Thanks Spitoon for correcting me.
As you say it comes from the AIP. But the 'note' attached to the DAMP description is very important. This is in agreement with EU-Ops.
'DAMP — the surface shows a change of colour due to moisture.
Note: If there is sufficient moisture to produce a surface film or the surface appears reflective the runway will be reported WET.'
All we need is to somehow ensure that on wet days, the runway is correctly reported as 'DAMP' unless unless 'the surface appears reflective' as above.
Then we can legally operate in and out of LCY in all but heavy rain.
See: FODCOM 3/2009
