In site-specific installation or situated media, a significant part of the "I" in NIME is the environment, the site and the implicit features of site such as humans, weather, materials, natural acoustics, etc. These could be viewed as design constraints, or features, even agency determining the outcome of responsive sound installation works. This paper discusses the notion of interface in public (especially outdoor) installation, starting with the authors’ Sculpture by the Sea Windtraces work using this recent experience as the launch-pad, with reference to ways in which others have approached it (focusing on sensor, weather-activated outdoor installations in a brief traverse of related cases, e.g. works by Garth Paine, James Bulley and Daniel Jones, and David Bowen). This is a dialogical paper on the topic of interface and ‘site’ as the aetiology of interaction/interface/instrument and its type of response (e.g. to environment and audience). While the focus here is on outdoor factors (particularly the climatic environment), indoor site-specific installation also experiences the effects of ambient noise, acoustic context, and audience as integral agents in the interface and perception of the work, its musical expression. The way in which features of the situation are integrated has relevance for others in the NIME community in the design of responsive spaces, art installation, and large-scale or installed instruments in which users, participants, acoustics play a significant role.