A320 Delta ISA on SD

[applecrumble]

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?

[JABBARA]

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