Some random comments cribbed from the Internet on when and why runners might decide to convert to shirtlessness:
A) When it rains
"If it is REALLY downpouring, I will sometimes go shirtless, just because all my running shirts get heavy and feel suffocating."
- by runner librarian, in a blog comment: click here to access original
B) When it's hot and breezeless
1) "In my opinion, bare-chested with a sunscreen that doesn't block the pores (greaseless) is the way to go. When all is said and done a light shirt offers about spf 15 protection at a huge cost of the body's ability to get rid of heat."
- by Wesley Best, on a forum: click here to access original
2) "I never thought I’d run shirtless but I knew the day was going to be hot and I decided to give it a whirl. In hindsight, it was a great idea!"
- by Natalie in a blog posting: click here to access original
C) To be comfortable
"I enjoy running shirtless because its more comfortable, but only when I'm doing my own, peaceful jog. I don't like to go shirtless around others, it makes you look like you're trying to show something off."
- by runnerboy in a Yahoo! Answers entry: click here to access original
D) To avoid injury
"Whenever possible, I like to run shirtless because it's more comfortable and doesn't make my nipples bleed!"
- by supermanz in a blog comment: click here to access original
E) To gain approbation
"[A friend] (shirtless) had passed us going the other direction and was oohed and aaahed at by the girls with his magnificent physique. So I decided to run shirtless for the last section but receieved no swooning from anyone. Ce La Vie!"
- by Peter Giddy in a blog post: click here to access original
May 2015: First up, though I still try to put up blog content whenever I can, it has been easier to more regularly visit the the Twitterverse. Follow me at @barethomas10 and let's keep the shirtless running flag flying. Of course, the blog still attracts very interesting comments, and good discussion. Keep it up.
Second, in the years since this venture launched, and as shirtless running among women has gone increasingly mainstream, the term "jogbra" has clearly declined in use. I will thus prefer "sportsbra" henceforth - as has already been the case on Twitter, and in recent posts here.
I continue to welcome guest posts (sent to firstname.lastname@example.org) on any related topic, including from those who would discourage stripping to the waist. I am myself of course a fervent convert to the joys of running bare. But let all voices be heard!