If I recall correctly, the lower one has right of way because visibility from lower to higher is blocked compared to higher to lower. It is easier for the one

If I recall correctly, the lower one has right of way because visibility from lower to higher is blocked compared to higher to lower. It is easier for the one further up in the air column to spot and react to the lower one. Going up is also likely a less dangerous proposition than going down. This is half remembered from a Snowmass balloon rally conversation.