Report - London Borough of Hillingdon

WINTER SERVICE OPERATIONAL PLAN
APPENDIX 9
39BWeather Terminology
40BAccretion
The build-up of snow on objects such as overhead cables, road signs and tree branches. It
occurs when wet snow with temperatures close to freezing is accompanied by a strong wind.
The wet snow freezes on to objects under pressure of the wind. The build up of snow can be
quite large, and can cause damage to cables and trees.
41BAir Frost
This occurs when air temperatures, (measured between one and two metres above the
ground) fall below 0 o C.
42Black Ice
Clear ice forms on roads due to the freezing of standing water. Occasionally it may be
formed by the transformation of hoarfrost, under pressure from car tyres. The name black
ice is used, as the road blacktop can be seen through the clear ice. The term is much
beloved by the media, but is used less frequently in road weather forecasts.
43Blustery
Used to describe showers that are accompanied by strong gusts of wind.
4BClimatic domain
An area of a county with broadly similar climatic characteristics, e.g., an urban area, or a high
level area, or a coastal area (see Forecast site).
45BCondensation
This is the change of state of water vapour to liquid water, thus forming a thin film or mist of
water on surfaces such as roads. During the process, heat is released (see Dew point).
46BConfidence Factor
Used by weather forecast organisations to give guidance to highway engineers on the
likelihood of forecasts having to be subsequently amended. Confidence HIGH means that
amends are unlikely, and confidence LOW that amends are likely. Some use is made of
MEDIUM confidence, although usage is discouraged, as it can be confusing.
47BDamped
This is the thermal map type that occurs on cloudy, windy nights. Temperature differences
along a stretch of road are at a minimum, (see Thermal map).
Deposition
This term covers the change of state from water vapour to ice without going through the
liquid water stage, (see Hoar frost).
48BDew
Liquid water formed on a surface by condensation from the atmosphere.
49BDew Point
The temperature to which a sample of air must be cooled for condensation to take place.
Dew point can be measured directly by instrumentation, e.g., road sensors.
50BDrifting
The movement of snow (usually powder snow) under the influences of wind. Snow need not
be actually falling for drifting to take place.
65