W is neg!
Until forecast wind data above 60 km is available, NLC apparent motion is defaulted to be E to W at 15 m/s.
Motion of low cloud (below 1500 m) not shown: obscures more interesting info, and rarely is confused with NLC.
The arrows do not have weather map meaning. It is the shift you would see on your images or in binoculars in 30 seconds or a minute.
Historical data (useful for looking at last week's event) is available starting in May 2019.
Data outage from June 12-17 2019, Mar 21-Jun 05,2021 inclusive. My apologies.
Code will break for southern hemisphere. Mobile app being considered. Other ideas welcome.
Project initiation: late September 2018