This is a complicated subject which I have been dealing with the past day or two.
There are sensors which report temperatures and there may be 10 of them. I think CPU Core temperature is one of the most useful but also a bit tricky to access. I am currently using MacsFanControl (see it in the menu bar) which lets you see the proximity temperature which tends to underestimate core temperature by about 5°C, or diode temperature which over-estimates core temperature by about 5°C. Hardware Monitor does a thorough report but when I ran it I was experiencing strange behavior in other things on my computer so I decided to go with a simpler tool.
Temperature will also vary depending upon computer activity and room temperature. Right now while just typing ASC replies my MacBook has a diode temperature of 57°C. Yesterday when processing video it was at 89°C.
Apple specifications include operating temperatures but those are ambient (room) temperatures. From what I have read online having a core temperature in the low-use range I see is perfectly normal. 89°C is high but when I do video the CPU is working 100% so may be expected. I'll just have to make sure if I do it not to do it on a day when it is 90°F in my room. If things really get too hot a computer will (should) automatically shut down.
MacsFanCotrol also reports fan speed. Right now mine is working just above minimum which I interpret it thinking 57°C doesn't need reducing much. When I was doing video it was running at maximum 6000 rpm.
These are some of the things I tried to let my late 2008 MacBook run a bit cooler:
- I got 4 rubber stoppers ("corks") and put them under the corners so there is more clearance under the computer and the case can be cooled. This obviously only works when you have the computer sitting undisturbed. I also try to have my room fan blowing in this direction so there's air moving under it.
- I removed the back case completely (requires a tiny screwdriver) and used canned air to blow dust off the fan blades and clear the exhaust vents. Be careful you don't freeze parts with canned air.
- I have my computer also sitting on a cooling rack which is basically a couple of fans in a plastic tray which blow air onto the base of the computer. The model I have is almost useless but I got it for $1 at a garage sale to test. You can get better models for $20+ (check reviews on Amazon).
Obviously don't sit your computer on a fuzzy surface, or let anything block the exhaust vent at the back.
All that said, I haven't actually felt the keyboard since I only use my computer in clamshell mode.