Is there a way to set an alarm or something for when my Ubuntu computer is about to overheat?
My computer has some temperature issues, so when it's about to overheat I'd like to turn on an external fan.
You can use psensor to monitor the computer temperature. This is a GUI application that readout the CPU and Mainboard sensors.
sudo apt-get install psensor
After starting psensor, go to the "Sensor preference Menu", choose your CPU sensor and click on Alarm - Activate Desktop Notifications. You can modify the temperature limit for your needs:
Can also start after login: