In the event of an imminent nuclear apocalypse, we suspect that many people who have access to private jets will immediately take to the skies and escape city centers. This site tracks this indicator in realtime. The current emergency level is reported on a scale of 1 to 5, with 5 being an indicator of a likely imminent apocalypse.
And it only triggers if enough other people are already on their way to safer places.
I think the assumption is that the ultra-wealthy will hear about it before us peasants, so when oligarchs all flee in their private jets it gives the rest of us a warning.