I wear a base layer when its below 75F ... armwarmers below 68F
That said there have been only 2 days (2 days during our recent Jesusita wildfire) when I haven't worn a base layer since last Oct. Prolly only been 6 rides when I haven't worn armwarmers since then.
Right now - 2 pm , 63F at SB airport, 57F here in Goleta and heavy Marine layer; been that way since forever... sun comes out about 4:30p and the wind blows at a steady 15...
although it don;t look like we're getting any sun today