I do apprieciate that the braking action measurements become unreliable in wet snow but at what satage will the BAA say to you guy's enough.
It's a difficult question to answer simply so it's well put.
Firstly, though, we have no manner or means at present to measure braking action. All we can measure is surface friction. The two are often confused but are different animals. There may well be some sort of rough correlation between the retardation of brake application and the friction of the surface; much work is being done at the moment to establish what this might be, but it ain't done yet.
I guess the point at which I will decide when to declare the runway closed for contaminant removal will in part depend upon when pilots refuse to continue using it and what the forecast is. We had a light shower of snow the other day which produced a covering on the runway which made it look white but it was very thin (<1mm) and no-one reported a loss of braking action. As both the air & ground temp were >+4C it melted very quickly and I decided to take no action. However, another scenario might be that the forecast is for the snow to continue, in which case I might decide to close early and start sweeping to prevent a build-up that will be more difficult to remove later.
We have a system at LGW where GAL, ATC the major airlines and the Handling Agents all get together to co-ordinate timings of this operation and what the order of flight will be when we re-open.
An alternative to sweeping, where falls are expected to be relatively light, is to apply ant-icing fluid instead of sweeping. This takes a closure of 12 mins to apply fluid that will melt falling snow up to 10mm down to a temp of -5C. The holdover of this fluid is quite good provided it doesn't get washed away by rain before the snow starts (which is quite common in SE England). The fluid itself may reduce surface friction slightly but not below that experienced with a wet runway so we normally declare 'wet, wet, wet' after treatment.
As I expect you all know by now, we're not allowed to publish friction readings during adverse weather, so forget that 'icy' table. Our policy now is to keep working on the runway once it's closed until we get back to black and it's in the 'wet, wet, wet' band.
Cheers,
TheOddOne