If the Eskimos have scores of different words for snow (in fact they don’t, apparently it is the Sami people of northern Europe who have a impressively wide vocabulary on this particular subject) then highways engineers have scores of different permutations to wade through when it comes to dealing with the white stuff when it falls on our roads.
The highway engineer’s bible on this topic is the Well-maintained Highways – Code of Practice for Highway Maintenance , Appendix H Winter Service Issues, as amended 29 November 2011.
This document is nothing if not thorough but for the layman it does not make for an easy read, full as it is of flow-charts, tables, diagrams and seemingly limitless scenarios relating to cold weather and salt.
Perhaps the first thing to appreciate is that mined rock salt (though marine salt can also be used) can be prepared by milling to two sizes, 6mm and 10mm. The difference is, well let’s refer to the manual:
“The larger particle size requires less processing and can therefore be produced faster. Thus significantly more 10mm salt than 6mm salt can be produced in the same period of time. If more orders are placed for 10mm than 6mm then the pressure on UK production has the potential to be lessened. However, smaller particles go into solution much more rapidly than larger particles; particularly in situations of low humidity. A 6mm particle size is suggested for precautionary salting, giving a faster reaction time and better opportunities for salt rate reduction, whereas 10mm is more effective for ice and hard-packed snow.”
As for the effectiveness of salt, the manual says it:
“…melts ice and snow at temperatures as low as -21°C, but below -5°C the effectiveness of the salt is reduced and below -10°C the amount needed increases to become environmentally and economically undesirable.”
Crucial to the whole de-icing process is moisture. Without some wetness the salt does not form a solution and so its melting properties are severely limited. Some local authorities pre-wet their salt but this takes time and money spent on extra equipment.
Prior to forecast snow and ice precautionary salting can take place, and then in the wake of the inclement weather ploughs skim off as much of the volume as possible before the remaining layer is again treated with salt. The danger is that it becomes too compacted for the salt to work effectively, often leaving the only practicable option of mixing grit (sand) with the salt to at least provide some extra grip on the snow.
I could go on but with the manual running to 33 pages it would take me a while and leave you overwhelmed with technical data. Suffice to say the information and instructions are out there. Whether or not people take them in is another. You, the driver can decide as you tiptoe your way around our frozen road network.