Report - London Borough of Hillingdon

WINTER SERVICE OPERATIONAL PLAN
74BPeriod
Used to describe the length of cloud breaks or amounts of sunshine, generally of two hours
or more duration.
75BPowder Snow
The form of snow that occurs when air temperatures are well below freezing, (-2 o C or less).
This form of snow is very fine (like sugar crystals), drifts very easily, but does not tend to
stick to objects, (no accretion). It can be handled by snow blowers. Salt is usually less
effective.
76BPrecipitation
A general term that covers all water (or ice), which ‘falls’ from the skies. As well as rain,
sleet, snow and hail it also includes dew, hoar frost and fog.
7BPrecipitation Type
The individual type of precipitation. In winter maintenance activities, this will be one of rain,
drizzle, sleet, wet snow, dry snow, hail, freezing rain and freezing drizzle, (see Precipitation).
78BProlonged
Used to describe showers that merge together producing a spell or continuous precipitation,
generally lasting more than one hour and covering a relatively large area.
79BRain
Water droplets that fall from clouds. Rain takes many forms, and can be of many different
intensities and duration’s. Within road weather forecasts there will often be differentiation
between rain and showers, the former usually referring to longer-lived but light intensity
precipitation, and the latter to short duration but heavy intensity.
80BRelative Humidity
This is the amount of actual water vapour held in a sample of air at a given temperature,
divided by the maximum amount of water that could be held in that sample of air at that
temperature, expressed as a percentage. Within fog or heavy rain, humidities may reach
100%. On a sunny, warm afternoon in summer, humidities may fall to 30%. On an average
night in winter, humidities rarely fall below 80%, (which is the minimum humidity at which salt
crystals will start to absorb water).
81BRime
Deposition of ice from freezing fog. It is a white form of ice, similar to hoar frost, but has a
finer (at times feathery), structure. On roads, tends to be more of a problem at higher levels
than lower levels.
RST
Common abbreviation for road surface temperature. Saturated Air is said to be saturated
when, at a given temperature, it holds the maximum amount of water vapour possible. Any
cooling below its current temperature will result in condensation. The relative humidity of
saturated air is 100%.
Saturated
The rate at which air temperature falls with height within adiabatic lapse saturated air, e.g.,
within fog or cloud. It is less than the rate dry adiabatic lapse rate, 0.49 o C per 100 metres or
around 1.5 o C per 1,000 feet.
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