Report - London Borough of Hillingdon

WINTER SERVICE OPERATIONAL PLAN
London and the suburbs and on occasion’s differences of 8 degrees Celsius in air
temperature can occur. Winds blow most frequently from the west or southwest over London
so that the Heat Island extends northeast from the City itself when long-term averages are
examined. Again variations on a given night occur and, for example, with easterly winds the
Heat Island is displaced westwards.
Road meteorology in the winter
General comments
This analysis has again used air temperature data to delineate the domains, but it would be
unwise on a given night to infer a road temperature from air temperature measurements.
The actual road temperatures observed across a domain or borough will be affected by
numerous factors such as road construction, shading of the road and elevation, as well as
meteorological factors, and it is not possible to sub-divide finely enough to cover all stretches
of road. Even within the domains there will be variations with local hot and cold spots. For
example, well shaded roads will be colder by day but overnight, reduced sky-view in heavily
built-up areas, will keep roads relatively warm compared to well exposed roads. The weather
on an individual night will determine the distribution of temperature, but the climatic domains
based on average overnight temperatures remain a useful sub-division.
A possible sequence of events overnight
On calm, clear nights lower areas will in general be colder and the London Heat Island effect
will be at its greatest, with large temperature differences between Domain 1 and the colder
domains. On windy, cloudy nights, higher ground will be coldest, and the Heat Island effect
will be least evident. Warning signs for a cold night are a clear sky, low humidity (dry air)
before sunset and little or no wind.
Dry air cools more rapidly whereas moister air may allow fog to form before the air
temperatures dips to zero, with air and ground temperatures steadying off or even rising as
the fog thickens. In the evening frost may form on grass and other surfaces that cool rapidly
eg cars. The depth of this cold and possibly frosty air will increase as successive layers of
the air are cooled.
The temperature of different surfaces
A frost can occur and hoar frost deposits may be seen on grassy surfaces, car windscreens,
car roofs and house roofs long before road temperatures dip to zero and indeed, frost can
occur on such surfaces without road temperatures falling sub-zero on a given night. As a
general rule of thumb road temperatures are warmer than most other surfaces due to a
reservoir of heat beneath the road surface and the temperature over a grass surface can be
3 to 6 degrees Celsius colder than a nearby road surface.
Blades of grass are poor conductors and do not conduct heat from the ground and air tends
to be still between the blades themselves. Although metal is a good conductor of heat,
parked cars are also cold, being thermally insulated from the ground, and cars can be one of
the coldest surfaces observed overnight by up to 8 degrees. The air coming into contact with
the grass or car will be cooled and dew is then deposited, with hoar frost forming if the
temperature of the surface falls sub-zero.
Hoar frost deposits are a good visual sign of a sub-zero surface, but absence of hoar frost
deposits does not however mean a road, or indeed any other surface, is above freezing.
These dry frosts occur when the humidity of the air is very low, typically with winds from the
east and to a lesser extent the northwest.
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