On one bike forum I frequent, it seems that most of the people (in the USA) who commute, live in the northern areas of the country--where it's coldest. They often claim that riding is easier to do year-round in the cold than it is in the heat, because you can dress for the cold (adding or opening/removing clothes as you need while riding) but there's no really good way to keep cool in the heat.

I know I live in central Illinois right now, and on days when it's ~100 degrees and 90% humidity, extended riding isn't very practical or fun. Doing anything outside--even just standing in the shade--causes you to sweat enormous amounts.
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