Winter Weather – Useful Information
Sources Of Moisture.
In very general terms, moisture comes from four main sources: 
Precipitation from clouds (rain, sleet, snow and hail).

Condensation from the atmosphere (dew, frost and fog).

Melting lying snow adjacent to the road.

Non meteorological (seepage, burst pipes, springs etc).
Ice.
Ice (sometimes referred to as black ice, because the road blacktop can be seen through it) is formed
when road temperatures fall below freezing and there was previously standing water on the road
surface, or it was damp. The dampness or standing water almost exclusively is due to precipitation
(rain, sleet snow or hail).
Black Ice.
Black Ice is comparatively rare due to pre-salting and other factors.
Roads tend to dry up very quickly because the camber of the road encourages draining, and vehicle
tyres are designed to spray water away from the vehicle track. A normal road given good drying
conditions will be dry within the hour and even in relatively bad drying conditions, the crown of the road
is normally dry within two hours.
Therefore, as a general rule, unless there are compensating local factors, significant black ice will only
form if road surface temperatures fall below freezing within 2 hours of rain stopping.
Hoar Frost, Dew and Flash Frosts.
Hoar frost needs a combination of the following: 
Road temperature below freeing point.

Road temperature less than the dewpoint.

A slight breeze overnight.

Long nights (late November to mid January).
Initially, hoar frost forms a highly visible white deposit on the roads. As drivers can see it, they usually
take care. However, the hoar frost does not remain in this state for long. Pressure from car tyres
causes temporary melting of the hoar frost, which will re-freeze afterwards if the road remains below
freezing. Thus, the white hoar frost is transformed into a sheet of clear ice. This may only be a shortlived phenomenon before the road temperature rises above freezing, but it can be highly dangerous.
Flash frosts are likely if the following occur under the following conditions: 
A calm, clear night.

A late sunrise (usually November to January).

An increase in traffic at sunrise.

Surface near the road facing the rising sun, e.g. embankments.

Hazy or misty conditions, maybe with shallow fog banks (although the clear sky must be
visible).

A thin bank of cloud on the south-eastern horizon that can temporarily block the sun (although
most of the sky should be clear).
The combination of a slightly falling road temperature, and constant replenishment of the air in contact
with the road surface will cause a surprisingly quick build up of hoar frost (it can form within 15 minutes).
Fog and Freezing Fog.
Fog is essentially cloud that forms at ground level. It can either form in situ on clear nights (radiation
fog), or from a lowered cloud base (hill fog), or be due to warmer air traveling over colder ground. The
main points to note about fog are: 
Road minimum temperatures will occur much earlier (normally in the first stage of fog
formation).

Once fog has formed road temperatures will be static or even rise a little.

Serious ice resulting from freezing fog on low level roads is rare. It needs air and road
temperatures to be below freezing accompanied by a breeze.

Serious ice from freezing fog on higher level roads is more common.
Freezing fog occurs when the temperature of the fog droplets falls below freezing. They remain in their
liquid state, but on contact with something (tree branches, car windscreens, roofs etc) they freeze very
rapidly. Thus, freezing fog can build up deposits of opaque white ice called rime. The deposition of
rime directly on the road surface is comparatively rare, because of the warming effect of the fog and the
road temperature is usually equal to or higher than the air temperature, which discourages deposition.
Deposition may be encouraged by an increase of wind, but if the wind increases too much, the fog will
disperse. Thus, although build up of ice may be considerable on tree branches and grass verges, the
amount on low level roads will usually be much smaller and easily prevented by a pre-salt.
Rime is likely to be more of a problem on higher roads than lower roads because winds may be slightly
stronger (encouraging deposition). The fog droplets themselves will be colder and higher roads will be
nearer the top of the fog layer so that the warming effect of the fog present at lower levels is much less
effective.
Sleet, Hail and Freezing Rain.
Sleet is partially melted snow. It occurs with air temperatures close to, but just above freezing point. As
sleet contains ice crystals, which melt on contact with the road surfaces, it will actually cool the road
surface down more than pure rain falling at the same temperature. If a shower of sleet occurs during
the day, road temperatures normally recover quite quickly. However, if a sleet shower occurs in the
evening, road temperatures will stay depressed and as a result may fall below freezing several hours
earlier than first thought. If sleet becomes heavy, it may readily turn to snow.
Hail is essentially frozen raindrops and is a more dense form of ice than snow. It occurs in shower
clouds and is rarely prolonged (although there may be several showers, one after the other). Similar to
sleet, hail will have a cooling effect on the road. It can occasionally produce quite treacherous road
surfaces, as a carpet of melting hail is very slippery. Fortunately, hail is rarely that heavy in winter. As
with sleet, the most dangerous time for hail to form is during the evening, as melting hail may quickly
freeze after the shower has passed over. Hail is more common in the second half of the winter,
(February to April), and occurs more near coasts than well inland.
Freezing rain is a rare but exceptionally dangerous condition. It occurs when rain, from clouds in
warmer air aloft, falls through a layer of air below freezing. The rain drops do not freeze immediately,
but only after contact with a surface. The rain does not freeze until hitting something and the ice that
forms is of the classic ‘black’ kind, i.e., clear sheet ice. It can build up very quickly and covers the
complete road surface. Freezing rain tends to occur at the end of a cold spell, when most surfaces are
below freezing, but milder air is pushing in. There is no way that freezing rain can be anticipated, other
than taking guidance from a weather forecasting organization.
Salt Wash-off.
Light rain or drizzle encourages the salt to go into solution when salt spreading provided that it is only
enough just to dampen the road surface. However, if the rain is heavier, it may remove the salt
completely. As a general rule, if the rain has been falling for more than half an hour and has been
heavy enough to cause standing water on the road surface, then the salt is likely to have been washed
off. One way to check is to look at the chemical factors on the road sensors. These will normally show
if there has been much dilution of the salt.
Weather forecasting organisations will issue updated forecasts if salt wash off is expected.
However, if the forecast is for showers overnight, then it is as well to keep in touch with the weather
forecast organisations, and to monitor road sensors.
Other Sources Of Moisture.
Melting lying snow – either snow that has been ploughed, the compacted snow at the side of the road or
snow on adjacent grass verges and both may be quite persistent. For, during the day, when
temperatures are normally above freezing, the snow will release water as it melts, thus wetting the road
surface. Overnight, if road temperatures fall below freezing, widespread ice may form even though
there has been no rain.
Seepage - after a period of prolonged rain, e.g. two or three days or longer, water tables may rise close
to the surface. In cuttings, water may seep from the banks and dampen the road surface. Thus the
situation to watch out for is a frosty spell following a period of prolonged rain.
Other sources – this includes things like natural springs, burst pipes, car washes and industrial
processes. The weather forecast organisations can give no guidance here and local knowledge is
paramount.
Snow.
Types of Snow.
Snow is formed from tiny ice crystals which interlock to form the snowflake. It is less dense than rain
and as a general rule, produces accumulation depths 5 to 10 times greater than rain.
Wet Snow.
This is the more common type of snow in the UK. It falls when the air temperature is close to freezing
(either just above or just below).
It readily melts under pressure, e.g. a car tyre, but may refreeze when the pressure is released. This
property means that it cannot be handled very well by snow blowers, which tend to become clogged. It
can be a very sticky type of snow. For instance, the pressure of wind may melt the snow temporarily as
it presses against walls, roofs etc, but it refreezes when the gust of wind temporarily lulls. Thus, this
type of snow can have a high nuisance factor when it builds up on road signs etc, (a property known as
accretion). The advantage of this type of snow is that it does not drift very much. The snowflake will
drift in the wind as it falls, but once it hits the ground it usually stays there, thus, although drifts may build
up whilst the snow is falling, once the snow has stopped falling, no further drifting takes place. This type
of snow is responsive to salt in small accumulations (under 2cm), but any more than that tends to
swamp the melting effect, particularly if traffic is forced to slow down. The rate at which this snow falls
largely governs how it will lie on roads, lighter snow is less likely to settle on roads than heavy snow, as
it is more likely to melt.
Powder Snow.
This type of snow is much rarer than wet snow, but can lead to many more problems, especially with
drifting. It occurs with air temperatures well below freezing, typically minus 2 degrees Celsius or less. It
is very fine, having a consistency similar to sugar crystals and does not melt so easily under pressure so
it is not sticky. Accretion is not normally a problem and snow blowers can be used quite successfully.
Salt is less successful with this type of snow, mainly because of the lower air temperatures and also
because pressure from traffic is less likely to melt snow (and encourage the salt to go into solution).
The main problem with powder snow is drifting, unlike wet snow, which only drifts whilst falling, powder
snow can carry on drifting long after the snow has stopped. As the snow is very fine, it can penetrate
through gaps in hedges quite easily. Powder snow will continue to drift for as long as air temperatures
remain below freezing and there is sufficient breeze. It is also less dense than wet snow and the rainfall
equivalent is much less. Factors that suppress drifting include air temperatures rising above freezing, a
drop in wind speed and bright sunshine. Bright sunshine may melt the surface of the snow temporarily
and when it freezes a crust is formed on the surface, thus suppressing drifting.
To summarise, drifting of snow is most likely when: 
Air temperatures are below freezing.

There is sufficient wind (normally 8 knots or higher).

The sky is overcast.
Powder snow is fairly rare. It tends to occur with cold easterly winds from the continent with January
and February being the most likely months. At higher altitudes powder snow may also occur with
northerly winds (as temperatures usually decrease with height above sea level).
Local Variations of Snow.
Snow rarely falls uniformly across an area. There are usually considerable variations in snow depth
which are mainly due to two factors - altitude and precipitation intensity.
Altitude.
Air temperatures normally decrease with increasing height above sea level so snow is more likely at
higher altitudes than at lower altitudes, purely because of the lower temperatures.
Precipitation Intensity.
Heavy rain or sleet may soon turn to snow, whereas lighter precipitation remains as rain. This can
sometimes lead to bizarre situations, where isolated snow may occur on lower ground (purely because a
heavy shower occurred there) but the higher ground may see very little.
Factors Affecting Snow Accumulation on Roads.
It is comparatively rare for snow to accumulate on a road surface and even when it does, the snow will
melt relatively easily. However, once snow does start to accumulate it presents quite considerable
problems to traffic.
The accumulation of snow on road surfaces is greatly helped if the road surface temperature is below
freezing which in an average winter is mainly on clear nights. For road temperatures to go below
freezing underneath cloud it requires air temperatures to be below freezing with a moderate wind, which
tends to occur with strong easterly winds from the continent. If a road surface is below freezing, then
any snow that falls will in the absence of salt accumulate even if the snow is very light.
In most cases, the road will be above freezing when the snow starts. However, it is possible under
these circumstances for snow to accumulate. When a snowflake hits a road surface above freezing, it
will melt and as it melts it takes in heat (the latent heat associated with a change of phase of water).
This evaporative cooling is aided if air is dry i.e., the dewpoint less than the air temperature. Thus heat
is removed from the road surface. If the snow is light, the amount of heat removed from the road can be
replaced by heat conducted from the sub road layer (the ground heat flux). However, if the snow is
heavier, the latent heat flux will dominate and the road temperature will fall to freezing point. Generally,
light snow less than 1cm per hour will not settle on roads, but moderate or heavy snow, greater than
1cm per hour, will bring the road temperature down to zero degrees Celsius and the snow will begin to
accumulate. Really heavy snow greater than 4cm per hour will start to accumulate almost
instantaneously.
As a general rule, snow accumulation is unlikely (unless the snow is heavy) with a depth temperature
greater than 3 degrees Celsius. After a prolonged cold spell, with depth temperatures close to freezing,
snow accumulation is more likely. Once snow has stopped falling, it will usually start to melt quite
quickly. The ground heat flux will warm the road surface below the snow cover, encouraging thawing
and if air temperatures are also above freezing, this will also aid thawing.
During the day, sunshine can also be very effective at removing snow. Freshly fallen white snow is
good at reflecting sunshine and therefore prevents thawing, dirty, churned up snow on a road surface is
less effective at reflecting sunshine.
Factors which encourage snow to remain on roads include air temperatures below freezing and/or clear
skies overnight. Clear skies overnight after snow result in temperatures falling quite quickly and any wet
snow or slush may soon freeze.
To summarize, the following conditions increase the likelihood of snow accumulating on roads. The
more conditions that apply, the more likely snow is to settle on roads: 
Road temperatures below freezing (at the start of the snow).

Depth temperatures below 3 degrees Celsius.

Moderate or heavy snowfall (greater than 1cm per hour).

Air temperatures below freezing with a moderate breeze.

Dewpoint below freezing and 2 degrees or more below the air temperature.

Once snow has settled on roads, thawing of the snow will be delayed if: Air temperatures remain below freezing.

During daylight skies remain overcast.

Overnight skies become clear.

The 30cm depth temperature is close to freezing.

Dewpoints are 2 degrees or more below the air temperature.
Traffic is a very important factor in snow accumulation. As said in a previous section traffic tends to
warm the road surface, which in itself will discourage snow accumulation. The traffic also churns up the
snow, allowing the salt to mix quite effectively and encourage thawing. Thus, the busier a road is, the
less likely snow is to accumulate. Purely as a result of traffic, snow is more likely to accumulate
overnight than during the day.
Rain Turning to Snow.
This is perhaps one of the most difficult situations to deal with in winter road maintenance. The amount
of presalting that can be done in advance is minimal as the rain is likely to wash the salt away.
Rain turning to snow is never an exact occurrence, therefore, weather forecast organisations talk in
terms of snow probabilities. A snow probability coupled with a forecast snow depth is a good way of
assessing the risk. For example, a 10% probability of 5cm of snow, (in other words a 90% chance that it
will stay as rain), will produce different planned action from a 90% chance of 1cm of snow.
In the first case snow is unlikely but if it does occur disruption could be high, whereas in the second
case snow is highly likely but should be easily controlled.