Ack-ack
How do tracer bullets work?
• Tracer rounds are used to indicate a target to other members of your troop or to give machine-gunners confirmation that they are shooting in the right direction, and also to help with directing fire at airborne targets. A soldier will load a single round when he needs to indicate a target. The standard ratio in machine-gun ammunition belts is one in five, although one in three is sometimes used in an anti-aircraft role.
The bullet has the normal cupro-nickel jacket and lead or steel core, but the core is reduced and partially replaced at the rear by a pyrotechnic compound which is ignited by the heat of the propellant charge when fired. This burns with a bright light so the bullet’s track can be seen. Red is the normal colour, but others are sometimes used. The NATO 7.62-millimetre tracer is designed to light up about 100 metres from the muzzle to conceal the firer’s position, and burns out after about 800 metres.
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Great care is taken to ensure the ballistics of tracer and general-purpose rounds match, because it is clearly pointless being able to see where the tracers are going if the other 80 per cent of bullets are going elsewhere.
J. Monk
Walton-on-the-Naze, Essex, UK
• Tracer bullets – which can be seen as they are fired from a gun to their target – are normally used in machine guns to help the shooter see if they are hitting their target. They are also used for the last few rounds in the magazine or belt to indicate it is time to reload. They are most commonly made in military calibres. However, in the US they are available for civilian and police use in all calibres from 0.22 inches to 0.50 inches and also in shotgun gauges.
There are three types: bright trace, subdued bright trace and dim trace. The standard bright tracer bullet starts burning at the muzzle, while the subdued bright tracer bullet doesn’t burn at full brightness until 100 metres or more after it has left the muzzle to avoid giving away the shooter’s position. Dim tracer bullets produce a trace that is difficult to see with the unaided eye but is visible with night vision equipment.
The combustion products of tracer bullets foul up barrels so tracer bullets are only used in a ratio of one tracer bullet to four or five non-tracer standard bullets to prevent clogging. To make the ratio easy to determine, tracer bullets are plainly marked. For example, it is US military and NATO practice to identify bright tracer bullets by a red or orange tip and dim tracer bullets by a violet tip.
Tracer bullets can start fires and should only be used under controlled conditions. Because of this they are illegal in many locations.
Ross Firestone
Winnetka, Illinois, US
Sink or Swim
I have just been on holiday to Jamaica. Leaving aside the question of pollution, why do the waters in the Caribbean appear a clear, almost transparent azure blue while the North Sea at Grimsby always seems grey and murky, even on the brightest, sunniest day?
• Tropical oceans are the clearest because the lack of nutrients severely limits the growth of phytoplankton, and hence the rest of the local food chain. The tropical sun we all love so much warms the surface layer – and consequently makes the swimming so good. But deeper waters remain cold, setting up a strong, permanent thermocline that blocks the circulation that would otherwise bring nutrients from the depths up to the sunlit layer.
In temperate oceans, such as the North Sea, the thermocline is not permanent. During colder times of the year, the surface water is just as cold as the depths and the thermocline vanishes altogether. Nutrient-laden water from the bottom can then come to the surface, replenishing the nutrient supply and allowing the phytoplankton (and all that feeds on them) to grow to such numbers as to make the ocean that lovely murky grey. Just think – if it weren’t for all that plankton, the North Sea off Grimsby would be just as beautiful a blue as the Caribbean.
Ivy Whitehorne
Lower Sackville, Nova Scotia, Canada
• The oceans are, for the most part, a nutrient desert. But photosynthetic organisms such as blue-green algae and cyanobacteria, which form the base of the oceanic food web, require the same things that all plants require: sunlight, water and, of course, nutrients. The oceans have plentiful supplies of sun and water, but they are missing the nutrients. Why is this?
This is where the situation gets complicated. The oceans are warmed by the sun, so the surface is the warmest place to be. However, water does not form a smooth temperature gradient. It divides into sharp delineations of temperature called thermoclines. These layers can be a consistent temperature for many metres and then change by several degrees within the space of a few millimetres. Here there is also often a change in the direction of the current. These two factors mean that as an organism dies or releases waste, the nutrients fall through the layers into the cold depths. After decomposition the nutrients are prevented from mixing with the sunlit surface waters by these thermoclines.
Two conditions in the ocean can circumvent thermoclines. The first takes places on the western edges of major land masses, where the Coriolis effect causes upwellings of cold, nutrient-rich waters. Most of the world’s oceanic biomass comes from these places. The other occurs in the winter in temperate and Arctic waters where the surface temperature is close to the temperature at the seafloor. In fact, in the first few weeks of spring, photosynthetic organisms use up almost all the available nutrients. The remainder of the warm months see a fall in the biomass of the phytoplankton.
There is another reason to be grateful to thermoclines. Without them, photosynthetic organisms in the ocean would rapidly deplete the carbon dioxide from the atmosphere, plunging us into a deep ice age. In all likelihood, life could not exist without them.
Martin Sherman
London, UK
This week’s question
Long shot
My five-year old daughter wants to know if you spit a cherry stone while swinging on a swing, would it go farthest if you spit it while you are at the lowest point of the swing (because you are moving so fast), or would it be better to spit it at the highest point?
Angela Koehler
Bremen, Germany