Smoke Grenade
General information
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.
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 with as much as three times the obscuring power.
Usage notes
During deployment
The most popular 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.
During scouting
Smoke can greatly reduce dangers of advancing across open ground without cover. Keep a cloud between your troops and the aliens, if their positions are known, or just keep throwing them in front of your front-line troops and advance inside the smoke cloud. When aliens are spotted, 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 units you wish to blind does not work so well because the other side will only have half as much smoke to look through.
The use of smoke in this manner will help to 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.
Outside UFOs
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 individually.
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.
Inside UFOs
The confined spaces inside a UFO greatly diminish 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.
Map limitations
Perhaps the biggest drawback of the use of smoke is the limited availablity of smoke "squares" that can be displayed on the map. Only approximately 400 squares of smoke 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, although this has not been formally tested.
Visual range
The smoke cloud produced by a smoke grenade provides the best screen within the first three turns after the detonation. The initial visibility through the cloud is 4-6 squares. Though the initial cloud may look small, it provides the heaviest cover because the smoke particles are tightly packed together. 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. A good rule of thumb is that every turn allows the alien (and you) to see one tile further into the smoke, until all the smoke particles clear.
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.)
Stunning with smoke
Smoke grenades can double as method of stunning units. There are two methods:
The first method simply involves choking units in dense clouds of smoke. Be warned however that smoke is filtered by armour. So the more armour the target has, the less likely it will choke. Rank and difficulty influence armour greatly -- superhuman commanders are less likely to fall unconscious than beginner soldiers.
The second method of stunning with smoke is to saturate all units that you want to stun with smoke. Then, set off as many incendiary effects as you can. It doesn't matter what you hit as long as the target, or targets, you want to stun are in smoke. Due to the odd nature of fire, stun damage is done to all units standing in smoke every time an incendiary explosion occurs. Use the Auto Cannon for the best effect! (Note: this claim is currrently in dispute. IC explosions do trigger stun damage to soldiers standing in smoke, but apparently not to aliens in smoke.)
Other tips
- Smoke will extinguish a raging fire. However, a unit fire cannot be snuffed by smoke.