I was intrigued to read the environmental requirements of the iPhone (and the iTouch, of course). There's a bit of ambiguity in the temperature department.
It states that the operating temperature should be from freezing (32° Fahrenheit, 0° Celsius) to 95°F (35°C). However, then follows the "nonoperating temperature" range, which is 4°F to 113°F (-20°C to 45°C). That's quite a span. My guess is that Apple means you should avoid operation between 4°F to 32°F and 95°F to 113°F.
Meanwhile, on its battery life-cycle page, Apple offers a "hot tip:"
If you use your iPod, iPhone, or notebook in temperatures higher than 95°F (or 35°C), you may permanently damage your battery’s capacity. That is, your battery won’t power your device as long on any given charge. You may damage it even more if you charge the device in these temperatures. Even storing a battery in a hot environment can damage it irreversibly.
Now, the popular hot Bikram Yoga classes (and other styles) are conducted in temperatures between 95°F and 105°F. While you might not be listening to your iPod or iPhone during the class (and certainly not using your MacBook), those devices could be in a bag nearby.
And if you drove to the class, you might not be better off leaving the devices out in the car.
According to National Highway Traffic Safety Administration research (and warnings), car temperature can climb quickly, even on a mild day.
Cars parked in direct sunlight can reach internal temperatures up to 131°F - 172°F (55°C – 78°C) when outside temperatures are 80°F – 100°F (27°C – 38C).” Even outside temperatures in the 60s can cause a car temperature to rise well above 110°F. When the outside temperature is 83°F, even with the window rolled down 2 inches, the temperature inside the car can reach 109°F in only 15 minutes.
Of course, this time of year, any concern about outside temperature would be on the low end. Using an iPhone in a very cold environment can also decrease battery life. However, Apple says that this is a temporary condition.
Once molecules in the battery warm up, the battery will return to its previous capacity.
So what to do?
According to the Battery University site, lithium-ion battery users should avoid hot cars and keep operating and storage conditions "cool."
The group also suggests that users "consider removing the battery from a laptop when running on fixed power."
That practice might be good for the battery, but it would stop your MacBook from having the automatic backup power supply functionality in case the power goes out — or the MagSafe adapter falls out. It happens.
Still, my office has several windows and the sun sometimes strikes my desk. I use a notebook stand that provides plenty of air space underneath the MacBook Pro, but I've never considered the combination of environmental and operational temperature. Perhaps I will pull out the battery when I'm not using the laptop as a mobile machine.
If you're interested in safety issues with lithium-ion batteries, I point you to an American Chemical Society article on burning batteries released last week. It says the rate of hazardous failures for lithium-ion batteries is rare, between one in 1 million and one in 10 million.
Here's what happens:
At other times, batteries fail violently. Given that lithium-ion cells contain a flammable electrolyte solution consisting of lithium salts in organic solvents such as ethylene carbonate and ethyl methyl carbonate, the heat generated from an internal electrical short could, under some circumstances, ignite the liquid or rapidly raise its vapor pressure until the cell bursts. Likewise, the cathode and anode surfaces can support unwanted reactions or begin to decompose under conditions that lead to thermal runaway, the onset of an unstoppable and potentially violent chain reaction.