I am a brand new Tempest WeatherStation user. The UV index seemed to work fine and read correctly for the first 8 hours, but has now been stuck at a reading of 4.8 for the last 18 hours including overnight. I am using an iPhone 11 Max Pro, iOS 13.5.
My Tempest had the same issue 3 weeks ago. You can try a reboot or ask @eric to give it a remote reboot if it’s not easily accessed. A remote reboot fixed it for me and it’s been fine since.
Even a reboot didn’t solve that. I can’t do much more than what I did. Time to contact support (link in top menu). Response time is little slow but they will help you out.
Don’t forget to mention your station ID. See here how to obtain it.
I did reboot it but that did not solve it. Your Tempest isn’t giving a correct value for sure. Open a support ticket as I’m on vacation right now with limited access. Ask them for a FW upgrade of the Tempest (older one and some had a glitch on the light sensor), maybe this can solve the issue, else they will help you out for sure.
Sorry I can’t be of more assistance right now.