By Roger Brugge
This tongue-in-cheek look at Reading’s weather in winter and its relationship to cycling safety was prompted by a couple of ice-related accidents experienced by members of staff during the cold and foggy weather of the morning of 24 January 2017 while cycling over the new cycle/pedestrian bridge in the town.
The University’s Atmospheric Observatory, along with several other UK climatological stations, has been recording concrete minimum temperatures since about 1971 (1972 is actually the first year at the university with a complete annual record). Such measurements were begun in the days when road traffic was beginning to increase in volume and the impact of cold weather on road surfaces was identified as being a potential problem. A greater understanding of the climatology of road surface conditions was therefore sought – in addition to the traditional ‘state of ground’ and snow depth reports.
The measurements are made in a similar way to that used in determining the grass minimum temperature – measurements are made using a grey concrete slab about 90 x 60 cm, and about 5 cm thick. If snow falls and covers the concrete slab, then the slab is swept clear of the snow (without disturbing the index of the thermometer) – and the concrete minimum thermometer replaced on the slab.
Figure 1. Annual incidence of concrete minimum temperatures below 0 °C (blue) and below -5 °C (red) at the University of Reading during 1972-2016.
As with many winter-time meteorological measurements at Reading there are large inter-annual variations. Figure 1 show that a concrete frost (when the reading drops below 0 °C during a 24-hour period) has varied between about 40 and 100 nights in a calendar year, while a sharp frost (a reading below -5 °C) has occurred on between 2 and 33 days in a year.
Figure 2 shows the distribution of concrete frosts within the year. An average of 75 concrete frosts and 13 sharp concrete frosts can be expected in a 12 month period. Concrete frosts have been recorded as late as the first week of June (1991 and 1994) and as early as 16 September (in 1975).
Figure 2. Average monthly incidence of concrete frosts at the University of Reading, averaged during 1972-2016.
On the morning of the accidents in question, the concrete minimum recorded at the university was -4.7 °C – slightly higher than on preceding days. The lowest recorded concrete minimum temperatures at the site are those of -14.9 °C on 14 January 1982, -13.4 °C on 15 January 1982 and -12.5 °C on 10 February 1986. Surface temperatures on a bridge are likely to be lower than those on a road surface in contact with the ground as the latter will be fed by heat from the underlying ground. Of course temperature per se is not a danger to cyclists (unless it is accompanied by a wind that is strong enough to cause significant wind chill to the fingers if the cyclist). This was not the case in the events above, when the presence of (frozen) moisture on the road surface (possibly deposited by freezing fog) was the root cause of the incident. So, thinking of the observations made each morning, which might be used as an indicator of frosty surfaces? The following spring to mind:
- Lying snow – we record the state of the ground and the depth of any lying snow, so an indicator of days with lying snow (at 0900 UTC) is easy to determine. In this analysis the presence of any snow cover is included, not just the usual ‘at least 50% cover’ days.
- Surface ice – we record this when there is no snow lying on the grass in the observatory. Sometimes lying ice is the result of melting snow that has subsequently frozen, rather than the result of North American-type ice storms.
- Frozen ground – this can result (as was the case in January 2017) in frost persisting and building up on the surface throughout the day if the surface remains in shade and the daytime air temperatures do not rise too much.
- Freezing fog at 0900 UTC (or in the preceding hour) – if thick enough and prolonged enough this can lead to the deposition of rime and ice. Unfortunately, we only have visibility measurements at 0900 UTC along with indications of fog in the preceding hour if it has cleared by this time.
The incidence of these four conditions were extracted from the daily register of 0900 UTC observations made at the university and the occurrence of these ‘hazardous days’ throughout the year was then examined – see Table 1.
|Number of days with a type of weather hazard||Total days affected by at least one hazard||Likelihood of a hazard day, %|
|Period||Lying snow||Surface ice||Frozen ground||Freezing fog|
Table 1. Occurrence of hazard days at the University of Reading, 1972-2016.
Table 1 suggests that such weather hazards occur on about one day in five to one day in three in the three winter months in Reading, with frozen ground and/or lying snow being the most prevalent type of hazard.
Clearly with these occurrence frequencies, periods of consecutive days with weather hazards are more than likely in most winters. In particular, during 1972-2016 there were 25 instances of spells of at least three consecutive days each of which had greater than 50 % snow cover (an average of more than once every second winter season).
In addition there were 156 of spells of at least three consecutive days each with an instance of hazardous weather in the same period – or 3.4 spells each year on average.
Interestingly, Reading Borough Council in their Winter service plan 2016 – 2017 (see page 10, section 7.5; ) state:
The Council does not promote the use of bicycles during periods of prolonged hazardous conditions. With the exception of shared carriageway routes on a primary or secondary precautionary salting route shared footway/cycleways and remote cycleways are not salted when frost, ice or prolonged hazardous conditions are forecast. Being part of the carriageway, shared carriageway/cycleway routes on the primary and secondary precautionary salting route networks will be salted by default in accordance with the Winter Service Plan.