The following quote is taken from FAA document ‘Airplane Performance Harmonization Working Group, Task 1 – Airplane Performance Operating Limitations', circa Nov 1997.
The conclusion provided in the FAA Report for Project 308-3X typifies the results shown by the data in the other reports: “The absolute values of friction coefficient between the low reflective surface (damp) and high reflective surface (wet), where there were no large areas of measurable standing water, were approximately the same.” This conclusion is echoed in Engineering Sciences Data Unit Item Number 25, paragraph 5.2.2, which states, “In damp conditions, with the exception of surfaces such as I in Figure 7 [which is a surface with an open macro-texture and harsh micro-texture, such as a grooved or porous friction course surface], the coefficient of friction is noticeably reduced from the dry surface value, the effect becoming most marked on surfaces such as IV in Figure 7 [which is a closed macro-texture, smooth micro-texture surface].
i.e. Damp = Wet, and the grooved / porous exception is not declared as dry.
The document notes at that time that the JAA allowed a damp runway to be interpreted as dry, whereas the FAA (with minor exceptions) did not. It was suggested that the JAA should align JAR-OPS with the FAA position in order to harmonize the regulations.