Within the Urban Observatory ontology we have defined various Classes (available here). For example we define an ObservableProperty class. The Urban Observatories will create multiple instances of each of these classes. Example instances for the ObservableProperty class could be air-temperature or pm10-mass-concentration.
Because a lot of these instances will be used by more than one Observatory it makes sense to list common instances to ensure we all refer to the same definition. The last thing we want is one Urban Observatory using an instance called air-temperature and another called temperature-of-the-air with virtually the same definition. This would make it far more difficult for users to extract comparable data from several Observatories.
Our instances are defined in JSON files. Here's a link to instances for each class: