On the Importance of Cleanliness
But even a relatively clean computer suffers from dust. You don't need giant dust bunnies clogging your CPU fan to see a significant impact on its cooling effectiveness. The thin, translucent coating of dust that settles onto anything after a few weeks is actually a pretty good insulator. It's like wrapping your heat sync in thermal underwear.
Here is what happens when you clean off that thin little layer of dust:
I have the polling rate for the temperature set at 1/20 Hz, so this is actually a significant length of time. I mention this to demonstrate that the temperature drop wasn't caused by the compressed air.
It's also worth noting that the hotter a semiconductor (or, in most cases, an ordinary conductor) gets, the higher its resistance. The increased overall resistance will cause larger voltage drops within the gate logic. Semiconductors require a minimum voltage to work reliably. So, you have two options; raise the supply voltage, thus dissipating more power, or run the risk of the voltage dropping too low somewhere in the gate logic, thus causing a logic fault. Either way, it's bad.
If the machine adjusts the supply voltage to avoid a logic fault, the part will get hotter, causing its resistance to go up even more, which requires another increase in supply voltage. One hopes that the cycle damps out before it runs away and the part explodes. So, even a relatively small improvement in heat dissipation can lower the temperature significantly due to this compounding effect.
So, boys and girls, remember to keep your computer clean. It will run cooler, last longer, and use less electricity.