I've used my Montbell Tachyon windshirt or my Patagonia Nine Trails windshirt for temps below 60F when it's cool and wind is present. Above those temps for me I find I need very little in the way of wind protection as the air temperature at that range is comfortable. Windshirts are quite different than nylon button up hiking shirts which are far from windproof, rather they're intended to be layered over a baselayer, midlayer, or insulation piece. I wouldn't use the Tachyon against your skin if you're on the go, it would stick to you Glad like Press and Seal, become clammy, and likely feel like you're wearing a trash bag. Can you use the Tachyon at 70F? I suppose so but 70F is rather "warm", if it were a cool 70, overcast, with wind present, I believe the Tachyon would nearly cut all the wind and breathe well enough to keep you comfortable. If you're moving again, I'd find the Tachyon too warm and would likely throw it in my pack.