avoid detection is the fact that a tank is a large metallic object with a distinctive, angular silhouette that emits copious heat and engine noise. A tank that is operating in cold weather or which needs to use its radio or other communications or target-detecting electronics will need to start its engine regularly to maintain its battery power, which will create engine noise. Consequently, it is difficult to effectively camouflage a tank in the absence of some form of cover or concealment (e.g., woods) it can hide its hull behind. The tank becomes easier to detect when moving (typically, whenever it is in use) due to the large, distinctive auditory, vibration and thermal signature of its engine and power plant. Tank tracks and dust clouds also betray past or present tank movement. Switched-off tanks are vulnerable to infra-red detection due to differences between the thermal conductivity and therefore heat dissipation of the metallic tank and its surroundings. At close range the tank can be detected even when powered down and fully concealed due to the column of warmer air above the tank and the smell of diesel or gasoline. Thermal blankets slow the rate of heat emission and some thermal camouflage nets use a mix of materials with differing thermal properties to operate in the infra-red as well as the visible spectrum. Grenade launchers can rapidly deploy a smoke screen that is opaque to infrared light, to hide it from the thermal viewer of another tank. In addition to using its own grenade launchers, a tank commander could call in an artillery unit to provide smoke cover. Some tanks can produce a smoke screen. Sometimes camouflage and concealment are used at the same time. For example, a camouflage-painted and branch-covered tank (camouflage) may be hidden in a behind a hill or in a dug-in-emplacement (concealment). Deception Some armoured recovery vehicles (often tracked, tank chassis-based "tow trucks" for tanks) have dummy turrets and cannons. This makes it less likely that enemy tanks will fire on these vehicles. Some armies have fake "dummy" tanks made of wood which troops can carry into position and hide behind obstacles. These "dummy" tanks may cause the enemy to think that there are more tanks than are actually possessed. Armour To effectively protect the tank and its crew, tank armour must counter a wide variety of antitank threats. Protection against kinetic energy penetrators and high-explosive anti-tank