A downward nod has always felt more like a gentle “hello” gesture where as an upward nod is more of an aggressive “What’s up”. Or even a “what! I didn’t hear you“or “what you looking at!?”. It’s very rare I give stranger’s an upward nod. Someone commented that sometimes they do both in the same motion upward & then downward… I hate when that happens…lol! Honesty the very few times someone has done that it always leaves me with a weard confused awkward feeling that lasts for a gew moments… ha,ha. Don’t do both at the same time.. that’s so weird when it happens lol. Just pick one. I’ve noticed the biggest problem with nodding upward at strangers is that it usually follows with a, “what’s up” or ‘how’s it going”, & sometimes strangers take it as an invitation to start talking to you and share there whole life story lol. I don’t recall this ever happening with a downward nod… Best advice I can give is experiment with it, nod at strangers & then talk to them lol.