Brown Willy Effect (Peninsular Convergence)
Brown Willy Effect (Peninsular Convergence)
Description
The Brown Willy effect is a particular meteorological phenomenon know also as peninsular convergence, which sometimes occurs in the south-west of England. It leads to heavy showers developing over high ground of Bodmin Moor in Cornwall, which then often travel a considerable distance downwind of their place of origin affecting flight operations at airports in their path.
The effect is named after the hill, Brown Willy, the highest point on Bodmin Moor and in Cornwall as a whole.
Causal factors
Cornwall lies in the path of prevailing mild, moist westerly or south-westerlly winds that blow in from the Atlantic Ocean. As these hit land, they are slowed by the effect of friction. Winds near the south coast tend to be diverted to a more northerly direction (backed), this creates a convergence zone near the central/northern spine of the peninsular, leading to uplift and condensation of the moist air.
In the summer, the land heats up more then the sea causing sea breezes to establish during the day. Converging sea breeze fronts from the north and south coasts of Cornwall can lead to rapid convection developing over the moor, which may result in very heavy showers or thunderstorms.
Brown Willy effect as seen from weather radar on 30 July 2007 [Source: Weatherquest]
Once the showers have formed, they often travel in an easterly or north-easterly direction, driven by the prevailing wind. This causes a thin line of showers to occur along the length of the south-west peninsular, all originating from the Bodmin Moor area, which are often intensified further by the downwind high ground of Datrmoor and the Blackdown hills. Places directly in the path of these showers suffer a series of heavy downpours, whilst nearby locations remain dry. On 27 March 2006, a continuous line of showers stretched from Bodmin Moor to Burford, Oxfordshire - a distance of about 140 nm.
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