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why do planes fly in the stratosphere

t would be reasonable to wager that the majority of air passengers have never questioned why the traditional flight altitude is 35,000 feet. The number has entered aviation lexicon as the catch-all height at which most aircraft travel from A to B. But do all planes fly this high Б and if so why? Why do planes fly so high? One of the central reasons behind aircraft altitude is that, as the air gets thinner with every foot climbed, planes can travel more easily and therefore move faster and burn less fuel, saving money. The Бsweet spotБ of flying is regarded as between 35,000 and 42,000 feet Б too high and the oxygen becomes too sparse to fuel the engines, too low and the air resistance is greater. This optimum height is linked to the usual weight of a commercial jet Б that is, heavier planes would fly lower, and lighter higher. It would actually be most efficient to be forever climbing, rather than plateau when reaching say, 35,000 feet, cruising altitude, as the weight of the aircraft decreases due to fuel usage and the air thins. БEach individual aircraft has an optimum altitude (for minimum cost or minimum fuel burn) which will be based on its individual weight,Б explains Peter Terry, a commercial airline pilot of 30 years. БConcorde flew at much higher altitudes Б 50,000/60,000 feet Б where there were no other aircraft and so were able to cruise climb [that is keep climbing]. Б
Doug Morris, a captain with Air Canada, explains that the general rule is the higher the better Бbecause the thinner air imposes less dragБ. БThere is a trade-off between fuel efficiency and power,Б he said. HowБs the weather up there? Flying thousands of feet above the ground also means aircraft avoid much of the bad weather people on the ground are subjected to. You know the feeling when you see nothing but bluebird skies from your window seat, only to descend into your destination airport to dreary drizzle. The troposphere Б that is the atmospheric layer closest to the ground Б is home to most of the worldБs weather phenomenons.


Usually measured up to 36,000 feet, this is where clouds are most likely, as well as heavy rains and high winds. Aircraft prefer life in the stratosphere, which means less turbulence. Flying so high also means that aircraft are able to avoid other airborne traffic, such as light aircraft or helicopters, which fly lower, as well as insects and birds. Light aircraft do not have pressurized cabins, therefore stick below 10,000 feet. Any higher and the pilot is required to don an oxygen mask to keep conscious. Should something bad happen to an aircraft at 35,000 feet, like losing power in its engines, the pilot has much longer to deal with the situation, than if the aircraft was just at 10,000 feet. This may sound silly, but remember that planes can б Б so having more time to get your ducks in a row before attempting such a manoeuvre could save lives. How cold is it up there? The higher you get, the colder it gets, up until 40,000 feet. If the temperature at ground level was 20C, at 40,000 feet it would be -57C. At 35,000 feet the air temperature is about -54C. Is there a minimum flying height? Has anyone seen comedian Eddie IzzardБs Glorious, where he does? He jokes that the pilotБs announcement is thus: БWelcome to Flight One from here to there. WeБll be flying at a height of ten feet, going up to 12 and a half feet if we see anything big. Б Well, why not fly at the minimum height required to clear any obstacles? Because there are laws that govern just that. Known as the lowest safe altitude (LSALT), the value has been applied by aviation bodies around the world, including the British Civil Aviation Authority (CAA) and the US Federal Aviation Administration. б БThis applies much more to light aircraft than airliners, but it is illegal to fly below 1,000 feet when over a built-up area, or 500 feet over any person, vehicle or structure,Б said Richard Taylor of the CAA. БThat is as much for environmental reasons such as noise as it is for safety.


Б Aircraft must not fly less than 1,000 feet above the Бhighest fixed objectБ beneath them and pilots must make sure that they are high enough to clear the congested area below in the event of engine failure. Obviously, these don't apply to take-off and landing when aircraft are in controlled flight paths. б What about a maximum? There is no maximum altitude for flying, however, engines will struggle as the oxygen levels fall, and communication with the ground will become a greater challenge. The record altitude for a jet plane is 123,520 feet, set by Alexandr Fedotov in 1997 flying a military Soviet MiG-25M. Incidentally, the record for a paper airplane is 89,590 feet, which did not fly to that height but was released by a helium balloon. The stratosphere is a. It is the second layer of as you go upward. The, the lowest layer, is right below the stratosphere. The next higher layer above the stratosphere is the. The bottom of the stratosphere is around 10 km (6. 2 miles or about 33,000 feet) above the ground at middle latitudes. The top of the stratosphere occurs at an altitude of 50 km (31 miles). The height of the bottom of the stratosphere varies with latitude and with the seasons. The lower boundary of the stratosphere can be as high as 20 km (12 miles or 65,000 feet) near the equator and as low as 7 km (4 miles or 23,000 feet) at the poles in winter. The lower boundary of the stratosphere is called the tropopause; the upper boundary is called the stratopause. Ozone, an unusual type of oxygen molecule that is relatively abundant in the stratosphere, heats this layer as it absorbs energy from incoming ultraviolet radiation from the Sun. Temperatures rise as one moves upward through the stratosphere. This is exactly the opposite of the behavior in the troposphere in which we live, where temperatures drop with increasing altitude. Because of this temperature stratification, there is little convection and mixing in the stratosphere, so the layers of air there are quite stable. Commercial jet aircraft fly in the lower stratosphere to avoid the turbulence which is common in the troposphere below.


The stratosphere is very dry; air there contains little water vapor. Because of this, few clouds are found in this layer; almost all clouds occur in the lower, more humid troposphere. Polar stratospheric clouds (PSCs) are the exception. PSCs appear in the lower stratosphere near the poles in winter. They are found at altitudes of 15 to 25 km (9. 3 to 15. 5 miles) and form only when temperatures at those heights dip below -78` C. They appear to help cause the formation of the infamous holes in the ozone layer by "encouraging" certain chemical reactions that destroy ozone. PSCs are also called nacreous clouds. Air is roughly a thousand times thinner at the top of the stratosphere than it is at sea level. Because of this, jet aircraft and weather balloons reach their maximum operational altitudes within the stratosphere. Due to the lack of vertical convection in the stratosphere, materials that get into the stratosphere can stay there for long times. Such is the case for the ozone-destroying chemicals called CFCs (chlorofluorocarbons). Large volcanic eruptions and major meteorite impacts can fling aerosol particles up into the stratosphere where they may linger for months or years, sometimes altering Earth's global climate. Rocket launches inject exhaust gases into the stratosphere, producing uncertain consequences. Various types of waves and tides in the atmosphere influence the stratosphere. Some of these waves and tides carry energy from the troposphere upward into the stratosphere; others convey energy from the stratosphere up into the mesosphere. The waves and tides influence the flows of air in the stratosphere and can also cause regional heating of this layer of the atmosphere. A rare type of electrical discharge, somewhat akin to lightning, occurs in the stratosphere. These "blue jets" appear above thunderstorms, and extend from the bottom of the stratosphere up to altitudes of 40 or 50 km (25 to 31 miles).

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