70 is a bit nippy…if you’re used to warmer weather. It isn’t the wind… it’s the humidity that will affect your strings. If you’re using sheet music outdoors in the wind, you need to anchor it so it won’t blow around….better to play all memorized rep, if you can. Just like going into an air-conditioned church or hall, it takes a while for the strings to adjust to the temp and humidity. Once they have settled, you can tune to the new atmosphere. The wind may cause the strings to sing as it blows thru them. I’m just speaking from my own experience, as I don’t know what others do, but sometimes the wind makes a whooshing sound thru the mic. Just hope there’s no rain. Keep your cover close by in case it starts to sprinkle.