Use 'impalpable' for things you can't physically feel, not just what you can't see. The impalpable chill in the air made the room feel haunted.
Pro Tip 2/3
Subtlety of Effect
'Impalpable' suggests a subtle or delicate influence that's hard to notice. She had an impalpable influence on his decisions, hardly noticeable but ever-present.
Pro Tip 3/3
Abstract Qualities
'Impalpable' often describes abstract qualities like feelings, ideas, or atmosphere. The impalpable sense of dread lingered long after the story ended.