(Here in the UK, yesterday was the hottest July day on record, with temperatures recorded at London Heathrow exceeding 36˚C, after a long and very cool start to the ‘summer’. I therefore think that this piece, written in the winter of 2005-6, is particularly appropriate.)

Whether you think the Met Office was right in forecasting an unusually cold winter, or perhaps you’re glancing through these pages on a Caribbean beach, you will surely agree that we are sensitive to the temperature.

A few degrees too cold, and we are soon fidgeting about and shivering; a few degrees too warm, and we break into a sweat and feel sleepy or irritable. Volvo and others have invested huge sums of money and plenty of good science into optimising our thermal environment, even if crystal palace office blocks continue to pose insurmountable demands on air conditioning plant.

Given this universal experience, it is surprising that purveyors of ‘new’ technology such as laptops and games consoles are still tripping up on the same cooling problem. As Apple starts the migration from hot PowerPC processors towards Intel chips, in its quest for faster cooler PowerBooks, Microsoft’s launch of the Xbox 360 (famously using PowerPC processors) was marred by complaints probably originating in over-optimistic thermal engineering. This is doubly ironic, as Microsoft has recently hired one of the chief architects of Cray supercomputers, Burton Smith, who had to solve much tougher cooling problems than those in the Xbox 360.

Computer cooling is going to be one of the key factors limiting the performance of many systems. If you had a G4 ‘Mirrored Drive Doors’, an early G4 Xserve, or have ever tried to use a PowerBook G4 on naked flesh, you will already have experienced this limit.

Faster processors, with more transistors squeezed into smaller volumes and running at higher clock rates, put out more heat. Early processors such as the Mac Plus’s 68000 strolling at a leisurely 8 MHz – just 0.4% of the speed of a modest modern 2 GHz component – were comfortably chilled out with a small heat sink looking like a hairbrush for a hedgehog.

As PowerPCs have got faster, general cooling fans gave way to noisy turbo blowers, then the aerodynamic ducting and multi-fan engineering of the G5. Crays have commonly used huge liquid cooling systems, and sometimes even opted to immerse their motherboards in special electrically non-conductive coolant in their bid to stay cool. Xserve clusters, such as the massive System X at Virginia Tech, may spend more on cooling plant than they do on the computers themselves.

You can cheat, particularly for laptops, by using special low-voltage processors, such as the PowerPC 7447A of late PowerBooks. Lower supply voltages mean that the processors turn fewer watts of electical power into heat, as the wattage is the current consumed (amps) times the voltage: halve the voltage for the same current consumption, and you should halve the amount of heat to be dissipated.

But even then, your laptop will still need cooling fans, which increases its power consumption, lowers the battery endurance, increases its weight and thickness, and really upsets the sound world when the whisper-quiet laptop suddenly flashes up those fans.

With Apple’s switch to Intel processors, it is tempting to think that Intel has solved all these problems, and that we will all have 5 GHz PowerBooks in the near future. Sadly this is not the case: Intel may be more willing to take on the challenge of making processors for Apple, but it still has the same basic physics to overcome.

If it is any consolation, when the boiler goes on the blink, you can dig out all your old Macs, fire up the rack of Xserves, and huddle round that high-tech stove. Once your hands have warmed up, you should be able to play on your Xbox 360.

Updated from the original, which was first published in MacUser volume 22 issue 3, 2006. Sure enough, we are not much closer to a 5 GHz MacBook Pro, and the new Mac Pro is ingeniously designed around the need to cool its processors, which can stretch to 3.7 GHz at a push. At least you can buy a rack full of Xserves for a modest sum now.