The squid season
The most dangerous day to ride in a t-shirt is the hot one
More bikes are out, more skin is showing, and the surface you would slide across is hotter and harder than ever.
Summer is the season riders talk themselves out of gear. The jacket feels like a sauna at the lights, the armour is sweaty, and a t-shirt is right there. The problem is that heat changes how you feel, not what happens in a crash. Bitumen at road speed takes skin off in under a second, and it does that whether it is 15 degrees or 40.
There is a second, quieter risk. Heat stress builds while you ride - dehydration and a rising core temperature chip away at concentration and reaction time long before you feel genuinely unwell. So the hot day is the one where the least gear is worn, the most riders are out, and the rider is least sharp. That is not an argument for suffering in a winter jacket. It is an argument for gear that is built to breathe.
A slide does not get gentler at 38 degrees. The only thing that changes is how much skin you left exposed to it.
The fix is not to accept bare arms as the price of staying cool. It is to change what "gear" means in summer - airflow where you need it, protection that stays put, and a couple of cheap tricks that make a hot commute genuinely comfortable. The rest of this guide is how.