grit is useless after -7 and the roads are hard ice so the grit is pointless, sand might give some traction.
Well I agree that sand or ash would be better but the Carrickfergus rock salt that NI Roads Service uses contains quite a lot of grit.
Not sure where -7 comes from - the eutectic point for sodium chloride & water is -18C = 0F in fact that is what defines the zero point on the Fahrenheit scale. Obviously salt is less effective as the temp falls but this is offset quite a lot by the crystalline structure of ice becoming increasingly amporphous as the temp drops.
The real fun will come when the top surface of the ice melts ... that's why it's so important to get the grit down so there is something to give some traction.
BTW
max here today was -4.1C, temp at the moment is a record -11.2C with partly clear sky & patchy fog. Bear in mind I'm only 200 metres from the ocean with a surface temp around +10C at this time of year.
