A Smoke grenade provides a portable smoke-screen that you can deploy to hide your troops. It operates the same way as a conventional grenade, but on detonation releases a medium sized dense cloud cover. The cloud will eventually thin and spread out over the course of several turns.
Smoke is difficult to use effectively in X-COM, although when properly deployed, it can be even more effective than armor. A well-placed smoke screen can help you to see the aliens before they see you and allow you to fire upon them without risk of return fire. It will also limit the number of aliens that can spot your troops at any given moment, allowing you to concentrate your firepower on the aliens nearest to you.
Smoke should not be permitted to cause complacency. While it can mean the difference between dead units, and units which can be kept up with medikits until the end of a mission, smoke is still not a complete substitute for physical cover. There are videos on YouTube by otherwise very good players, who still show said players leaving their units in smoke with no physical cover, and then being surprised when said units get shot by aliens. Use smoke to cover rapid charges across open terrain, but get your units back behind physical cover again as quickly as possible.
Smoke is also a double edged sword. While it reduces firing accuracy for the aliens, it does so for your own soldiers if they have to shoot through it as well. If you are going to use it, practice to the point where you know when you should use it, and when not to.
Conventional explosive (HE) weapons and incendiary weapons will produce a smoke cloud, but it is believed that only smoke grenades will produce a high-density cloud, which has as much as three times the obscuring power.
A simple and highly-effective use of smoke grenades is to throw one at the base of your transport's ramp on the first turn of deployment (do not deploy until Turn 2, after the grenade has detonated). This will mask your soldiers' movements as they emerge from the transport, greatly reducing the chance of their being killed by as-yet-unseen aliens. Additionally, aliens have their full Time Units at the start of combat; on subsequent turns they will usually have walked around a bit, making their reaction fire less deadly.
If the transport has landed in a high-risk area (facing a building, a UFO door, or several visible aliens), you may wish to drop an additional grenade inside the transport, to reduce the aliens' ability to fire upon your undeployed troops.
Smoke can also reduce the dangers of advancing across open ground without cover. Keep a cloud between your troops and the aliens, if their positions are known, or throw them in front of your front-line troops and advance inside the smoke cloud. When you spot aliens, utilize standard scout/sniper tactics: have the scout stop moving, and fire upon the alien(s) using snipers hidden by the smoke.
For best results, place the smoke grenade between yourself and the attackers. Placing it on yourself or on the aliens you wish to blind does not work as well because the aliens will have only half as much smoke to look through.
Protecting exposed soldiers
If a soldier is moved and ends up stranded in an open field with an alien in full view, the chance this soldier of surviving tend to be very slim. You can alter the odds very slightly by throwing a 0 turn smoke grenade between the soldier and the alien to provide cover.
Even though the alien be facing and may already be attacking the soldier, as long as the smoke cloud is up by the time the alien move begins, it will affect the alien's ability to target the soldier and may prevent it from attacking.
Rockets are an instant means to provide similar support, but the cloud generated will not be as dense.
Multiple aliens will often engage your troops from within the UFO after one alien has stepped through the door. To prevent this, you can detonate a smoke grenade just outside a UFO door. Any aliens that step out will have to engage your troops one at a time.
Furthermore, if your troops are a sufficient distance away (10 - 15 squares), any emerging alien will have to travel several paces through the smoke before they can engage your troops, reducing their TUs and giving your troops a much greater chance of firing first.
The confined spaces inside a UFO reduce the usefulness of smoke. However, smoke may still be of use in limiting aliens' visibility down long corridors, across larger rooms, or placed in front of inner doors (as per "Outside UFOs", above).
During base defense
If you have built your base properly, placing a smoke grenade at the "choke point" (outside the Access Lift) will allow you to use scout/sniper tactics while engaging the invasion force. Have one unit advance through the smoke until an alien is spotted, then have it retreat back out of the alien's visual range and open fire using troops hidden by the smoke.
Soldiers' visual range at night is only 9 squares, but aliens' night-time visual range is the same day or night: 20 squares. Thus, you might be able to even the odds a bit using well-placed smoke grenades.
Perhaps the biggest drawback of the use of smoke is the limited availability of smoke "squares" that can be displayed on the map. Only 400 squares of smoke or fire can be displayed on the map at any one time; this is equivalent to about three smoke grenades, or less than that if smoke or fire has been generated by HE or Incendiary rounds. If the UFO has crash-landed, there may be even less smoke available for display.
It takes 15-20 turns for the cloud from a smoke grenade to clear, during which time you may be unable to create additional clouds in desired locations. The game seems to store "undisplayed" smoke -- as smoke from earlier explosions clears, smoke from more recent explosions appears, even if it was not displayed at first.
It is believed that "undisplayed" smoke does not provide any cover, and this has been tested to some extent.
Smoke spreads around after an explosion. While smoke itself is stored in the savefile SMOKREF.DAT, perhaps information as to where the smoke will go to is stored in SMOKBIT.DAT - Explaining why "undisplayed" smoke seems to "reappear" later on... - BB
The smoke cloud produced by a smoke grenade provides the best screen within the first turn after detonation. The initial visibility through the cloud is 4-6 squares. Over time the smoke will spread out and cover a larger area, but by then the smoke will have grown thin and units and allies alike will be able to see much farther through the smoke than when it was a tight mass. Smoke has a variable starting effectiveness, which falls off as time passes. A good rule to know is that every turn allows the aliens (and you) to see one tile further per 3 tiles of smoke, until all the smoke particles clear.
The formula is at SMOKREF.DAT
If you take the amount of turns a given tile of smoke is due to last, divide that by three and round the result down, you have the amount of tiles worth of visibility it'll reduce. However, there is no way to tell a cloud's age just by looking at it. Typically each tile produced by a freshly detonated smoke grenade will block from one to four tiles.
Be warned that the smokescreen produced is not an impenetrable barrier. It will hinder visibility not just for your enemies, but for your allies as well. However, this trait allows the use of scout/sniper tactics over short distances or in confined spaces: if you spot an alien while wandering through smoke, move back one square out of its visual range again and then fire at it. It will not return fire. (See reaction fire triggers for more details.)
Unarmoured units will take stun damage whenever they end their turn inside a cloud of smoke. The stun damage to an unarmoured unit is 1-3 points per round. Soldiers in Power Suits or Flying Suits are immune to stun effects from smoke. Even unarmoured troops will have to spend a long time in smoke before they become unconscious, so as long as you remain aware of your soldiers' Stun levels (the narrow white bar within the Health bar), it should not present major problems.
A more serious hazard emerges from a bug in the way the game handles fire, smoke and Incendiary effects. Fire and smoke are interdependent in the game. One unintended consequence is that stun damage is done to all friendly units standing in any smoke area, every time an incendiary explosion occurs anywhere on the map. This happens instantaneously, affecting soldiers who are in smoke at the time of detonation. Therefore, be very careful in your use of IC munitions if you wish to move unarmored units through smoke, or hide them in smoke.
Unfortunately this is another major drawback that prevents smoke from fulfilling its powerful tactical potential. A few turns maneuvering through smoke, followed by an inadvertent burst of Incendiary rounds from an Auto-Cannon, is capable of knocking out half of your force. Of course this becomes less of a problem once all your troops are outfitted with heavy armour and immune to these effects.
Note - under no circumstances will enemy-controlled units take stun damage from smoke, or from incendiary action while standing in smoke. If aliens are under X-COM mind control, they will take stun damage from smoke, but never when enemy-controlled.
Note - You can use this bug to your advantage, to save the life of the soldier who first goes through the door of a medium scout. One clever solution given by forum-ites was to have some snipers stationed a bit further away from the UFO. Then one unit bursts into the UFO but is not fired upon due to mutual surprise. You then KO this scout with a stun launcher, then snipe the aliens within the UFO from out of vision range. However, early game, you don't have access to stun launchers. However, you can have someone toss a smoke grenade, putting the interior of the UFO in smoke, then fire off some AC-IN rounds elsewhere to intentionally KO him.
- Smoke will extinguish a raging fire. However, a unit on fire cannot be extinguished by smoke.
- Dropping an activated smoke grenade instead of throwing it saves TU, using 50%+2 TU instead of 75+%.
- Smoke does not travel vertically, upwards or downwards.