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آمار و اطلاعات

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103/1/10
9:59 ص

 

When we measure air temperature with a common liquid thermometer an incredible number of air molecules bombard the bulb transferring energy either to or away from it. When the air is warmer than the thermometer the liquid gains energy expands . And rises up the tube the opposite will happen when the air is colder than the thermometer .The liquid stops rising (or falling)when equilibrium between incoming and outgoing energy is established.At this point we can read the temperature by observing the height of the liquid in the tube.
It is impossible to measure air temperature accurately in direct sunlight because the thermometer absorbs radiant to energy from the air molecules.The thermometer gains energy at a much faster rate than it can radiate it away and the liquid keeps expanding rising until there is equilibrium between incoming and outgoing energy .Because of the direct absorption of solar energy the level of liquid in the thermometer indicates a temperature much higher than the actual air temperature and so a statement that says today the air temperature measured 100 degrees in the sun has no meaning .Hence a themometer must be kept in a shady place to measure the temperature of the
air accurately.
Of these instruments measure the electrical resistance of a particular material.Since the resistance of the material chosen for these thermometers changes as the temperature changes a meter can measure the resistance and be calibrated to 
represent air temperature.*
Air temperature may also be obtained with instruments called
infrared sensore or radiometers.
Radiometers do not measure do not measure temperature directly;rather they measure emitted radiation (usually infrared).By measuring both the intensity of radiant energy and the wavelength of maximum emission of a particular gas ( either water vapor or carbon dioxide);radiometers in orbiting satellites are now able to estimate the air temperature at selected levels in the
atmosphere.
A bimetallic thermometer consists of tow different pieces of metal (usually brass and iron) welde together to form a single strip .As the temperature changes the brass expands more than the iron causing the strip to bend.The small amount of bending is amplifred through a system of levers to a pointer on a calibrated  
scal.
The bimetallic themometers is usually the temperature sensing part of the thermograph an instrument that measures and records temperature (see
fig.3.16).
(chances are you may have heard someone exclaim something like Today the thermometer measured 90degrees in the shade!Does this mean that the air temperature was higher in the sun?If you are unsure of the answer read the Focus section above before reading the next section on
instrument shelters.)
Thermometers and other instruments are usually housed in an instrument shelter (see fig
3.17).
The shelter completely encloses the instruments;protecting them from rain snow and the sun;s direct rays.

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