Please describe in details about the standard atmosphere (ISA) ?

Question

Please describe in details about the standard atmosphere (ISA) ?

Answer 1

Basically, the ISA is just an average temperature/pressure that a given horizontal slice of the atmosphere possesses at any theoretical time.  

For instance, the average annual temperature at 30,000 ft (normally about 250 mb) is -44.4 deg C.

Answer 2

International Standard Atmosphere, (ISA), is used as a base  reference for an atmospheric condition. This base is used generally for forecasting and in my experience, in aviation, is a base condition used by ICAO, ( International Civil Aviation Organization), wordwide as a start point to calculate jet engine, (turbine) performance.

A standard day is a Temperature of 15degrees C, a Surface  Pressure of 29.92 inches of mercury (1013.2 mb), at Sea Level. These values and variations thereof are needed to find any given Density Altitude.

Density Altitude along with ambiant Humidity, will tell aircraft and engine manufacturers along with operators (pilots),

the amount of oxygen molecules in a given volume of air (cold air dense-warm air thin) which when mixed with a given mass of jet fuel allows one to calculate power outputs and turbine temperatures at any given time. For flight planning purposes one can know before hand what allowable power and performance maximums and restrictions to expect, when calculating runway take off distances that will be required along with associated climb performance under normal and emergency conditions.

I hope this has been of some help!