Smoke screen

A smoke screen is smoke released to mask the movement or location of military units such as infantry, tanks, aircraft, or ships.

Smoke screens are commonly deployed either by a canister (such as a grenade) or generated by a vehicle (such as a tank or a warship).

Whereas smoke screens were originally used to hide movement from enemies' line of sight, modern technology means that they are now also available in new forms; they can screen in the infrared as well as visible spectrum of light to prevent detection by infrared sensors or viewers, and they are also available for vehicles in a super-dense form used to block laser beams of enemy laser designators or rangefinders.

These are canister-type grenades used as a ground-to-ground or ground-to-air signalling device. The body consists of a steel sheet metal cylinder with a few emission holes on the top and/or bottom to allow smoke release when the smoke composition inside the grenade is ignited. In those that produce colored smoke, the filler consists of 250 to 350 grams of colored (red, green, yellow or violet) smoke mixture (mostly potassium chlorate, sodium bicarbonate, lactose and a dye). In those that produce screening smoke, the filler usually consists of HC smoke mixture (hexachloroethane/zinc) or TA smoke mixture (terephthalic acid). Another type of smoke grenade is filled with white phosphorus (WP), which is spread by explosive action. The phosphorus catches fire in the presence of air, and burns with a brilliant yellow flame, while producing copious amounts of white smoke (phosphorus pentoxide). WP grenades double as incendiary grenades.

Artillery and mortars can also fire smoke generating munitions, and are the main means of generating tactical smokescreens on land. As with grenades, artillery shells are available as both emission type smoke shell, and bursting smoke shell. Mortars nearly always use bursting smoke rounds because of the smaller size of mortar bombs and the greater efficiency of bursting rounds.

Very large or sustained smoke screens are produced by a smoke generator. This machine heats a volatile material (typically oil or an oil based mixture) to evaporate it, then mixes the vapor with cool external air at a controlled rate so it condenses to a mist with a controlled droplet size. Cruder designs simply boiled waste oil over a heater, while more sophisticated ones sprayed a specially formulated oily composition ("fog oil") through nozzles onto a heated plate. Choice of a suitable oil, and careful control of cooling rate, can produce droplet sizes close to the ideal size for Mie scattering of visible light. This produces a very effective obscuration per weight of material used. This screen can then be sustained as long as the generator is supplied with oil, and—especially if a number of generators are used—the screen can build up to a considerable size. One 50 gallon drum of fog oil can obscure 60 miles (97 km) of land in 15 minutes.

Whilst producing very large amounts of smoke relatively cheaply, these generators have a number of disadvantages. They are much slower to respond than pyrotechnic sources, and require a valuable piece of equipment to be sited at the point of emission of the smoke. They are also relatively heavy and not readily portable, which is a significant problem if the wind shifts. To overcome this latter problem, they may be used in fixed posts widely dispersed over the battlefield, or else mounted on specially adapted vehicles. An example of the latter is the M56 Coyote generator.

Many armoured fighting vehicles can create smoke screens in a similar way, generally by injecting diesel fuel onto the hot exhaust.