Environment
Parameters
Uhi neighborhood radius
80
How far around a home its land cover is read when scoring that home’s heat. Ziter et al. measured Madison street by street and found tree canopy only starts cooling once it covers a meaningful share of a roughly 60–90 m neighbourhood — a single tree outside one window changes nothing, a canopied block changes everything. Setting the read radius to that scale is what makes planting a GROVE work and planting a token tree not.
Uhi night impervious warming
3.5
How much warmer the night is in a neighbourhood that is ENTIRELY paved and built, versus open country. This is the headline heat-island number and the reason the mechanic exists: asphalt, concrete and masonry store the day’s sun and release it for hours after sunset, so the city never gets its night back. EPA puts typical US night-time heat islands at 1.1–2.8 °C and a multi-city study found large cities peaking near 3.7–4.5 °C; 3.5 at FULL pavement lands a realistic dense district in that band once the neighbourhood is never quite 100% hard surface.
Source: US EPA — What Are Heat Islands? (typical day and night ranges)
Uhi day impervious warming
1.2
The same fully-paved neighbourhood’s DAYTIME penalty. Much smaller than the night’s, and that asymmetry is the whole physical story rather than a balance choice: by day the sun swamps everything and afternoon air mixes freely, so measured daytime heat islands sit near 0–1.5 °C. The city’s temperature problem is a night-time problem.
Source: Angevine et al. — multi-city atmospheric urban heat island study (diurnal magnitude)
Uhi day canopy cooling
2.5
How much cooler the afternoon is under a neighbourhood of FULL tree canopy. Trees cool two ways at once — they shade the surfaces that would otherwise store heat, and they evaporate water, which costs the air real energy. Fully paved (+1.2) to fully canopied (−2.5) spans 3.7 °C, matching the ~3.5 °C spread Ziter et al. measured across a single city on one afternoon. This is the strongest daytime lever the player has.
Source: US EPA — Using Trees and Vegetation to Reduce Heat Islands
Uhi night canopy cooling
0.5
Full canopy’s much weaker effect on the NIGHT. Shade is worth nothing after dark and evapotranspiration nearly stops, so Ziter et al. found night-time temperature tracked impervious cover rather than canopy. Keeping this small is what forces the two levers apart: plant trees to fix your afternoons, take up pavement to fix your nights. A player who only ever plants trees will watch the day number improve and the night number sit still.
Source: Ziter et al. (2019) — canopy cools by day, impervious cover governs the night (PNAS)
Uhi day water cooling
1
Daytime cooling from a neighbourhood entirely surrounded by water. Real but modest and strictly local: evaporation and the sheer thermal mass of water hold the afternoon down near a shore and stop mattering a few blocks inland. Deliberately weaker than canopy, so a waterfront is a pleasant place to live and not a substitute for planting anything.
Source: US EPA — Heat Island Compendium: evaporative and water-body cooling
Uhi night water cooling
0
Water does NOT cool the night, which is why this is zero rather than absent. The same thermal mass that holds the afternoon down releases it after dark, so a large water body keeps its shore slightly WARMER overnight and roughly cancels its own daytime gift. Modelling that as a small warming would be defensible; zero is the honest simplification, and it keeps the player from believing a lake fixes a paved district’s nights.
Source: US EPA — Measuring Heat Islands (surface vs atmospheric, diurnal behaviour)
Uhi canopy tile spread
2.2
How many tiles of canopy one planted tree is credited with. A tree occupies one 10 m tile but its crown is 9–23 m across (see the tree blueprint), so it shades well past its own square and neighbouring crowns close over the gaps. Crediting ~2.2 tiles is what lets a normally-spaced row of street trees add up to real canopy instead of scoring as a few isolated pixels of green.
⚠️ Source pending
Uhi lawn canopy equivalent
0.35
What a tile of open park grass is worth against a tile of tree canopy. Grass transpires and is not asphalt, so it genuinely cools — but it casts no shade, and shade is most of what a tree does on a hot afternoon. At about a third, a lawn is clearly better than pavement and clearly worse than trees, which is the ranking a player should be able to feel.
Source: US EPA — Benefits of Trees and Vegetation (shade vs evapotranspiration)
Uhi calm wind reference
8
The wind speed at which the heat island is at FULL strength; above it the island is progressively mixed away as sqrt(reference / wind). Oke’s classic result is that maximum heat-island intensity falls off with roughly the inverse square root of wind speed — wind stirs the warm city air into the cooler air above it, and a calm clear night is when the island is strongest. Pinned to the calm map’s mean wind, so an ordinary map sees the full effect and windy Velaria (24 km/h) runs at ~58% of it. That is the same wind that makes Velaria’s fires worse: the breezy map trades a fire problem for a cooler, cleaner one.
Source: Oke (1973) — City size and the urban heat island (UHI intensity vs wind speed)
Uhi factory waste heat
0.3
How much one fully active factory warms its immediate surroundings by waste heat alone, on top of whatever its pavement already does. Machines, ovens and cooling plant dump their energy into the street, but a Philadelphia modelling study put city-average anthropogenic heat below 100 W/m² against a summer solar load many times that — so industry is a real but MINORITY contributor, and it must never out-shout the pavement. Shares the contamination plume’s distance falloff, since it comes out of the same building.
Uhi max anomaly
5
Hard ceiling on how far above the regional reading any neighbourhood can sit, after every term is added. Nothing in the model should reach it — a fully paved district on a calm night with factories next door lands near 4 — so it exists to guarantee that some future contributor cannot quietly produce a 12 °C city. Observed extremes for large cities under heatwaves top out around this figure.
Source: Angevine et al. — heatwave amplification of the large-city night-time heat island
Hot day temperature
32
At or above this daytime high, the day counts as a HOT DAY in the statistics. 32 °C is 90 °F, the round number US heat reporting has always used and the point where heat starts showing up in health data. Counted on the CITY’s high, not the region’s, so a heat island can add hot days to a summer the countryside found ordinary.
Source: US EPA — Climate Change Indicators: High and Low Temperatures
Warm night temperature
20
At or above this overnight low, the night counts as a WARM NIGHT — what European climatology calls a tropical night, defined at exactly 20 °C. This is the statistic the heat island actually moves, and the one that matters for health: the body recovers from a hot day during a cool night, and a night that never drops below 20 °C is a night it does not get to. A dense city manufactures these out of nights the region passed comfortably.
Source: WMO / European climatology — tropical night (Tmin ≥ 20 °C) definition
Contamination decay distance
600
The e-folding distance of a factory’s PM2.5 plume: exposure falls by a factor of e every 600 m. Four times the measured near-source gradient — elemental carbon fell from 22.8 µgC/m³ at 192 m to 3.02 at 500 m, an e-fold of about 150 m — because the reading has to be city-wide: at the measured curve a district of forty factories was invisible from housing a kilometre away. At 600 m one plant is still felt at a fifth of its fence strength a kilometre out and is under 2% by 2.5 km, so a big industrial district taints the whole city while a single plant across town stays a whisper. The practical shape: brutal next door, a quarter off a block later, a fifth left at a kilometre.
Source: Near-source industrial and roadway concentration gradients (PMC4308952)
Contamination range
3000
Distance past which a factory’s PM2.5 is ignored entirely. Five decay lengths out the plume is under 1% of its fence-line strength, well inside the rounding of a 0–100 score. Three kilometres is 300 tiles — most of a city — which is the point: the reading is city-wide by design, and only a plant on the far side of a big map is out of earshot. Real fine particles travel enormously further than this; the map is a local planning tool and does not pretend to model regional transport.
Source: US EPA — What is Particle Pollution? (atmospheric lifetime and transport)
Pm25 fine fraction
0.25
Share of a factory’s particulate output fine enough (≤2.5 µm) to travel as PM2.5 rather than settle out with the coarser fraction. The rest becomes PM10_COARSE. Grinding, cutting and material handling — the activities CONTAMINATION’s particulate weight represents — throw off mostly coarse dust: German industrial-plant stack measurements found PM10 exceeded 90% of total particulate (PM2.5 under 10%) in most readings, and construction-type dust sources typically run near 21% fine. 25% lands a blended light-manufacturing plant between that mechanical-dust floor and a pure-combustion source, which would run close to 100% fine.
Pm10 coarse decay distance
300
The e-folding distance of the COARSE (2.5–10 µm) fraction’s plume — half PM2.5’s 600 m, stretched from the 100 m the near-source literature supports by the same factor as the fine plume so the two keep their shape relative to each other: gravity pulls coarse particles out of the air faster than it does fine ones. Coarse dust is still the near-fence problem: it makes up most of the reading right at the plant and is mostly settled a few blocks out, leaving the lingering PM2.5 fraction as nearly the whole story across town.
Pm10 coarse range
1500
Distance past which the coarse fraction is ignored entirely — five coarse decay lengths, and half PM2.5’s 3 km for the same reason PM10_COARSE_DECAY_DISTANCE is shorter: coarse particles settle out of the air instead of riding the wind indefinitely the way fine particles do.
⚠️ Source pending
Contamination fence line score
100
The exposure score at the fence of ONE fully active widget factory, in still reference air. This is the anchor the whole 0–100 scale is defined against, so the number always means the same thing: 100 is ‘as bad as living next door to a working factory’, 50 is half that, and several factories can only stack to 100 because the score is capped there. Deliberately NOT an air quality index — AQI is defined per pollutant over specific averaging periods and this model earns none of that precision.
Source: AirNow — AQI Basics (why an index is pollutant- and period-specific)
Contamination wind reference
8
The wind speed the fence-line score is quoted at; concentration scales as reference / wind, floored so still air cannot divide by zero. Every dispersion model from the simplest Gaussian plume upward has concentration inversely proportional to wind speed — twice the wind spreads the same emission through twice the air. Pinned to the calm map’s mean like UHI_CALM_WIND_REFERENCE, so windy Velaria breathes about a third easier while a stagnant week anywhere gets noticeably worse.
Source: US EPA — Air Quality Dispersion Modeling: preferred and recommended models (AERMOD)
Contamination max wind relief
2.5
The most a strong wind may divide the score by, and its reciprocal is the most stagnant air may multiply it. Without a clamp the 1/wind law sends a still day to infinity and a gale to zero; real dispersion stops improving once the plume is well mixed, and real stagnation is bounded by the depth of the air it is trapped under. 2.5 keeps the weather worth noticing without ever letting it be the whole answer — the factory is still the problem.
Source: NOAA Global Monitoring Laboratory — Temperature Inversions and Air Pollution
Contamination canopy reduction
0.12
The most a neighbourhood of full tree canopy can take off its own contamination score. Leaves really do intercept particles, but published removal rates for urban forests run to a few percent of ambient concentration — a useful trim, nowhere near a cure. Capped this low on purpose: trees must never become the answer to a factory, because the answer to a factory is siting it away from homes or not running it.
Source: Nowak et al. — Air pollution removal by urban trees and shrubs in the United States
Contamination high threshold
40
The score at or above which a resident counts as living in HIGH contamination, and a day counts as a bad-air day. One full-tilt factory holds its neighbours above this line out to roughly 300 m at the reference wind — about three blocks — so the statistic responds to genuinely bad siting rather than only to a home sharing a fence with a plant, and moving housing a few blocks further is a real fix. A game rule, stated plainly in the tooltip, and named ‘high contamination’ rather than borrowed from any health or regulatory scale this model has not earned.
⚠️ Source pending
Contamination sickness multiplier
0.45
How much more likely a resident at the FULL 100 score is to fall ill on a given day — 1.45x, scaling linearly from 1x at clean air. Long-term exposure to particulate air pollution genuinely raises respiratory and cardiovascular illness, and the direction and rough size are not controversial; the exact figure is a balance choice sized so a badly sited industrial district is a visible, fixable public-health problem rather than a death sentence or a rounding error.
Source: WHO — Global Air Quality Guidelines (health effects of PM exposure)
Warm night severe temperature
26
The overnight low at which the heat health effect reaches full strength, phasing in linearly from WARM_NIGHT_TEMPERATURE. A bedroom that never falls below the mid-20s is where heat-health warning systems start counting excess deaths, and a city can only reach it by adding its own heat island on top of an already hot night — which is the entire point of putting the health effect here.
Source: US EPA — Climate Change and Extreme Heat (night-time minima and heat mortality)
Heat sickness multiplier
0.35
How much more likely a resident is to fall ill on a night at or above WARM_NIGHT_SEVERE_TEMPERATURE — 1.35x, phasing in from 1x at the warm-night line. It rides on the NIGHT alone, not the afternoon, because that is where the evidence is: the body recovers from a hot day during a cool night, and it is the night that never comes that fills emergency rooms. That also makes the mechanic cohere — the heat island’s whole effect is on the overnight low, so the thing the player builds is the thing that makes people ill, and unpaving is the cure.
Source: US EPA — Climate Change and Extreme Heat (health effects of warm nights)