Blank temperature in iOS 8 Weather app (showing just underscores) Since upgrading to iOS 8 on iPhone, the Weather app will occasionally show a "blank" temperature (using an underscore: _ for each empty digit, so two underscores: __). The blank temperature also appears in Notification Center.

It is like this about 40% of the time. I imagine this doesn't affect everyone, but that it is instead affecting only temperature data for certain cities? Has anyone else experienced this and know how to fix it? 

It's been like this ever since iOS 8 first came out on September 17, 2014, and since then I have tried killing the Weather app, restarting my device, etc. and none of those worked.
If the blank temperature is associated with a page that also has a blank city, then the problem might be that you have not allowed the Weather app to "always" access location data. 

You can check the setting by going to "Settings" --> "Privacy" --> "Location Services".