One of my favorite apps for Lumia phones is still LockTemp. The app is super old and hasn't been updated since April 2014, but it still works and serves a purpose on my Lumia 950 or Lumia 950 XL. I've been using for years, writing the same tip back in 2013 and I still swear by it.
The single function of this app is to show the current temperature with a nice little thermometer on your lock screen. The best part is it even works with Glance so that you can just peek at your phone to know the temp.
LockTemp is simple enough. You download and install it. Run the setup and enable geolocation and then add it to your quick status area under your lock screen settings. The phone displays the temperature in Fahrenheit or Celsius and updates once an hour.
And that's it. Sure, you can use a full weather that takes over your lockscreen or use the native Weather app's detailed status setting (which is truncated and looks terrible, by the way), but sometimes the simplest solution is the best.
LockTemp has a free trial and otherwise costs 99 cents.