Dress Code for Summer

My wedding is taking place in the evening in summer (ceremony for family-only at 6pm, followed by cocktail hour/reception at ~6:30pm).

My mom seems to think "formal" should be the dress code and from what I've read most evening weddings are(?) However it'll be 90° at the time of our ceremony and will still be ~80° at sunset around 8:30pm. I don't feel comfortable demanding people wear floor length gowns and multi-piece suits with that kind of temperature and, honestly, I don't necessarily want people feeling they need to dress up THAT much.

We've listed the attire as semi-formal. Thoughts? TIA

Edit: I should mention it's outdoors but the reception is in a tent with at least some AC capability