67هستش. - همه جوره

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103/2/8
9:32 ع

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NA.GH[79]

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بایگانی وبلاگ

اسفند 88[1] اردیبهشت 90[5]

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عاشق آسمونی فانوسهای خاموش جیغ بنفش در ساعت 25 نوری چایی_بیجار مذهب عشق مرجع اس ام اسSMS امپراتوری هخامنشیان ME&YOU شرکت نمین فیلتر دریچه در فراق گل نرگس درختری از جنس باران

عناوین یادداشتهای وبلاگ

Hadrese file injast bardar salam shirin jan 70هستش 69هستش عناوین یادداشتها[11]

67هستش.

NA.GH

Is there some where in our atmosphere where the air temperature can be exceedingly high (say above 500 c or 900 f)yet a person might feel extremely cold .There is a region but is s not at the earth s suface.
You may recall from Chapter that in the upper of our atmosphere ( in the middle and upper therm osphere) air temperatures may exceed 500 c . How ever a thermometer shielded from the sun in this region of the atmosphere would indicate an exltremely low temperature.This apparent discrepancy lies in the meaning of air temperature and how we measure it.
In Chapter 2 we learned that the air temperature is directly related to the average speed at which the air molecules are moving –faster speeds correspond to higher temperatures.In the middle and upper thermosphere air molecules are zipping about at speeds corresponding to extremely high temperatures.However in odere to transfer enough to heat something up by conduction (exposed skin or a thermometer bulb); an extremely large number of molecules must collide with the .In the ;thin; air of the upper atmosphere ;air molecules are moving extraordinarily fast but there are simply not enough of them bouncing against the thermometer bulb for it to register a high temperature .In fact when
properly shielded from the sun the thermometer bulb loses far more energy than it receives and indicates a temperature near absolute zero.
This explains why an astronaut when space walking will not only survive temperatures exceeding 500 c but will also feel a profound coldness when shielded from the sun s radiant energy.At these high altitudes the traditional meaning of air temperature (that is regarding how ;hot; or ;cold something feels)is no longer applicable.
By the body must be equal to the heat it loses surroundings .There is therefore a constant exchange of heat –especially at the surface of the skin –between the body and the environment.
One way the body loses heat is by emitting infrared energy. But wenot only emit radiant energy ;we absorb it as well. Another way the body loses and gaings heat is by conduction and convection;which transfer heat to and from the body by air motions.On a cold day a thin layer of warm air molecules forms close to the shin protecting it from the surrounding cooler air and from the rapid transfer of heat .Thus ; in cold weather when the air is calm the temperature we perceive- called the sensible temperature –is often higher than a thermometer might in dicate. (Could the opposite effect occur where the air temperature is very high and a person might feel exceptionally cold?If you are unsure read the focus section above).
Once the wind starts to blow the insulating layer of warm air is swept away;and heat is rapidly removed from the skin by the constant bombardment of cold air when all other factors are the same the faster the wind blows the greater the heat loss is usually expressed. As a wind –chill factor. The wind –chill charts (Tables 3.2and3.3)translate the ability of the air to take heat away from the human body with wind (it s cooling power)in to a wind –chill equivaent temperature with no wind.For example notice that in Table 3.2an air temperature of 20 f with a wind speed of 30mi/hr produces a wind –chill equivalent temperature of -18 f .This means that exposed skin would lose as much heat in one minute in air with a temperature of 20 f and a wind speed of 30 mi/hr as it would in calm air with a temperature of -18 f .Of course how cold we feel actually depends on a number of factores; including the fit and type of clothing we wear and the amount of exposed skin.
High winds in below –freezing air can remove heat from exposed skin so quickly that the skin may actually freeze and discolor.The freezing of skin called frostbite usually occurs on the body extremities first because they are the greatest distance from the source of body heat.

90/2/3::: 3:32 ع

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