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 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. Naval methods Warships have sometimes used a simple variation of the smoke generator, by injecting fuel oil directly into the funnel, where it evaporates into a white cloud. An even simpler method that was used in the days of steam-propelled warships was to restrict the supply of air to the boiler. This resulted in incomplete combustion of the coal or oil, which produced a thick black smoke. Because the smoke was black, it absorbed heat from the sun and tended to rise above the water. Therefore, navies turned to various chemicals, such as titanium tetrachloride, that produce a white, low-lying cloud. Infrared smokes The proliferation of thermal imaging FLIR systems on the battlefields necessitates the use of obscurant smokes that are effectively opaque in the infrared part of electromagnetic spectrum. This kind of obscurant smoke is sometimes referred to as "Visual and Infrared Screening Smoke" (VIRSS). To achieve this, the particle size and composition of the smokes has to be adjusted. One of the approaches is using an aerosol of burning red phosphorus particles and aluminium-coated glass fibers; the infrared emissions of such smoke curtains hides the weaker emissions of colder objects behind it, but the effect