[Eddy_Viscosity2]

> how come we can measure the temperature of the microwave background radiation

Because that's the average temperature over an region not the temperature of a finite point. It's kind of like pressure; it really only makes sense at a macro level where you are averaging over a volume or region. The pressure at a singular finite point will also be zero because that singular infinitely small point will likely be in the empty space that makes up an atom.

The original comment was about scalar fields in which there is a value at every finite point. Temperature and pressure are not good examples of these. A better example would be like a magnetic or gravitational field which would have a specific value at every point in space.