It's hot in summer like any jacket with sensible level of protection, but only when stationary. It has a vent zipper in each bicep, plus 1 down each side of the back, which hide perforated leather panels. This has 2 effects...1 allows air flow and 2 makes the jacket a little bit larger so it's not clinging. Also for warm weather, the lapels can be opened and secured with poppers (to stop them flapping around). It then looks more like a traditional Highway jacket (Marlon Brando style). It was 32degC when I was in in Barcelona and, whilst my legs were dripping with sweat (I was wearing Triumph Cordura jeans), the Samford was fine with just a Tee under.
For cooler weather riding you can get a Triumph fleece neck and chest liner (not a full jerkin) that fixes to the inside of your Samford with velcro. Down to around 3degC I generally find a thermal vest + Tee + Micro fleece (all long sleeve) is satisfactory under my Samford.
