We’re officially into the “odd uneven time” of seasonal transition—when a cloudy morning could just as easily mean rain or a balmy afternoon.
Our suggestion: look to the perennially underrated henley.
It splits the difference between a tee shirt and a rugby, which is ideal in casual layering situations, but it can hold its own when the temps unexpectedly rise and your collar needs loosening. (A nautical stripe helps with the latter.) Some might even be adventurous enough to slip it under a blazer (what helps here: being Ewan McGregor).