What a 12 months: one minute we are having record temperatures (August 2003), drought conditions and crops shrivelling in fields, then six months down the line the country is blanketed in snow; while a week later flood warnings are the cry! What is happening? Are we in the midst of extreme weather? Today, Lancashire along with much of the UK woke up to water everywhere! The Environment Agency is putting flood warnings out by the dozen. Why? Basically our weather is by its very nature changeable. This is largely due to our proximity to the Atlantic Ocean, where weather system after weather system sweep in from the Americas. These weather systems are driven by the jet stream; a large river of air that rushes along at speeds of around 200 mph, 8-10 km above us. This jet stream steers these weather systems and often during the winter there is nothing to stop them battering our shores. Because these moisture-laden depressions (moisture is swept up into them as they cross the Atlantic) come from the west, much
of the moisture is dumped here in Lancashire and counties along western Britain. By the time they have risen up and dropped more of their load on the Pennines (due to a process called orographic uplift), there is little left to deposit on the eastern side of the country, thus eastern UK records rainfalls up to half of those experienced in counties such as Lancashire. Sunny winter days generally originate from cold high-pressure systems, which bring northerly, or easterly winds that drag cold air from the Artic or from cold continental landmasses such as Russia respectively. This is in stark contrast to the warm moist air that sweeps off the Gulf Stream out in the Atlantic or the warm southern winds that originate in the subtropics.
In early February 2004, 40.2mm of rainfall fell between 9am Monday 2nd and 9am Tuesday 3rd; simultaneously the temperature has been around 11-13 degrees centigrade. This combination of precipitation and warm air is due to the strong southwesterly winds that are prevailing presently. Moist warm air has been dragged from the subtropics across the Atlantic sweeping up moisture from the warm Gulf Stream on its way before reaching the western shores of Britain. This air is unusually warm for February due to the Atlantic being 2 degrees centigrade warmer than it would normally be at this time of the year.
While 40mm of rainfall is not a record (2nd Feb), it is only the 21st time since 1965 (almost 40 years) that the rain gauges at Hazelrigg Weather Station at Lancaster University have registered more than 40mm in a day (the record being 68mm on the 8th December 1983)! Thus, rainfall of this magnitude can be expected to occur on average, only once every two years. In December 2003, 131.3mm of rain fell, while this is above the average of 109mm, it is not too excessive and is less than half the record 268mm that was recorded at Hazelrigg in October 2000 during the wettest Autumn on record. This January has seen 143.6mm (long term average for January is 100mm) and the first four days of February a further 80mm (long term average for February 70mm) February is historically the third driest month at Hazelrigg (after April and May) and this is the wettest start to the month recorded here. It remains to be seen how the rest of the month develops, it is forecast to become clearer and colder as the winds shift towards a more northerly direction by the weekend. This February is in stark contrast to last year when just over 50mm of rain was recorded; but even that is in stark contrast to the driest February at Hazelrigg when a mere 3.2mm fell during the entire month. The wettest February on record in Lancaster in recent years was 1983 when 192mm was recorded. The river Condor was witness to the high rainfall as along with many other rivers and streams in the area it burst its banks and over-spilled onto its flood plain. Even though these areas only flood every few years it emphasises the importance of protecting and preserving the floodplain.
The main reason for the flooding has been the intensity of rainfall; the last five days (30th, 31st Jan and 1st, 2nd 3rd and 4th Feb) have seen over 120mm. This is 12% of Lancaster’s annual mean precipitation in less than 6 days. The ground simply can’t soak this amount of water in, and the consequence is a huge amount of overland flow resulting in the floods we have all witnessed recently. The only good news is this rainfall will help to top reservoirs up after last summer’s drought. However, the ideal intensity would be steady rainfall over a sustained period so more can soak into the ground and recharge the underground aquifers, which store much of our water reserves. In short, if we are to avoid a summer of water shortages and standpipes, we should embrace winter precipitation; however intense bouts of rainfall such as the last 4 days is not the best way to receive it.
Phil Leigh 10th Mar 2004.
 Phil Leigh<p.leigh@lancaster.ac.uk > 12/03/04 |
