Reading Comprehension

A forecast of a 30 percent chance of rain is among the most familiar and most frequently misread numbers in daily life. In the United States National Weather Service's definition, the figure is the probability that at least a hundredth of an inch of rain, the smallest measurable amount, will fall at any given point in the forecast area during the stated period. Forecasters sometimes describe it as the product of their confidence that rain will develop somewhere in the area and the fraction of the area they expect it to cover. On that account, 40 percent could mean near certainty of rain over two-fifths of a region or a 40 percent chance across all of it. Surveys have found many people reading the number as the share of the area that will be wet or of the period during which it will rain. A 2005 study led by the psychologist Gerd Gigerenzer found such readings common among people surveyed in several European cities. The same study noted that forecasts rarely say what the percentage refers to, whether days, places or hours. Other researchers have found that people read the figure more accurately when it is paired with the chance of no rain, although no wording yet tested has removed the misunderstanding entirely. Forecasters themselves disagree about whether confidence multiplied by coverage is the best way to explain the number, since the formal definition refers only to the chance at a single point.

Israeli forces are often credited with the first systematic use of small unmanned aircraft in combat, during the 1982 war in Lebanon, when propeller-driven drones such as the Scout and the Mastiff flew over the Bekaa Valley. Carrying television cameras that sent live pictures to the ground, the drones located Syrian surface-to-air missile batteries, and some served as decoys that led the missile crews to switch on their radars, revealing positions for Israeli aircraft to attack. Impressed by the campaign, the U.S. Navy arranged to buy a system derived from the Israeli designs, and the result, the Pioneer, entered service in 1986. Battleships launched it with a rocket booster, caught it in a net on its return and used it to spot where their heavy shells were landing. In the Persian Gulf War of 1991, Iraqi troops came to associate the sound of the small aircraft overhead with the bombardment that tended to arrive soon after it. On one occasion a group of Iraqi soldiers on Faylaka Island waved white cloths at a Pioneer launched from the battleship Wisconsin, an episode widely described as the first surrender of troops to an unmanned aircraft. The Pioneer's record helped persuade American commanders that drones were more than targets or curiosities, although critics pointed to the many aircraft lost in accidents during its early years.

Heavy bombers and fighters of the Second World War needed long, firm runways, yet the campaigns in which they fought often moved faster than concrete could be poured, and many of the places where airfields were wanted, from Pacific islands to the farmland of northern Europe, offered only sand, mud or coral. The answer the United States adopted was pierced steel planking, interlocking steel sheets that could be shipped in bulk and laid by hand over graded ground. Each plank was about ten feet long and fifteen inches wide, weighed some sixty-six pounds, and was stamped with rows of holes that cut its weight and let rainwater drain through, while hooks and slots along its edges locked it to its neighbors. The material took its popular name, Marston mat, from the village in North Carolina near which Army aviation engineers tested it in November 1941 by laying a runway of about 3,000 feet in eleven days. Engineer units, including the Navy's construction battalions, laid such strips in every theater of the war, often within days of an area being secured. The mat had drawbacks that crews learned to respect: it was slippery when wet, it could shift or buckle under repeated landings, and the soil beneath still had to be graded and drained, since the steel spread the load of an aircraft without carrying it.

Before 1941 most navies regarded the aircraft carrier as an auxiliary whose aircraft would scout for the battle line and perhaps slow an enemy fleet enough to be brought to action by guns. The Pacific war eclipsed that conception within months. A battleship could strike only as far as its guns could throw a shell, a distance measured in tens of thousands of yards, while a carrier could deliver an attack at a range measured in hundreds of miles. The side that found the enemy first could therefore strike before being struck. The engagements of 1942 made the consequence explicit, for in the first carrier battle the opposing surface fleets never came within sight of one another, and at Midway a few minutes of successful attack destroyed a force that no gun duel could have reached. Scouting became the decisive activity, and a fleet that divided its aircraft carefully between searching and striking held an advantage over one that committed everything to the attack. Naval thinking was reorganized around that fact, and the battleship, still valuable for shore bombardment and antiaircraft defense, ceased to be the measure of a fleet's strength.

Corrosion is not the slow dissolution it appears to be but an electrical process, in which one region of a wetted metal surface gives up electrons while another consumes them, and the metal is eaten away at the region that surrenders them. Whether that process ruins a structure or protects it depends on the oxide it leaves. Iron produces an oxide that takes up more room than the metal it replaced, so the layer flakes away and exposes fresh metal to the next round, and the reaction continues until nothing is left to consume. Aluminum, a far more reactive metal by any laboratory measure, produces an oxide that is thin, dense and tightly bonded, and that film halts the reaction almost as soon as it begins. Stainless steel obtains the same protection from the chromium dissolved in it, and its film reforms within moments wherever a scratch removes it, provided oxygen is at hand. A third strategy accepts corrosion and directs it, since a coating of zinc on steel corrodes in preference to the iron it covers, and the coating is consumed while the structure it guards is spared. Rust is therefore no measure of how reactive a metal is, only of how useless the product of its reaction turns out to be.