The problem has to do with the battery's ability to convert its stored chemical energy into electrical energy. Cold temperatures slow down that process.
Note: If your iPhone looks weird in the cold, it's because the cold weather is slowing down the liquid crystals in its LCD screen.
The bottom line? Keep your iOS device at temperatures that you think are comfortable.
But, don't think the summer is necessarily better for your iPhone. Check out this video of a former Apple employee warning you that an iPhone battery can be permanently damaged if it's extremely hot outside.