A320 Delta ISA on SD
 

Heya guys, why is it only once you have pulled standard on the captains EFIS selector that the delta ISA appears?
Why isn't it shown on the ground?

Because ISA DEV is the deviation from standard temperature at given pressure altitude. For example at 5000 standard altitude, standard atmosphere temperature should should be around 5 degree Celsius. If SAT is 10 degree, then ISA DEV is +5 degree.

To see what the standard altitude is, QNE (1013.25 hPa) should be set to altimeter (pull Standard)

I hope this helps