International Standard Atmosphere
A reference model of the atmosphere used to calibrate instruments and compare performance: 15 °C at mean sea level, a pressure of 1013.25 hPa, and a temperature drop of about 2 °C per 1000 ft. Real conditions are quoted as ISA plus or minus a number of degrees.
Practise real radio calls, decode weather and fly ATC scenarios — with an instructor that talks back.