Weather Blog

So why so much rain?

Some that watch Cwmparc weather station may question why we record such high rain totals in Cwmparc when stations a few miles south record much less. Where is the rain gauge? Under a waterfall?

This is something that perplexed me when I started taking records manually in Cwmparc - at that time using a glass instrument to measure the rainfall. I quickly noticed that sometimes two inches or more had sometimes fallen into my gauge when maybe only half an inch had fallen less than 20 miles away. I also noticed the nearest weather station to Cwmparc - Nantymoel - only a few miles to the South West as the crow flies - often recorded less rainfall - sometimes quite large amounts - particularly for a station so near.

Firstly I have to say the instruments I use are good ones, they are calibrated well, and sited to do the job they are meant to do well. But there has to be a reason so what was it? This is where I quickly learned an important lesson in topography.

The station to the South West of me is over on the other side of the Bwlch mountain - the first mountain in a series that the moisture laden sub tropical air from that direction encounters on the way to and over the Brecon Beacon's. As our wettest weather comes from the South West to Westerly quarter weather systems - and more important winds - pass over Nantymoel weather station first. But as the moisture laden wind approaches the Bwlch mountain air is forced up the steeply sided mountain that has a top height of 1600 foot. Simply put this warm moist sub maritime air is pushed up into the colder air above, where the moisture rapidly condenses into thicker clouds. Just over the other side of the mountain, on the other slope at 770 foot is Cwmparc weather station. And of course it here that the results of all that condensation is seen - in the form of heavy rain. Comparing conditions with the Nantymoel station and mine it can often be observed that it is dry in Nantymoel - while raining at Cwmparc.

Often was the case when I worked by the seaside about 15 miles away as the crow flies - for me to hear it is raining at home - even though I was basking in sunshine. Amazing isn't it. The price we pay for living on high ground. Weather here is all to do with Topography that give us very localised weather. Of course this means the recorded rain for Cwmparc being so very localised is not a general indication for South Wales in general.