A British Ice Pellet, Freezing Rain and Glaze Event During the Cold Snap of Late January 1996.

Introduction
A cold snap across Britain during late January 1996 saw a prolonged freezing rain, ice pellet and glaze event across a large part of central England and Wales during the night of  23rd/24th January as a frontal system decayed over southern England. This event was untypical of the usual freezing rain events experienced over the British Isles, particularly in its persistence and spatial extent across central England and Wales, and was probably comparable with other notable freezing rain events over southern Britain such as those of late January 1940, late January/early February 1986 and January 1987.

A typical atmospheric profile associated with a freezing rain eventFreezing rain in the UK tends to be a short-lived transitionary feature associated with the approach of a warm front or occlusion, often towards the end of a cold spell of weather, caused as the incoming milder frontal air overrides existing cold, stagnant low-level continental air forming a 'warm nose' of air above a layer of sub-freezing air near and at the surface. This 'warm nose' is a layer of air above 0C of sufficient depth to allow solid precipitation to melt or partially melt as it falls through the layer. The resulting rain and drizzle will then re-freeze into ice pellets or exist as drops of supercooled rain in the cold, sub-freezing layer between the 'warm nose' and the surface. The resulting precipitation will then reach the surface as ice pellets and freezing rain which turns to glaze on impact with objects at or near the surface. The diagram on the left illustrates a typical atmospheric profile associated with a freezing rain event.

In most British cases freezing rain will give way to rain or snow as the weather system approaches, with only a narrow zone tending to be affected by freezing rain at any one time. The time span of freezing rain events depends on the speed of the approaching system but will tend to be relatively short at any one place. More prolonged freezing rain and ice pellet events are far less common both in this country, and around north-western Europe in general, but may cause extensive damage, to trees and overhead cables for example, and accidents when and where they do occur. More notable examples of prolonged and widespread freezing rain and glaze events around the British Isles in recent times include that of late January 1940, late January/early February 1986 and January 1987.

A more unusual set of circumstances is required to ensure a prolonged or widespread spell of freezing rain and ice pellets. Optimum conditions for such a spell of freezing rain requires a longer-term maintenance of the cold air supply near and at the surface and the layer of subsaturated air in the 'warm nose', as illustrated in above. Cold continental air is notorious for its ability to persist at the surface and the cooling effect on the lower layer of air by snow-covered ground, sea-ice or a cold continent would also maintain this cold, sub-freezing layer of air. A subsaturated layer of air in the 'warm nose' caused by descent of air in this layer would reduce the melting rate of particles falling through the 'warm nose' thereby allowing more partially melted particles to reach and then refreeze in the colder lower level air below the 'warm nose', partially melted particles being more likely to refreeze than fully melted particles. Furthermore, in cases of freezing rain ahead of frontal systems, the delaying or prevention of the front arriving at any one place would also prolong a freezing rain/ice pellet event.

The Freezing Rain Event of 23rd - 24th January 1996
Area affected by ice pellets/freezing rain on 23/24 January 1996 based on three hourly synoptic observations and observations made by the Climate Observers Link (COL)A prolonged spell of freezing rain, ice pellets and glaze affected much of Wales and central England as well as parts of southern England on the night of Tuesday 23rd/Wednesday 24th January 1996. The shaded area on the map on the right shows the counties affected. Ice was responsible for a spate of road accidents in these areas, for example a 50% increase in hospital admissions was reported in parts of Birmingham (COL 1996), and also caused problems with power and telegraph cables in some regions, particularly in South Wales where severe icing disrupted power supplies (Eden 1996) for some time. Snow also fell in some regions, with the most persistent and notable fall of snow reported to be in the Cardiff, Bristol and Oxford areas. Snow caused some lanes of the M5 to be closed for a time during the evening. Many of the factors discussed in the previous section were present to enable this unusually extensive and persistent glaze/ice pellet/freezing rain event.
 
 
 
 
 

Synoptic situation 12GMT 23rd Jan 1996Synoptic situation 12GMT 24th Jan 1996The two charts to the left (which I kept from the time) show the synoptic situation over north-western Europe at 1200 GMT on 23rd January (extreme left) and 1200 GMT on the 24th January. The synoptic situation on the 23rd shows a complex area of low pressure to the south of Britain which led to heavy rain and thunder over parts of Spain, Portugal and France and interacted with high pressure over northern and eastern Europe to produce a brisk, bitterly cold east to north-easterly flow across the British Isles. This brought very cold continental air to much of the UK (although still colder air and blizzards followed later in the week as temperatures in the Netherlands dropped as low as -20C by the weekend). Maintening this cold low level flow was critical in making the ensuing freezing rain event possible. The frontal system over southern Britain on the 23rd was moving very slowly northwards and would go on to merge with the decaying warm front stretching from the Thames estuary to the Bristol Channel before ending up over northern England by 1200 GMT on the 24th. This frontal system also decayed and fragmented during its slow northward progression across England and Wales.

During the evening of the 23rd this frontal system became stationary over southern Britain and continuous moderate rain was affecting many southern areas of the country. In some areas rain turned to snow, most notably over South Wales, Avon and Oxfordshire where heavier snowfall caused chaos on some roads. Meanwhile, further north, intermittent and lighter rain affecting much of central England began to turn to ice pellets and freezing rain in places. By 0000 GMT Wednesday 24th, rain over southern England had become lighter and patchier as the frontal system rapidly decayed. However, spells of ice pellets and freezing rain continued to affect a large part of central Britain through the rest of the night and then intermittently through the following day, January 24th.

Profiles of the air over the southern half of Britian during the night of the 23rd/24th January revealed a layer of air above 0C (the 'warm nose') between 920mb and 820mb, the 'warm nose', and colder sub-freezing lower level air approximately between 1000mb and 920mb. Because the weather system responsbile for this severe weather was only moving very slowly, a feed of cold low-level air from the east north-east and a feed of slightly milder air in the 'warm nose' from the east south-east was able to persist for a reasonable length of time thus leading to prolonged fall of freeing rain or ice pellets. Had the weather systems been moving faster conditions would have been no-where near as severe. It seems likely that snowfall around parts of South Wales and south-western England during the evening of the 23rd January was caused by precipitation in this region becoming locally more intense, possibly due to some orographic enhancement, thus cooling the air in the 'warm nose' to below freezing, which in turn would allow precipitation reach the surface as snow.

References/Sources
BROOK, C.E.P. and DOUGLAS, C.K.M. (1956): Glazed frost of January 1940. Meteorological Office Geophysical Memoirs, 12, No. 98, HMSO London.
COL, (1996): Climatological Observers Link, issue 309.
EDEN, P. (1995): Weatherwise. First edition, Macmillan, London, 323 pp.
EDEN, P. (1996): Weather Log, Weather, 51.
HANESIAK, J.M. and STEWART, R.E. (1995): The mesoscale and microscale structure of a severe ice pellet storm. Monthly Weather Review, 123, pp. 3144-3162.
PIKE, W.S. (1987): The glaze accretions of 31 January - 1 February and 3 - 4 February 1986 in England and Wales. Journal of Meteorology, 12, pp.3-17.
TURNER, D.W. and MARRIOT, D.J. (1988): An unusual example of freezing rain. Meteorological Magazine, 117, pp. 255-257.

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Dan Suri, 5 March 2001