What do we need to describe a - simple - fluid locally?

- Macroscopic bulk velocity (or momentum )
- Two thermodynamic quantities, e.g.
- temperature and pressure
- or mass density and internal energy

- LTE: two thermodynamic quantities are sufficient to describe the local microscopic state of the fluid completely: element composition, ionization, occupation numbers, microscopic velocity distribution,
- we do not need to look at the atomic level: we have a continuous fluid.