s own heat pushed past 20 °C.
Room to Breathe
Room to Breathe goes to a city that, over twelve complete months in a row, averages 10,000 residents and keeps nine in ten of them out of High air pollution at home: no more than one resident-day in ten spent in High, scored in the standard breeze. Because the breeze is fixed, the city’s map wins the award, not a lucky windy year. Keep industry away from homes and plant trees near the rest. Progress shows on the Pollution layer and on the Energy map’s Year page.
Why heat has no award
The simplest way to keep a city’s nights cool is to spread it thin, which is not worth rewarding. Trees barely move night heat, and on the windy map the wind would decide the result for you. So heat stays on the map as information and as a health cost, and waits for a lever that does not reward sprawl.
Parameters
Uhi neighborhood radius
80
How far around a home its land cover is read when scoring its heat: 80 m. A bicycle survey of Madison, Wisconsin found tree canopy cools most when measured over a city block, 60–90 m around a point, and most strongly once canopy passes about 40%, so a grove works and a lone tree does not.
Basis: measured
Uhi day canopy threshold
0.4
The canopy share of a neighbourhood past which its trees start cooling the afternoon in earnest: 40%. Below it, shade falls in scattered patches over ground that still bakes. Past it, the patches join up and keep the air under them cool. In Madison, going from 0% to 40% canopy barely moved the afternoon, and going from 40% to 80% cooled it by about a full degree. So a grove cools its block and a lone tree does not.
Basis: measured
Hot day temperature
32
A city high at or above 32 °C (about 90 °F) counts as a hot day: US climate records have long counted days of 90 °F or more (90 °F is 32.2 °C). It is read on the city’s resident-weighted high, so the heat island can add hot days the countryside does not see.
Basis: measured
Source: NOAA NCEI — Comparative Climatic Data (days with a maximum of 90 °F or higher)
Warm night temperature
20
A night whose city low is at or above 20 °C counts as a warm night, after Europe’s ‘tropical night’ indicator, which counts nights whose low stays above 20 °C; the game’s ≥ 20 also counts exactly 20, a game threshold. The heat island moves this statistic most, and warm nights matter for health: the body recovers from a hot day overnight.
Basis: measured
Source: Climate-ADAPT (EEA) and Copernicus C3S — Tropical nights