Actual representations found online
I selected 6 online representations of pressure around an airfoil, mostly using inconsistent symbolization of the pressure field.
As visible, arrows can be found in nearly every combination of directions. On figure 4, pressures are about 2 hPa (200 Pa) and 5 hPa, which is not really understandable when pressure at sea level is about 1,000 hPa.
Last figure shows the pressure field in the cross-section, not only at the surface of the airfoil (which I believe is the case for the other figures).
Assumptions made to read the figures
By following the answers on this site, I know that the pressure on the lower side of the airfoil is greater than the pressure on the upper side. I also assume that the pressure on the upper side is lower than the ambient pressure, and the pressure on the lower side is greater than it. I believe the drawings are trying to depict these two aspects, but using different (and unspecified) conventions.
Probably the clearer convention is the one used on fig 6: Yellow is likely the ambient pressure, orange would be greater, green lower, blue even lower.
What are the usual pressures found around an airfoil, e.g. in cruise, at mid-span of a A320 wing (compared to the ambient pressure).
What is the most correct representation of the pressure field in the figures above?
Is there some conventional way used by engineers to depict the pressure field without ambiguity?