Odour intensity guide
Subtle: Odour is detected but might not immediately be recognisable, described and attributed to a source. You need to focus to recognise it, for example, by standing still, inhaling into the wind and concentrating but the odour will ultimately be recognised.
Obvious: Odour is recognisable, can be described and can be attributed to a source. You can easily smell it without any effort or focus on it.