Many of these comments focus on getting between car and building. I don’t worry too much about that, because both car and building will be warm. The brief journey won’t cool the harp enough to change wood or strings.
As to playing outside, my general rule is, if I’m comfortable, the harp will be OK. You must keep the harp out of direct sun, as sunshine really heats up an instrument, affecting both wood and strings. Your concern will be not just wood, but strings, especially if you have wound brass bass strings. They contract mightly in cold temperatures, which is your main risk for cracking a soundboard.