The International Standard Atmosphere, also called ISA, has been established by the ICAO.
The ISA is a specific standard weather condition used as a reference in aviation.
The ICAO Standard Atmosphere has a Mean Sea Level temperature of 15 degrees Celcius and a QNH of 1013.25 Hpa.
According to the ISA the temperature decrease with the Altitude with a Ratio of 1.98 deg celsius every 1000 feet.
This Standard temperature variation with the altitude gives you the possibility to calculate the temperature at every level.
If you want to know the Temperature at 20000 feet you need to multiply 20 by 1.98 (temperature variation every 1000feet).
The result will be the Temperature variation from the mean sea level to 20000 feet, then, you subtract this result to the 15 degrees (standard mean sea level temperature) and you will get the Temperature at 20000 feet according to the International Standard Atmosphere.
The pressure, according to ISA, decreases by 1Hpa every 27 feet.
So that means that at 27feet of Altitude the pressure will be 1012.25Hpa since the standard mean sea level pressure is 1013.25Hpa.
Knowing this pressure change rate you can calculate the Standard Atmospheric Pressure at any Altitudes using the same principle discussed before.
It is important to understand the meaning of ISA Deviation.
If your Airport is located at the mean sea level and the temperature is 5 degrees this means that on that day we have a temperature which is 10 degrees colder than the ISA one (which is 15 degrees).
We can also say that the temperature is ISA minus 10.
This is important because once you know your temperature and pressure ISA deviation you can calculate the performance of your aircraft.
If you want to know more about the Altimeter setting and QNH check the KZfaq video below:
• QNH, QFE and QNE - [A...
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Disclaimer:
THIS VIDEO IS FOR ENTERTAINMENT PURPOSES ONLY.
NOT FOR REAL-LIFE OPERATIONS.
PLEASE REFER TO THE OFFICIAL MANUAL AND DOCUMENTATION.
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