Orographic uplift and rain shadows

  • Figure 1: Google map illustrating topography of Wales and positioning of Aberystwyth.

    Figure 2: Cumulative radar for 24 hours 0600 GMT 8th June - 0600 GMT 9th June 2012.

    Figure 3: Total rainfall (mm) in 12 hours at 1800 GMT 8th June 2012.

    Figure 4: Total rainfall (mm) in 12 hours at 0600 GMT 9th June 2012.

    Photo of flooding in Aberystwyth taken from Twitter @Geraintrcymru

  • Orographic uplift and rain shadows
    09.06.2012 13:49

    Severe flooding in parts of central Wales over the last 24 hours has illustrated how the topography of the UK can impact the severity of a weather event. Aberystwyth a small town in Ceredigion, Wales has been affected by severe flooding, as well as several other villages in north-west Wales. But why have they been affected and other areas have not?

    A low pressure system gradually edged east across the UK yesterday, causing an occluded front to become almost stationary across central and northern parts of Wales, as well as parts of northern England. This led to heavy rainfall, across many central areas. Just south of Aberystwyth at Trawscoed, 81mm of rainfall was recorded between 0600 GMT 8th June 2012 and 0600 GMT 9th June 2012. In comparison the average June rainfall at Trawscoed is 78.5mm. In addition to the heavy rainfall, strong winds battered the coast; with wind gusts reaching above 50mph. Unsettled weather like this is not unusual for June in the UK. However, this particular low was deeper than would normally be expected in June; instead it is the kind of system expected to develop in autumn or even winter. 

    Aberystwyth is positioned at the bottom of a valley by the confluence of two rivers, the Ystwyth and the Rheidol, as illustrated in figure 1. In addition to this, it is positioned on the coast of the Irish Sea in Cardigan Bay. This makes it susceptible to strong winds and heavy rainfall. However, it is unusual for flooding to occur as extensively as it has done in the last 24 hours. The reason this depression has affected Aberystwyth so much is due to orographic uplift. 

    Orographic uplift is a phenomenon whereby rain is produced in the presence of high ground due to moist air being made to rise and cool, then condensing out as rain drops. In this instance it was heavier over the Welsh mountains than the West Midlands. Due to Aberystwyth being positioned at the bottom of the valley, as well as the already heavy rain falling, the additional heavy rain over the mountains flowed down to the town. The different intensity of the radar can been seen in figure 2 which illustrates the 24 hour accumulation of rainfall. It clearly illustrates the heaviest precipitation over the central Welsh mountains with over 125mm falling in places. 

    Figures 3 and 4 illustrate the different rainfall levels recorded in Wales yesterday daytime and last night. It is clear that north-west facing sites received more rainfall than eastern areas. The reason eastern areas became drier is due to the rain shadow effect. The rain shadow effect occurs when moist air is able to sink down slope, the temperature increases, causing the cloud to evaporate, therefore reducing the amount of precipitation. 

    Ororgaphic uplift and the rain shadow effect are also very common in parts in Scotland and the Moors in south-west England, depending on what direction the wind is blowing from. It will now be a dry day for those in Aberystwyth having to manage the flooding. A few showery outbreaks of rain will develop on Sunday and Monday, but luckily not as heavy as yesterday’s rain.

    By: Sally Webb