0C - 29C You might want to turn your computer on first!
30C - 49C Idling. No major tasks are running. Are you sure your miner is even on?
50C - 59C GPU is idling in a warm room, or GPU is underclocked. Might be able to crank a bit more power.
60C - 69C There's action happening. GPU is processing at a very acceptable rate!
70C - 75C Okay things are getting really warm. High graphics gaming or optimized mining. We shouldn't get any hotter.
75C - 79C Alright, this is questionable area. Some GPU can handle this range easily, but I'd rather play it safe and back off.
80C - 89C Like playing with fire. At this range, your GPU might seem like it's doing just fine (you don't smell burning plastic, right?) but always assume you're doing long-term damage to your GPU. Only run this high if you don't expect to use that GPU for another year or two!
90C - 99C There's definitely some damage happening. Don't breath in the burning smell, it's probably bad for you!
100C - 109C If you're lucky you can use your GPU after it get this hot, but treat it nice like an abused puppy.
110C+ Congratulations, your GPU is now a paperweight. However, let it cool off before you hang it on your wall