Weather & environment

In 1.13 the battlefield itself fights back. Sectors have their own weather, the day/night cycle drives how far anyone can see, gas clouds and fires reshape firefights, and water is more than scenery. Almost every rule on this page applies to both sides — the enemy suffers in a sandstorm exactly like you do, and the AI actively avoids gas, deep water and lit spots at night.

Everything below is taken from the current Ja2_Options.INI, game data and source code; settings live in the [Tactical Weather Settings] and [Environment Hazard Settings] sections unless noted (see the Ja2_Options.INI tour).

Dynamic weather

Early 1.13 could only make it rain — everywhere in Arulco at once. In June 2016 Flugente rewrote the system as localized weather (r8253): each day the game rolls a number of weather events, and each event then hits only some sectors, based on per-sector chances. Two squads a few sectors apart can fight in completely different conditions.

The current weather types:

Weather	What it is
Normal	Nothing falling from the sky.
Rain	Classic rain, with sound and visible raindrops.
Thunderstorm	When lightning is enabled, every rain shower has a 50% chance to be upgraded to a thunderstorm — louder, darker, with lightning and much bigger penalties.
Sandstorm	Devastating: vision drastically reduced, mercs can hardly hear anything over the noise, and even breathing gets harder. Stock map data puts sandstorm chances on desert sectors.
Snow	Rare in tropical Arulco (the stock map gives it to a handful of mountain sectors), but fully supported — mods with colder settings can use it broadly.

There is no fog — Flugente considered adding it to swamps but found no reasonable way to render the effect, so it was never built. Old descriptions that mention only "rain and thunderstorms" predate the 2016 rework.

What weather does

The INI defines four penalties per weather type. Defaults in the current game data:

Effect (INI key prefix)	Rain	Thunderstorm	Sandstorm	Snow
Weapon reliability reduction, 0–10 (WEAPON_RELIABILITY_REDUCTION_*)	0	1	4	2
Stamina regeneration lost (BREATH_GAIN_REDUCTION_*)	5%	40%	70%	20%
Sight range lost (VISUAL_DISTANCE_DECREASE_*)	5%	15%	40%	30%
Hearing lost (HEARING_REDUCTION_*)	30%	80%	60%	10%

• Reliability: weapons behave as if less reliable, so they get dirty and jam more readily — see weapon mechanics. Keep cleaning kits handy in storms.
• Hearing: reduced hearing works both ways. Rain is a gift for stealth players — a 30% hearing cut lets you move much closer before anyone notices, and gunfire is harder to place.
• Sight and stamina penalties hit every combatant, including militia and enemies.
• Mercs with the Survival trait shrug off 35% of weather penalties — see skills & traits.

There are no separate weather aim modifiers in CTHConstants.ini — weather degrades your shooting indirectly, through shorter sight ranges and (in storms) faster-fouling guns.

Lightning

During thunderstorms, lightning periodically lights up the whole map for a moment:

• Lightning reveals soldier positions for both you and the enemy — a stationary sniper can suddenly be spotted mid-flash.
• If someone is spotted during a flash in turn-based mode, the game pauses for a few seconds (DELAY_IN_SECONDS_IF_SEEN_SOMEONE_DURING_LIGHTNING_IN_TURNBASED = 5) so you get a good look.
• In turn-based combat lightning only strikes between turns, with a 35% chance per turn (CHANCE_TO_DO_LIGHTNING_BETWEEN_TURNS). In real time it strikes every 2–15 seconds (MIN/MAX_INTERVAL_BETWEEN_LIGHTNINGS_IN_REAL_TIME_SECONDS).

Where weather is configured

• In-game: the new-game options screen has Weather: Rain, Weather: Lightning, Weather: Sandstorms and Weather: Snow toggles that override the INI — see new-game options.
• Ja2_Options.INI: master switches and frequency per type — ALLOW_RAIN, ALLOW_LIGHTNING, ALLOW_SANDSTORM, ALLOW_SNOW, plus per-type *_EVENTS_PER_DAY, *_CHANCE_PER_DAY and *_MIN/MAX_LENGTH_IN_MINUTES keys. With stock settings rain is common (up to 5 events a day, 1–4 hours each), sandstorms less so, snow rolls only once per day. MAX_RAIN_DROPS scales the visual effect down for slow machines.
• TableData\Map\SectorNames.xml: per-sector <rainchance>, <sandstormchance> and <snowchance> tags decide which sectors a weather event actually reaches.

You can watch the weather from the strategic screen: the map border has a Show Weather filter button. Sectors are tinted by current weather — no colour = sunny, cyan = rain, blue = thunderstorm, orange = sandstorm, white = snow.

Fighting in rain

Plain rain barely hurts your eyes (−5% sight) but cuts everyone's hearing by 30% — it is the best weather in the game for sneaking up on a garrison or slipping away from a lost fight. A thunderstorm is a different animal: everyone is nearly deaf, stamina recovers 40% slower, and every lightning flash can expose your people — keep them prone and in cover, and expect to be seen anyway. In a sandstorm, vision is so short that fights collapse into brutal close quarters; if you have the better long-range team, consider simply waiting it out (they rarely last more than a few hours). After any storm, check your weapons for dirt and jams.

Time of day and light

Arulco runs a full day/night cycle. In the current source, dawn breaks at 6:47 AM (full daylight by 7:05 AM) and dusk starts at 8:57 PM (full night by 9:15 PM) — the transitions are quick, under 20 minutes of game time.

Sight distance is the base sight range (BASE_SIGHT_RANGE = 14, automatically doubled — so 28 tiles by day) scaled by the light level of the spot being looked at. The INI externalizes this as BRIGHTNESS_MOD_0 … BRIGHTNESS_MOD_15 (0 = brightest, 15 = darkest): normal day (light level 3) is 100%, normal night (light level 12) is 43%, and the darkest levels drop to 9%. In practice a merc who sees 28 tiles at noon sees roughly 12 tiles at night — less in deep shadow, more under a street lamp.

What matters is the light at the target's position: someone standing in a pool of light is visible from far away, while a merc in darkness is near-invisible. The enemy knows this too — with NEW_AI_TACTICAL enabled, elite enemies refuse to walk into lit places at night, and AI_PATH_TWEAKS makes the AI route around light (and gas, and deep water) when moving at night.

Night vision gear

• Night vision goggles come in generations I–IV in the current item set. They trade day vision for night vision — for example, Gen II NVGs give <NightVisionRangeBonus>20</NightVisionRangeBonus> but −20 day vision and −40 in bright light (Items.xml). Better generations see further.
• Sun goggles do the opposite, helping in bright light.
• Most face gear also narrows your field of view: with ALLOW_TUNNEL_VISION = TRUE (the default), characters cannot see behind themselves, and items with a <PercentTunnelVision> tag (including NVGs and many scopes) narrow the cone further.
• Shift+N performs a smart goggle swap — everyone equips the right goggles for the current time of day; Ctrl+Shift+N forces a uniform swap. With SMART_GOGGLE_SWAP and GOGGLE_SWAP_AFFECTS_ALL_MERCS_IN_SECTOR (both default TRUE) this covers the whole sector. See the hotkey reference.
• The Night Ops trait adds sight and hearing range in darkness and an interrupt bonus at night — see skills & traits.

Light sources

Anything that makes light changes the sight math around it:

• Flashlights — as a hand-held item, or mounted on a weapon (the Rifle LAM-Flashlight Combo, for instance). A flashlight needs batteries attached to work, only lights up at night or underground, and cannot cast light while you stand on a roof (an engine limitation). Press Shift+L to switch the flashlight in your merc's hand — or attached to the weapon in hand — on and off; in combat this costs a few APs. Like lasers, a lit flashlight makes you easier to spot — see attachments.
• Chemical break lights and flares — throwable light. Break lights burn where they land; trip flares can be rigged to tripwire to catch night attackers.
• Fires and explosions — a fire explosion leaves a burning light source behind (ADD_LIGHT_AFTER_EXPLOSION = TRUE), and regular outdoor explosions leave a small smoke cloud (ADD_SMOKE_AFTER_EXPLOSION = TRUE).
• Muzzle flash — every shot from an unsuppressed gun produces a muzzle flash that can give the shooter away in darkness. Flash suppressors and silencers (items with <HideMuzzleFlash> in Items.xml) prevent it — the single biggest reason night fighters want them; see attachments.

For flavour, RADAR_MAP_MODE_NIGHT / OVERHEAD_MAP_MODE_NIGHT in the INI can recolour the radar and overhead maps at night (dark, infrared or night-vision style).

Fighting at night

Night halves everyone's sight range, so whoever sees first wins. Give your point men NVGs (and swap the squad with Shift+N at dusk), fit flash suppressors or silencers so your own shots stay invisible, and leave the lasers and flashlights off until contact — they betray you as much as they help. Treat every light source as a trap: shoot from darkness into light, throw a break light past the enemy to backlight them, and never stand next to your own flare. Since night barely reduces hearing, sound discipline decides fights — walk or sneak instead of running, and let stealth-oriented mercs lead. Lightning during storms overrides all of this for a moment — assume you are visible whenever the sky flashes.

Smoke, gas and gas masks

Grenades, mortar shells and 20mm–43mm launcher rounds come in several cloud-producing variants (all present in the current Items.xml): smoke, tear gas and mustard gas hand grenades, 40mm/43mm grenades and cylinders, 20mm/25mm clips and 60mm mortar shells. Clouds spread over several turns — faster outdoors than indoors — then dissipate.

Cloud	Effect
Smoke	Concealment. Anyone inside a smoke cloud becomes extremely difficult to spot, but also loses a large chunk of their own sight range. The go-to tool for crossing open ground or breaking contact.
Tear gas	Breath (stun) damage each turn to anyone inside without a working gas mask. Hangs around longer than other gases.
Mustard gas	Wound damage each turn without a mask — the lethal option.
Creature gas	The noxious attack of the Crepitus (Sci-Fi mode) — gas masks do not help against it. See enemy classes.
Burnable gas	Fire-type cloud (e.g. from exploding gas cans). Counts as fire: gas masks are useless, armor fire resistance helps.
Signal smoke	Red marker smoke used to call in artillery strikes — see support roles.
Fire retardant	Sprayed by fire extinguishers; puts out fires and prevents tiles from catching fire.

Details worth knowing, all from the current source:

• Robots ignore smoke, tear gas and mustard gas; zombies ignore tear and mustard gas but burn nicely (enemies).
• A merc caught in gas without protection gets the gassed condition, which seriously widens the aiming aperture under NCTH (BASE_GASSED = -15.0, AIM_GASSED = -80.0 in CTHConstants.ini).
• Gas masks block gas damage completely while in good condition, but protecting against damaging gas wears the mask down a few points each time. Below 70% status a mask leaks, letting part of the breath damage through — repair or replace masks after a gas fight.
• Alt+Shift+N is the emergency command: every merc in the sector puts on a gas mask if they have one anywhere in their inventory (hotkeys).
• Enemy soldiers can wear gas masks too — soldier tooltips show NVGs and gas masks (SOLDIER_TOOLTIP_DETAIL_LEVEL), and your militia will grab masks, goggles and NVGs left lying in the sector if MILITIA_USE_SECTOR_EQUIPMENT_FACE is on.
• The AI paths around gas clouds (AI_PATH_TWEAKS), so don't expect enemies to blindly wade through — gas is better for area denial than for kills.

Gas is an attack

As far as civilians and militia are concerned, walking into a cloud of gas you created counts as being attacked by you (documented at the CAN_TRUE_CIVILIANS_BECOME_HOSTILE / CAN_MILITIA_BECOME_HOSTILE settings in Ja2_Options.INI). Gassing a town square can turn it hostile.

Fire

Fire-type explosions (incendiary grenades, burnable gas, exploding fuel) burn tiles and anyone on them:

• Gas masks give no protection against fire or burnable gas; armor with fire resistance reduces the damage.
• Burning tiles are light sources — a night ambush ends when the barn catches fire.
• Fire can spread to neighbouring tiles as it burns, but never across water.
• Fire extinguishers (and extinguisher grenades) create fire-retardant clouds that remove existing fire and stop tiles from igniting.

Water, swamps and snakes

• Shallow water slows you down; deep water forces mercs to swim — a swimming merc cannot fight, and the alternative weapon-holding stances are unavailable for pistols while standing in water. An IMP with the Nonswimmer disability should stay on dry land.
• Water damages equipment. While a merc is in deep water, every carried item flagged <WaterDamages> in Items.xml (most electronics and gear) has a 10% chance per check to lose up to 10 points of status; waterproof items are immune. Swim with as little as possible.
• Camouflage washes off in deep water, about 1% per tile (much slower with the Survival trait; under the new trait system, face paint survives wading through shallow water) — see stealth & camouflage.
• Snakes: with ALLOW_SNAKES = TRUE ([Environment Hazard Settings]), water in some sectors harbours venomous snakes (<snakechance> per sector in SectorNames.xml, evaluated anew each turn). Mercs have a small chance to evade an attack, improved by the Survival trait, a knife in inventory, or gear with a snake evasion bonus.
• Swamps are a health hazard rather than a combat one: swamp water is poisoned as a drinking source (see food & water), and swamp and tropical sectors breed disease-carrying insects if the disease system is on (drugs & disease).

Sources

• New feature: localized weather — Flugente, The Bear's Pit, June 2016 (feature thread; r8253 / GameDir r2325)
• Data-1.13\Ja2_Options.INI from the current 1dot13/gamedir repository — [Tactical Weather Settings], [Environment Hazard Settings], vision, tooltip, explosion and AI sections
• Data-1.13\CTHConstants.ini (gassed modifiers) and Data-1.13\TableData\Map\SectorNames.xml (<rainchance>, <sandstormchance>, <snowchance>, <snakechance>) from the same repository
• Data-1.13\TableData\Items\Items.xml (grenade/gas/flashlight/NVG items, <FlashLightRange>, <HideMuzzleFlash>, <WaterDamages>, vision bonus tags)
• Current 1dot13/source files verified: TileEngine/environment.cpp (weather forecast, day/night times), TileEngine/SmokeEffects.h (cloud types), TileEngine/Explosion Control.cpp (gas damage, gas masks, fire), Tactical/Items.cpp (water damage), Tactical/Soldier Control.cpp (flashlights, deep water), Tactical/Weapons.cpp (muzzle flash), Tactical/Turn Based Input.cpp (Shift+L, Alt+Shift+N handlers), Strategic/Map Screen Interface Border.cpp (weather map filter), i18n/_EnglishText.cpp (in-game texts)
• JA2 1.13 official hotkey reference PDF (r9389, 2022) for the goggle/gas mask hotkeys