Winter Service Decision Guide

Frost and Ice Pre-Treatment

RST Forecast	Dew point Forecast	Surface State Forecast	Interim Precipitation Forecast	Treatment	Rate	Comments
+1 to 0C	Dew < RST	Dry	Dry	None
	Dew < RST	Wet	Wet	None
	Dew = > RST	Frost	Dry	Liquid CMA	Normal
	Dew = > RST	Frost	Wet	None
0 to -2C	Dew < RST	Dry	Dry	None
	Dew < RST	Ice	Wet	Liquid or Solid CMA	Ice 1	If thin ice forms reapply CMA at Ice 1.
	Dew = > RST	Frost	Dry	Liquid CMA	Normal
	Dew = > RST	Ice	Wet	Liquid or Solid CMA	Ice 1	If thin ice forms reapply CMA at Ice 1.
-2C to -6C	Dew < RST	Dry	Dry	None
	Dew < RST	Ice	Wet	Solid CMA/Grit	Ice 2	A combined mix of solid CMA and grit or grit followed by solid CMA. If thin ice forms reapply CMA at Ice 1
	Dew = > RST	Frost	Dry	Liquid CMA	Ice 1	If thin ice forms reapply CMA at Normal rate
	Dew = > RST	Ice	Wet	Solid CMA/Grit	Ice 2	A combined mix of solid CMA and grit or grit followed by solid CMA. If thin ice forms reapply CMA at Ice 1
-6C and below	Dew < RST	Dry	Dry	None
	Dew < RST	Ice	Wet	Solid CMA/Grit	Ice 3	A combined mix of solid CMA and grit or grit followed by solid CMA.
	Dew = > RST	Frost	Dry	Liquid CMA /Grit	Ice 3	Apply liquid CMA followed by grit.
	Dew = > RST	Ice	Wet	Solid CMA/Grit	Ice 3	A combined mix of solid CMA and grit or grit followed by solid CMA.

Pre-treatments must be adjusted to take account of residual CMA present.

Snow Pre-Treatment

Weather Conditions	RST Forecast	Interim Precipitation Forecast	Snow Type Forecast	Pre - Treatment	Comments
Snow forecast	0C and rising	Any	All Snow	None
	0C and falling	Dry to moist	Light snow (<10mm/hr)	None
		Dry to moist	Medium/heavy snow	Pre-treat Liquid CMA at Ice 3 rate
		Wet or rain	Light snow	None
		Wet or rain	Medium/heavy snow	Grit

a) Attention must be paid where water may run across the pavement after heavy rains such as from adjacent land and drains which will wash the chemical from the road. These locations should be closely monitored and may require additional treatment in the evening and morning or at other times if the forecasted RST is 0C or below.

b) Frosts usually occur in the early morning. Where frosts are forecast treatment should be close as possible and up to 2 hours prior to the expected time of the frost.

c) When the road is dry and rain is not forecast liquid chemical may be applied more than 2 hours in advance but with caution as moisture will be attracted to the chemical if the dew point approaches RST giving reduced skid resistance. Additional traffic management may be required during this process. Where solid chemical is used this should be applied up to 2 hours in advance to reduce the likelihood of loss of chemical due to traffic or wind action.

NOTE: This decision guide is used as an aid to identifying the appropriate pre- treatment for various weather conditions and forecasts/predictions. Conditions can vary across the network therefore the pre-treatment will need to be assessed to suit the location. Thermal maps assist in identifying the variable RST’s on the network. The objective is to have the appropriate pre-treatment “in the right place – at the right time”.

Storm Treatment

Weather conditions	RST	Treatment	Plough	Comments
Snow covering less than 30mm	Any	None	Plough/broom
Snow covering exceeding 30mm	Any	Solid CMA	Yes	Plough to remove excess snow, slush and ice before chemical application
Hard packed snow/ice	Above -6C	Solid CMA	Yes	Plough until no further snowfall then plough to remove excess snow and ice before chemical application
Hard packed snow/ice	Below -6C	Solid CMA	Yes	Plough until no further snowfall then plough to remove excess snow and ice before chemical application. Chemical will react when RST rises to above -6C
Ice formed	0 to -6C	Liquid or solid CMA	No	Do not apply liquid CMA on ice so thick that the pavement cannot be seen.
Ice formed	Below -6C	Grit/solid CMA	No	Chemical will react when RST rises to above -6C

a) Snow, ice and slush should be removed by ploughing as far as possible before treating with chemical.

b) Time the initial and subsequent chemical applications to prevent deteriorating conditions or development of packed and bonded snow.

Useful Notes

Treatment

Type	Cost ratios
Gritting	1
Ploughing	0.3

CMA
Normal - 25ml/m2 - 7.5mg/m2	10
Ice 1 - 50ml/m2 - 15mg/m2	20
Ice 2 - 75ml/m2 - 22.5mg/m2	30
Ice 3 - 100ml/m2 - 30mg/m2	40

Terms

Black Ice. A thin coating of clear, bubble–free homogeneous ice which forms on a pavement with a temperature at or slightly above 0C when the temperature of the air in contact with the ground is below freezing point and small slightly super-cooled water droplets deposit on the surface and flow together before freezing.

Frost. Ice crystals deposit on the road surface when the road surface temperature (RST) falls below 0C and below the dew point of the air.

Snow. Snow occurs when the layer of the atmosphere from the surface through to cloud level is entirely below freezing.

Light snow. Snow falling at the rate of less than 10mm/hour.

Moderate or heavy snow. Snow falling at a rate of more than 10mm/hour. Visibility may be reduced.

Freezing rain. Occurs when there is a shallow layer of sub-freezing air at the surface underlying an above freezing layer of air above it. When it lands on a road surface with RST below freezing it will freeze on contact to form ice.

Sleet. Occurs when there is a warm layer of air above a relatively deep sub-freezing layer at the surface.

Hail. Forms as a by-product of strong updrafts as in thunderstorms with clouds at heights below freezing. The droplets of water rise upwards in the wind and collide with other droplets increasing in size. The droplets eventually freeze and when the hailstones grow too large to be suspended in the updraft, they fall to earth. Hail is usually not associated with winter storms.