Night Missions
Night missions are very challenging as the visibility is about a third of day missions. It is possible to avoid most night missions by delaying your landing until daytime. Night Terror Missions will disappear before daytime unless you can keep the site continually targeted with different aircraft. If you only have one Skyranger you may find yourself landing at night.
The most important additional operation in night missions is to manage your illumination. It comes in four varieties - flares, fires, artificial lighting, and personal lighting. But first, an important message about alien night vision.
Alien Nightvision
In full daylight, every unit in the game can see up to 20 tiles away (counting from zero where the unit is standing), not counting obstructions to Line of sight.
At night, your units can only see 9 tiles away, but aliens can still see 20. Aliens always see the full 20 (and night missions can end in disaster!) Anything you do to improve lighting only affects you. Your flares do not make you more obvious to aliens, etc. - aliens can always see just fine.
Although dusk or dawn battlescapes clearly have light levels intermediate between day and night, these are actually day battles for visual purposes (range of 20). Visual range is either 9 at full night, or 20 at all other times.
Note that visual range and lighting rules ignore elevation levels. If you can see 9 (or 20) when both you and your visual target are on the ground, you will also be able to see them when you're flying 4 stories high and they're on the ground 9 (or 20) tiles away. Elevation differences do not add to distance for these purposes.
Flares
Start combat with some of your soldiers holding flares. The first soldiers shown on the combat load-out screen will be the first out the door. The smallest number of inventory TUs are needed to move something from your shoulder to your hand (3 TUs), although the belt and leg are a close second (4 TUs).
Flares can be picked up and re-thrown. Try to leapfrog flares as you progress.
Flares cause a circle of light of radius 9 (not counting ground-zero tile), diameter 19 (including GZ). They cast the exact same amount/pattern of light as Personal Lighting (below). In real life, something seen in very dim light would probably only be visible to the closest person, but this is not the case in XCOM. If anyone can see an alien, everyone can (as long as they have Line of sight). I guess everyone's in radio contact.
Due to the fact that both soldiers and flares have radius 9 light patterns, flares should be thrown approx. 18 tiles away (laterally; less for diagonally) for complete coverage. Of course, this doesn't take into account that you will be moving, etc. Likewise, flares should be about 18 tiles away from each other. A hexagon of flares around a soldier will cause almost complete coverage out to approx. 27 tiles.
It is possible to see the outline of an alien against a light backdrop if you get lucky; this is even though no one can "officially" see him (and get the glowing red number). This most often happens if the alien is elevated. The edge of a lit area (9 from the flare or soldier) is extremely dim, making it impossible to see an alien just in front of an edge. But an elevated alien is in profile against farther, less-dim parts of the illuminated area. It can also happen in situations where an alien is just off to the side of a well-lit area, but light is blocked from hitting his tile by e.g. the corner of a building.
Using the throwing formula vs. flare weight of 3, the weakest soldier (STR 20) can throw a flare approx. 17 tiles, and the strongest soldier (STR 70) can throw it approx. 58 tiles - all the way across even the largest map (except diagonally). BUT, as pointed out by NKF, there is an unfortunate limit on how high an object can go before its arc intercepts the fourth-level "ceiling", which causes the confusing/annoying "Unable to throw here!" message even when the route is obviously clear. Ceiling interception appears to make the actual maximum throwing distance approx. 35 tiles. (More testing needed, pls.) Unfortunately, the stronger you are, the more a percent of your possible distance gets clipped, but even fairly weak soldiers may see some clipping.
As a parlor game: Flares illuminate 269 tiles (see inset). Small maps (3x3 tilesets for small and medium scouts) have 900 tiles and would need 3.3 flares to totally cover them, using the extremely simple (and unrealistic) math of 269/900. Large maps (5x5) have 2,500 tiles and would need 9.3 flares to totally cover them. This does not take into account any issues of properly joining/overlapping the patterns etc., nor that your soldiers or other sources produce light. Anyway, there it is.
For what it's worth, flares that are dropped or picked up do not change their light status until the start of the next turn. To wit: If you drop a flare and walk away, it does not give illumination until the next turn. (Not that it matters, since your soldier had the same range-9 lighting as the flare when he was standing there.) And if you pick up a flare and walk away, its lighting remains behind until next turn. Conversely, if flares are thrown, they do immediately light up their area.
Flares are the most fragile object in the game, and are always destroyed by explosions. So you might think twice before fragging an alien revealed by a flare. (Or might not!)
Personal lighting
As discussed under Flares (above), soldiers can see to a radius of 9 around them, and are effectively light sources exactly equal to flares. This does not matter to aliens; they always see the full 20, day or night.
If you have just mind controlled an alien, you will not get its personal lighting until someone on 'your' team moves (including your new pet alien, or any other MC'd aliens). For these purposes, 'turning in place' does not cause personal lighting to come on - only actual movement (from one part of board to another) does. Since movement of your new pet can trigger reaction fire, move one of your 'back field' boys if you want to light up the pet's area while playing it safe re: your new pet drawing reaction fire. Note that turning in place also does not cause reaction fire.
More specifically, aliens do not check for personal lighting status change, until someone on 'your' team uses energy. On a more abstruse note, this means that going up and down on an elevator by your backfield team will not cause newly-MC'd aliens to light up. On a more practical note, though, this also means that aliens that you MC'd last turn, will stay lit this turn, until you move one of your guys (so that one of them uses energy). This might be useful on night missions or in those dark, windy alien base 'gardens' - at the start of a turn, aliens you MC'd last turn stay lit until one of your soldiers moves. Then again, while all of this is true, you will still get the glowing 'alien seen' number, even if you do move - and it's the most important thing of all, by far. Perhaps these insights will help, some cold dark night.
Incendiaries
Incendiary ammo is as good as a flare, plus it can travel further and do some damage if it hits the enemy. It does not do very much damage, though - never expect it to kill an alien.
Load up the Auto Cannon with Incendiary before starting the mission. Use on snap-shot to light up the depths of the map, further than flares can be thrown. Give the soldier a back-up pistol.
Incendiary ammo is also available for Heavy Cannon and Rocket Launcher, but the Auto Cannon's fast shot makes it best for the mission - the soldier can still use his pistol the same turn.
Artificial Lighting
Certain items in the game produce their own light. This can be a big bonus on night terror city missions. Explosives and shots that can destroy the object will stop the lighting.
There are 11 terrain items that produce light:
- 7 alien base objects (several pulsing fuel pods and several 4-part round "museum containers"),
- 2 XCOM base ground tiles, and
- 2 city objects (street lamp and large standing room lamp).
Behind the scenes
For those techies who like to look Under The Hood, the XCOM engine handles varying levels of illumination quite cleverly. The palette is arranged so that the base colours of a tile can be added to the degree of illumination to get the dimmer colours.